Tropical and Subtropical Ostreidae of the American Pacific: Taxonomy, Biology, Ecology, and Genetics

Abstract

Oysters have throughout history been one of the most important marine invertebrate animals used, whether as human food or as a cultural base. Today, they represent one of the most exploited natural resources, are produced by aquaculture activities, and are one of the most studied groups of shellfish. This is most evident in temperate zones, whereas studies in tropical zones have been comparatively scarce and somewhat disorganized. The present review organizes the studies of a dozen species that are grouped as oysters of the tropical and subtropical American Pacific, considering their taxonomy and identification, as well as their distribution, and establishing an identification key using the characteristics of the shell. Aspects of their biology are described, referring to general anatomy with a type species, the Cortez oyster Crassostrea corteziensis, as well as the life cycle, taking as reference the recent studies carried out on the rock oyster Striostrea prismatica. In the same way, a description of their
populations and ecological interactions is provided, emphasizing the reproduction of the different species and ordering the description of the main stages in the gametogenic development of the populations in a latitudinal form. The main diseases and some uses of oysters as bioaccumulating organisms are also described, as well as phylogenetic and population genetic studies. Finally, possible future actions are discussed to provide a more comprehensive knowledge of oysters from the tropics and subtropics of the American Pacific based on the conservation and use of the resources.

Full text

TROPICAL AND SUBTROPICAL OSTREIDAE OF THE AMERICAN PACIFIC: TAXONOMY,
BIOLOGY, ECOLOGY, AND GENETICS
C´
ESAR LODEIROS,
1,2
* PAUL VALENTICH-SCOTT,
3
JORGE CH ´
AVEZ-VILLALBA,
4
JOS ´
E MANUEL MAZ ´
ON-SU ´
ASTEGUI
5
AND JOS ´
E MANUEL GRIJALVA-CHON
6
1
Grupo de Investigaci ´
on en Biolog ´
ıa y Cultivo de Moluscos, Escuela de Acuicultura y Pesquer´
ıa Facultad
de Veterinarias, Universidad T ´
ecnica de Manab´
ı, Calle Gonzalo Loor Velasco, Bah ´
ıa de Car ´
aquez
EC131459, Ecuador;
2
Instituto Oceanogr ´
afico de Venezuela Universidad de Oriente, Av. Universidad,
Cuman ´
a 6101, Venezuela;
3
Department of Invertebrate Zoology, Santa Barbara Museum of Natural
History, 2559 Puesta del Sol Road, Santa Barbara, CA 93105;
4
Centro de Investigaciones Biol ´
ogicas del
Noroeste (CIBNOR), Unidad Sonora, Camino al Tular Km. 2.35, Estero de Bacochibampo, Guaymas,
Sonora, Mexico C.P. 85506;
5
Centro de Investigaciones Biol ´
ogicas del Noroeste (CIBNOR), Calle
I.P.N. No. 195, Col. Playa Palo de Santa Rita Sur, La Paz C. P. 23090, M ´
exico;
6
Departamento de
Investigaciones Cient ´
ıficas y Tecnol ´
ogicas, Universidad de Sonora, Hermosillo, Av. Colosio s/n, entre
Reforma y Sahuaripa, Sonora 83000, Mexico
ABSTRACT Oysters have throughout history been one of the most important marine invertebrate animals used, whether as
human food or as a cultural base. Today, they represent one of the most exploited natural resources, are produced by aquaculture
activities, and are one of the most studied groups of shellfish. This is most evident in temperate zones, whereas studies in tropical
zones have been comparatively scarce and somewhat disorganized. The present review organizes the studies of a dozen species that
are grouped as oysters of the tropical and subtropical American Pacific, considering their taxonomy and identification, as well as
their distribution, and establishing an identification key using the characteristics of the shell. Aspects of their biology are
described, referring to general anatomy with a type species, the Cortez oyster Crassostrea corteziensis, as well as the life cycle,
taking as reference the recent studies carried out on the rock oyster Striostrea prismatica. In the same way, a description of their
populations and ecological interactions is provided, emphasizing the reproduction of the different species and ordering the
description of the main stages in the gametogenic development of the populations in a latitudinal form. The main diseases and
some uses of oysters as bioaccumulating organisms are also described, as well as phylogenetic and population genetic studies.
Finally, possible future actions are discussed to provide a more comprehensive knowledge of oysters from the tropics and
subtropics of the American Pacific based on the conservation and use of the resources.
KEY WORDS: oyster, bivalve, mollusc, reproduction, geographical distribution, life cycle, Crassostrea gigas,Crassostrea
sikamea,Dendostrea folium,Dendostrea sandvichensis,Undulostrea megodon,Saccostrea palmula,Ostrea angelica,Striostrea
prismatica,Ostrea conchaphila,Crassostrea aequatorialis,Crassostrea columbiensis,Crassostrea corteziensis
INTRODUCTION
Oysters are bivalve molluscs widely distributed globally and
are considered one of the most economically important and
appreciated seafoods in the world. In the tropical region of the
American Pacific, they are listed as commercially important
species within the invertebrates (D´
ıaz et al. 2014, Posada et al.
2014, Ross et al. 2014). Their demand, consumption, and use
today are mainly satisfied by aquaculture and are, thus, of ut-
most importance for the local fishing economies (Sevilla-
Hern ´
andez 1993). In 2017, the world oyster production was
approximately 5.9 million tons, with harvest production being
97.5% of the total production, and oysters are positioned as the
fourth most important seafood group globally, by volume of
production, after cyprinid fish, seaweed, and clams (FAO 2020).
In addition, to fulfilling their role as primary producers
within the trophic chain, oysters are ecologically a keystone in
marine ecosystems, providing ecological services as filter-
feeders, calcifiers, and reef-builders, and for this last reason,
they are considered bioengineers (Smaal et al. 2019); however,
oyster populations have been declining because of overfishing,
habitat destruction, and diseases (Beck et al. 2011), and their
protection and management require a sound biological
baseline.
In a taxonomic context, the shell plasticity of ostreid species
has been a troublesome matter, where some ‘‘forms’’ of several
species have been assigned different names by several authors
(Harry 1985, Guo et al. 2018). Reports on the difficulty of
proper species identification caused by the wide variation in
body size and coloration are frequent in the literature, so the
geographical distribution is somewhat uncertain. This taxo-
nomic uncertainty affects the fishery statistics because some
individuals combine multiple species as a single species, and this
gives the data little analytical value with which to draw con-
clusions and make fisheries administrative decisions.
The present review organizes the knowledge of the Ostreidae in the
tropical and subtropical American Pacific and presents an analysis of
aspects of their taxonomy, classification, biology, ecology, geographic
distribution, and genetics. Although there are a dozen species of
oysters in the region, most of the information in this study is focused
on two local species (Crassostrea corteziensis and Striostrea prisma-
tica) and on the introduced species (Crassostrea gigas).
Taxonomy, Identification, and Distribution
Oysters are members of the family Ostreidae (Rafinesque-
Schmaltz 1815), order Ostreida (F ´
erussac 1821–1822), subclass
*Corresponding author. E-mail: cesarlodeirosseijo@yahoo.es
DOI: 10.2983/035.039.0202
Journal of Shellfish Research, Vol. 39, No. 2, 181–206, 2020.
181

Pteriomorphia (Beurlen 1944), and class Bivalvia (Linnaeus
1758). Sixteen living genera with about 75 species are known.
Six genera are currently documented in the tropical eastern
Pacific. The family occurs as early as the Triassic period.
Despite being one of the most common and economically
important bivalve molluscs consumed around the world, oys-
ters are exceptionally difficult to identify. Oyster shell shape is
greatly influenced by the substratum to which they attach. They
also are frequently covered with epibionts, including other
oysters. That being said, there are reliable shell features that will
assist in the identification of most specimens.
Some molecular studies have reorganized the taxonomy of
oysters in the eastern Pacific. For instance, Polson et al. (2009)
demonstrated that Ostrea lurida (Carpenter 1864) and Ostrea
conchaphila (Carpenter 1857) are two distinct species despite
having nearly identical shell morphology. Raith et al. (2016)
have further defined the distribution of this species complex, as
well as synonymizing the genus Myrakeena (Harry 1985), and
the species Ostrea tubifera (Dall 1914). It is hopeful that these
landmark studies will be pushed forward to all oysters in the
eastern Pacific, and will also yield additional morphological
characteristics to separate them.
Additional genetic studies have led to the creation of new
generic and higher level names in the Ostreidae (Salvi &
Mariottini 2016, Guo et al. 2018). Following suggestions by
Bayne et al. (2017), the generic allocations have been main-
tained as outlined in Coan and Valentich-Scott (2012) and
Valentich-Scott et al. (2020) until the taxa defined in these ge-
netic studies have stabilized. A diagnostic characteristic for each
species and an explanation of the current taxonomy are pro-
vided. Detailed descriptions of each species are modified from
Valentich-Scott et al. (2020), Coan and Valentich-Scott (2012),
and Coan et al. (2000).
In the taxonomic synonyms in the following text, we use the
term ‘‘of authors.’’ This implies that an author or authors have
misapplied a species name in the literature.
Description of Shell Characteristics
Identification of oyster shells requires an understanding of a few
specific terms (Fig. 1A–E). The left/right orientation of an oyster shell
is not immediately apparent. The right valve is almost always the
upper valve, whereas the attached valve is almost always the left valve
(Fig. 1D), and the convexity is frequently a useful diagnostic char-
acteristic. The height of the shell extends from the beaks above the
hinge margin (dorsal) to the opposite (ventral) side (Fig. 1E). The
length is the maximum dimension approximately 90°from the height
measurement. In oysters the height is usually greater than the length.
The adductor muscle scar is mostly posterior of the midline of the shell
(Fig. 1E). Thus, the left valve has the adductor muscle scar mostly to
the left of the midline, and the right valve has a scar mostly to the right
of the midline.
The amount of shell attachment to the substratum is some-
times diagnostic. Some species have only a slight area of at-
tachment on the left valve, whereas others have extensive areas
of attachment. The shell margin might be strongly undulate
(Fig. 1C), or not undulate (Fig. 1D). The exterior valve sculp-
ture can be useful in identification if not heavily eroded or
encrusted with epibionts. Several species have strong radial
plicae (ribs) (Fig. 5E, F, I), some have lamellate sculpture
(Fig. 3H, K), and others are mostly smooth (Fig. 2A).
Internal shell characteristics are often more helpful than ex-
ternal ones in separating the tropical eastern Pacific oysters. The
presence or absence of small denticulations (chomata) along the
shell margin near the hinge can be very useful to separate genera
(Fig. 1A, B); however, sometimes chomata are greatly reduced in
certain specimens (e.g., Ostrea conchaphila).Onemustbecarefulto
make an immediate or hasty identification based on a single shell
character. Using the entire suite of the aforementioned character-
istics leads to an accurate identification.
Crassostrea aequitorialis
Figure 2A–F
[Synonyms: Ostrea aequatorialis (dÕOrbigny 1846: 672).]
Diagnosis
Shell usually long, narrow, attached to mangrove root; lig-
ament groove long; shell margin not undulate; chomata absent.
Description
Shell dorsoventrally elongate, some specimens more ovate;
shell thick, heavy; left valve shallowly concave; right valve flat
to slightly concave; left valve usually broadly attached to
mangrove roots; sculpture reduced in both valves, some with
irregular commarginal lamellae; ligament groove moderately
long, broad; chomata absent; exterior color cream with spo-
radic purple rays or blush in some; interior color cream to light
brown, with darker patches in some; height to 80 mm.
Distribution
Isla Luna, Guayaquil, Ecuador (2.2°S); intertidal zone, on
mangroves. Likely many other South American localities, but
the distribution has been confused with Crassostrea corteziensis.
Remarks
This species has been confused in the literature with Cras-
sostrea corteziensis. Compared with this species, Crassostrea
aequitorialis is less inflated, has a thinner shell, a longer ligament
groove, and is attached to mangrove roots. Its distribution is
undoubtedly extensive, but has yet to be resolved.
Crassostrea columbiensis
Figure 2G–L
[Synonyms: Ostrea columbiensis (Hanley 1846: 107); Ostrea
ochracea (Sowerby 1871: pl. 10); Ostrea tulipa (Lamarck, of
authors, not Lamarck 1819); Ostrea chilensis (Philippi, of au-
thors, not Philippi 1844).]
Diagnosis
Shell round to elongate, very thin, attached to mangrove
root; ligament groove short; chomata absent.
Description
Shell irregular in shape, typically ovate–elongate, usually thin
relative to other Crassostrea; both valves mostly flat, with slight
convexity in some; broadly to moderately attached by left valve,
usually to mangrove roots; left valve larger than right, frequently
flared over right valve; sculpture of both valves with fine com-
marginal striae and growth checks; ligament groove short, broad;
chomata absent; external color cream with sporadic purplish-red
LODEIROS ET AL.182

blush, and darker brown near beaks, some with rays of lighter
color; exterior of right valve frequently shiny; interior color white
to cream, often with dark purple margin; height to 80 mm.
Distribution
Bah ´
ıa San Bartolom ´
e, Pacific coast of Baja California Sur
(27.7°N) to Col ´
an, Piura, Peru (5.0°S); intertidal zone, particu-
larly in mangrove swamps, attached to roots and to each other.
Remarks
The species is recorded as early as the Pliocene of California
and Baja California.
Crassostrea corteziensis
Figure 3A–F
[Synonyms: Ostrea corteziensis (Hertlein 1951: 68); Ostrea
chilensis (Philippi, of authors, not Philippi 1844).]
Diagnosis
Shell very heavy; left valve deeply concave, broadly attached
to substratum; right valve mostly flat; chomata absent.
Description
Shell irregular in shape, from ovate to dorsoventrally elongate,
some arcuate; shell very thick, heavy; left valve moderately to deeply
concave; right valve flat to slightly concave; narrowly to broadly at-
tached by left valve, more broadly attached to shells; sculpture of both
valves with irregular commarginal lamellae, left valve with fluted la-
mellae in some; ligament groove broad, long; chomata absent; exte-
rior color cream with sporadic brown blush in some; interior color
cream to white, with purple patches in some; height to 250 mm.
Distribution
Bah ´
ıa Kino, Sonora, Mexico (28.8°N), to Isla Can
˜as, Pan-
ama (9.0°N); intertidal zone, on rocks and shells.
Remarks
The species is recorded as early as the Pliocene of California
and Baja California.
Crassostrea gigas
Figure 3G–L
Figure 1. Shell characteristics of Ostreidae. (A) Hinge with smooth margins and long ligament groove; (B) Hinge with chomata and broad ligament; (C)
Left (bottom) valve nearly flat, margin strongly undulate; (D) Left valve deeply convex, margins not undulate; (E) right (top) valve, adductor muscle scar
mostly posterior of midline.
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 183

[Synonyms: Ostrea gigas (Thunberg 1793: 140); Ostrea
laperousii (von Schrenck 1861: col. 411); Ostrea talien-
whanensis (Crosse 1862: 149); Ostrea gravitesta (Yokoyama
1926: 388); Ostrea posjetica (Razin 1934: 36); Lopha
(Ostreola) posjetica beringi, posjetica zawoikoi, posjetica
newelskyi (Vialov 1946: 523).]
Figure 2. Oyster species of the eastern Pacific: (A–F) Crassostrea aequatorialis, Ecuador, Guayaquil, Isla Luna. BMNH 1854.12.4.823 (holotype),
length 44 mm, height 80 mm. (G–L) Crassostrea columbiensis, Ecuador, Guayas, Santa Elena. SBMNH 120,192 (neotype), length 62 mm, height
59 mm. (A, G) Exterior right valve; (B, H) Exterior left valve with mangrove root adhering to shell; (C, I) Interior of the right valve; (D, J) Interior of the
left valve; (E, K) Lateral view of both valves; (F, L) Hinge of the left valve.
LODEIROS ET AL.184

Diagnosis
Shell height usually much greater than length; left valve
deeply concave; sculpture of long thin lamellae; chomata
absent.
Description
Shell irregular in shape, from ovate to more typically
elliptical; left valve deeply concave; right valve flat to
slightly convex; usually narrowly attached by left valve;
Figure 3. Oyster species of the eastern Pacific: (A–F) Crassostrea corteziensis,M´
exico, Sonora, Bah ´
ıa Kino. CAS 064,947 (holotype), top valve length
83 mm, height 140 mm and bottom valve length 103 mm, height 152 mm. (G–L) Crassostrea gigas, Mexico, Sonora, Bah ´
ıa la Choya, intertidal. SBMNH
359915, length 61 mm, height 119 mm. (A, G) Exterior right valve; (B, H) Exterior left valve; (C, I) Interior of the right valve; (D, J) Interior of the left
valve; (E, K) Lateral view of both valves; (F, L) Hinge of the left valve.
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 185

sculpture of commarginal frills; frills frequently abraded,
but much prolonged in some habitats; ligament groove
long, narrow to broad; chomata absent; external color gray
to purplish, sometimes in radial bands; internal color white,
generally with light-colored adductor muscle scar; height
to 450 mm.
Distribution
Native to the western Pacific from Sakhalin Island, Russia, to
Pakistan; introduced into Japan, Europe, and Australia. On the
American Pacific coast, it has been introduced into the USA,
Canada, Mexico, Guatemala, El Salvador, Honduras, Nicaragua,
Costa Rica, Panama, Colombia, Ecuador, Peru, and Chile.
Remarks
Following the recommendations by Bayne et al. (2017), we
are continuing to place this species in the genus Crassostrea.
Crassostrea sikamea
Figure 4A–F
[Synonyms: Ostrea gigas sikamea (Amemiya 1928: 335); Crassostrea
gigas ‘‘kumamoto,’’ of authors, infraspecific [ICZN Code Art. 45f (iv).]
Diagnosis
Shell height not much greater than shell length; left valve
deeply concave; right valve flat; chomata absent.
Description
Shell tear-drop shaped, relatively small; left valve deeply
concave; right valve flat to slightly convex; sculpture reduced,
primarily of commarginal growth checks and irregular radial
folds (primarily on left valve); ligament groove long, narrow;
chomata absent; external color cream to dark gray; internal
color white, adductor muscle scar dark purple; height to 60 mm.
Distribution
Native to the western Pacific, and brought to the West Coast
from southern Japan; raised commercially from Puget Sound, WA
(47.5°N), to Tomales Bay, CA (38.3°N), and also in Australia.
Remarks
Following the recommendations by Bayne et al. (2017), we
are continuing to place this species in the genus Crassostrea.
Dendostrea folium
Figure 4G–J
[Synonyms: Ostrea folium (Linnaeus 1758: 699); Ostrea serra
(Dall 1914: 2, not de Lamarck 1819); Ostrea dalli (Lamy
1929–1930: 252, new name for Ostrea serra Dall, not
Lamarck); Ostrea frons (Linnaeus, of authors, not Linnaeus
1758); with additional synonyms in Inaba et al. (2004).]
Diagnosis
Shell height usually much greater than length; exterior sur-
face with a few strong radial plicae; inner shell margin densely
denticulate especially near hinge.
Description
Shell ovate–elongate; left valve flat to slightly convex,
broadly attached; right valve convex, fitting on top of left
valve; sculpture of strong radial plications that interlock;
ligament groove short, narrow; weak chomata present on
both sides of ligament, not present in all specimens; exterior
color rose to purplish-red, sometimes eroded to cream; inte-
rior color white, light brown to rose on margins; height in the
easternPacificto24mm.
Distribution
This Indo-Pacific species has a discontinuous distribution in
the tropical eastern Pacific at Cabo Pulmo, Baja California Sur,
Mexico (23.5°N); Bah´
ıa de Panam ´
a, Panama; Isla del Coco,
Costa Rica; Isla de Malpelo, Colombia; Isla Gal ´
apagos,
Ecuador; in 8–37 m, attached to calcareous substrata, including
sea urchin spines (Eucidaris thourasii and Eucidaris gal-
apagensis), gorgonians, gastropods, and corals.
Remarks
Molecular studies are needed to understand the relationship
of the two eastern Pacific species of Dendostrea with their Indo-
Pacific analogs.
Dendostrea sandvichensis
Figure 5A–D
[Synonym: Ostrea sandvichensis (Sowerby 1871): pl. 27,
Figure 66].
Diagnosis
Shell height usually much greater than length; exterior sur-
face with a few weak radial plicae; inner shell margin densely
denticulate over most of shell.
Description
Shell ovate to ovate–elongate; left valve attached by large
area, flat to slightly concave, especially on margins; right
valve flat to slightly convex, fitting on top (not inside) left
valve; sculpture of numerous conspicuous sharp radial un-
dulations or plicae, especially on margins, usually overgrown
with encrusting organisms; fine crenulations often found over
entire margin; ligament groove short, narrow; exterior color
light brown; interior color white with green patches, margins
white; height in eastern Pacific to 43 mm.
Distribution
Another Indo-Pacific species that is occasionally found in
the tropical eastern Pacific. It has been reported at Bah ´
ıa de los
Muertos, Baja California Sur (23.9°N) and Golfo de Tehuan-
tepec (approximately 16.0°N), Mexico; attached to coral and
rocks; at a depth of 2–3 m.
Ostrea angelica
Figure 5E–J
[Synonyms: Ostrea angelica (Rochebrune 1895: 241); Ostrea
vespertina (Conrad, of authors, not Conrad 1854); Ostrea
cumingiana (Dunker, of authors, not Dunker, in Philippi 1846).]
LODEIROS ET AL.186

Diagnosis
Shell length and height subequal in many; with weak cho-
mata near hinge; exterior surface of both valves with many
strong radial plicae; shell margin strongly undulated.
Description
Shell subovate to subcircular; left valve concave, narrowly or
broadly attached; right valve flat to slightly convex, fitting
slightly inside left valve; both valves with heavy radial plica-
tions, resulting in an undulating margin; ligament short to long,
narrow; chomata weak to strong; exterior color grayish-white
to dark brown; interior color white, frequently with large olive
green regions, some with dark margins; shell height to 100 mm.
Distribution
Rocas Alijos, Pacific coast of Baja California Sur (25.0°
N), to Bah ´
ıa Santa Elena, Guanacaste, Costa Rica (10.9°N);
low intertidal zone to 50 m. This species has been reported
from as early as the late Miocene of California and the Pli-
ocene of Baja California.
Figure 4. Oyster species of the eastern Pacific: (A–F) Crassostrea sikamea, USA, OR, Lincoln County, Yaquina Bay. SBMNH 145309, length 48 mm, height
66 mm. (A) Exterior right valve; (B) Exterior left valve; (C) Interior of the right valve; (D) Interior of the left valve; (E) Lateral view of both valves;(F)Hingeof
the left valve. (G–J) Dendostrea folium, Mexico, Baja California Sur, Punta Pescadores, attached to urchin, 3–6 m. SBMNH 83500, length 8 mm, height
17 mm. (G) Exterior right valve; (H) Urchin host with figured oyster attached to the top spine; (I) Interior of the right valve; (J) Interior of the left valve.
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 187

Remarks
Harry (1985) erected a new genus, Myrakeena, and selected
angelica as the type species. Raith et al. (2016) have shown that
angelica fits well within the Ostrea clade and formally recog-
nized it as Ostrea angelica.
Ostrea conchaphila
Figure 6A–F
[Synonyms: Ostrea conchaphila (Carpenter 1857: 161);
Ostrea lurida of authors, not Carpenter (1864).]
Diagnosis
Shell usually higher than long; chomata present in most; exterior
surface without strong radial plicae; interior color green to white.
Description
Shell circular to dorsoventrally elongate; left valve flat to
slightly convex, frequently cemented to substratum by a broad
Figure 5. Oyster species of the eastern Pacific: (A–D) Dendostrea sandvichensis, Mexico, Baja California Sur, Bah ´
ıa de los Muertos, 3–6 m. SBMNH
149691, length 18 mm, height 28 mm. (A) exterior left valve, (B) oyster attached to coral branch, (C) interior of right valve, (D) interior of left valve.
(E–J) Ostrea angelica, Mexico, Sonora, NE of Guaymas, Bah´
ıa Batuecas, intertidal. SBMNH 135082, length 41 mm, height 42 mm. (E) exterior right
valve, (F) exterior left valve attached to rock, (G) interior of right valve, (H) interior of left valve, (I) lateral view of both valves, (J) hinge of left valve.
LODEIROS ET AL.188

area, but attachment may be smaller; right valve flat to slightly
convex; ventral margin flat to slightly undulating; sculpture of
obscure commarginal lamellae, frequently abraded; ligament
groove very short, broad; chomata fine, not visible in some
specimens; exterior color grey, purple, or white, with dark rays;
interior color green to white; height to 85 mm.
Distribution
Laguna San Ignacio, Baja California Sur, Mexico (26.9°N),
to Puerto Pizarro, Tumbes, Peru (3.5°S); intertidal zone with
shells reported as deep as 100 m. The species has been recorded
as early as the Pliocene.
Figure 6. Oyster species of the eastern Pacific: (A–F) Ostrea conchaphila, Mexico, Sonora, Estero El Soldado. SBMNH 20357, length 39 mm, height
51 mm. (G–L) Saccostrea palmula, Mexico, Oaxaca, Golfo de Tehuantepec. BMNH 74.12.11.260 (syntype of Ostrea mexicana), length 39 mm, height
51 mm. (A, G) Exterior right valve; (B, H) Exterior left valve; (C, I) Interior of the right valve; (D, J) Interior of the left valve; (E, K) Lateral view of both
valves; (F, L) Hinge of the left valve. (M) S. palmula, Mexico, Baja California, Puertecitos. SBMNH 468301, length 52 mm, height 62 mm, depth
34 mm, lateral view depicting the deep left valve and concave right valve fitting inside left.
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 189

Remarks
Genetic studies have shown that the northeastern Pacific
Ostrea lurida (Carpenter 1864) is separable from this species,
but no one has yet advanced morphological characters to tell
them apart (Polson et al. 2009). Raith et al. (2016) reported that
the southern limit of Ostrea lurida is Guerrero Negro, Baja
California, Mexico, and the northern limit of Ostrea con-
chaphila is Laguna San Ignacio, Baja California Sur.
Figure 7. Oyster species of the eastern Pacific: (A–F) Striostrea prismatica, Panama, Veraguas, Guacamya. BMNH 1912.6.18.40 (syntype), length
63 mm, height 80 mm. (A) Exterior right valve; (B) Exterior left valve; (C) Interior of the right valve; (D) Interior of the left valve; (E) Lateral view of both
valves; (F) Hinge of the left valve. (G–L) Undulostrea megodon, Peru (precise locality unknown). BMNH unnumbered (syntype), length 43 mm, height
102 mm. (A, G) Exterior right valve; (B, H) Exterior left valve; (C, I) Interior of the right valve; (D, J) Interior of the left valve; (E, K) Lateral view of
both valves; (F, L) Hinge of the left valve.
LODEIROS ET AL.190

Saccostrea palmula
Figure 6G–J
[Synonyms: Ostrea conchaphila palmula (Carpenter 1857:
163); Ostrea amara (Carpenter 1864: 363); Ostrea mexicana
(Sowerby 1871: pl. 16); Ostrea tubulifera (Dall 1914: 3); Ostrea
cumingiana (Dunker, of authors, not Dunker 1847).]
Diagnosis
Shell height usually greater than length; shell margin
strongly undulates; right valve exterior surface usually smooth,
with strong plications on margin, frequently fitting inside left
valve; chomata strong.
Description
Shell irregularly shaped; narrowly to completely attached by
left valve, frequently deeply cupped, sometimes almost flat;
right valve concave, flat, or somewhat convex, fitting inside left
valve; shell margin slightly to strongly undulating; sculpture
of a few conspicuous sharp plications, especially in left valve;
ligament groove moderate to long, narrow; chomata strong;
exterior color cream to purplish-brown, some with radial rays;
interior color white with green or purple patches, many with
purple margins; height to 80 mm.
Distribution
Laguna San Ignacio, Pacific coast of Baja California Sur
(26.8°N), to Bayovar, Piura, Peru (5.9°S); also the offshore
islands of Isla del Coco, Costa Rica and Islas Gal ´
apagos,
Ecuador; on rocks or mangroves. The species has been recorded
in the Pleistocene and Pliocene of Baja California.
Remarks
In Coan and Valentich-Scott (2012), Ostrea tubulifera was
listed as a separate species, differentiated by its hollow (hyote)
spines. Raith et al. (2016) presented genetic evidence that O.
tubulifera is in fact a morphological variant of Saccostrea
palmula.
Striostrea prismatica
Figure 7A–F
[Synonyms: Ostrea prismatica (Gray 1825: 139); Ostrea iri-
descens (Hanley, 1854: pl. 2); Ostrea panamensis (Carpenter
1864: 362); Ostrea lucasiana (Rochebrune 1895: 241); Ostrea
turturina (Rochebrune 1895: 242); Ostrea spathulata (Lamarck,
of authors, not de Lamarck 1819).]
Diagnosis
Shell thick, heavy; weak chomata present in most; interior
iridescent, golden to dark brown; sculpture of commarginal
lamellae, without radial ribs or plicae.
Description
Shell ovate to dorsoventrally elongate, heavy, thick; left
valve moderately to deeply concave, often broadly attached, but
with raised margin; right valve flat to moderately convex; liga-
ment groove broad, usually short; chomata heavy, few in
number; sculpture of dense commarginal lamellae, many with
broad, low undulations; exterior color brownish-purple; inte-
rior iridescent gold with olive-brown patches; height to 190 mm.
Distribution
La Paz, Baja California Sur (24.2°N) [SBMNH], and
Mazatl ´
an, Sinaloa (23.2°N) [SBMNH], Mexico, to M ´
ancora,
Tumbes, Peru (4.1°S) [SBMNH]; in the intertidal zone, in ex-
posed rocky areas. The species has been recorded in the Pleis-
tocene of Mexico and the Pliocene of Peru.
Undulostrea megodon
Figure 7G–L
[Synonyms: Ostrea megodon (Hanley 1846: 106); Ostrea gallus
(Valenciennes 1846: pl. 21); Ostrea cerrosensis (Gabb 1869: 35).]
Diagnosis
Shell crescent shaped; anteroventral margin of both valves
with three to four strong, broad undulations; chomata present;
interior color tan to dark green.
Description
Shell arcuate; both valves moderately inflated; left valve
often narrowly attached; sculpture of three to four broad
anteroventral undulations, each with stacked commarginal la-
mellae; ligament groove long, narrow, oblique; chomata strong,
frequently extending well down the anterior and posterior
margins; exterior color tan to purple to reddish-brown, some-
times with darker rays; interior color dark cream with green
patches; height to 103 mm.
Distribution
Bah ´
ıa San Bartolom ´
e, Pacific coast of Baja California Sur
(27.7°N), to Bah ´
ıa de la Independencia, Ica, Peru (14.3°S); low
intertidal zone to 110 m. This species has been recorded from as
early as the Miocene of southern California.
Remarks
It has been followed Valentich-Scott et al. (2020) and
maintaining the use of Undulostrea for this species until addi-
tional studies are completed that include morphometric and
genetic data.
BIOLOGY AND ECOLOGY
General Anatomy
Internal anatomy of the oysters in the tropical and subtropical
regions of the American Pacific is similar to that of other Ostreidae
molluscs, and we take Crassostrea corteziensis as an oyster model.
According with Ch ´
avez-Villalba et al. (2005) and Flores-Higuera
(2011), two lobes forming the mantle cover the organs and shape
the pallial cavity. The gills divide the space in inhalant and ex-
halant chambers where seawater circulation takes place. The
mouth is covered by the labial palps and is oriented to the liga-
ment, whereas the anus borders the adductor muscle. The gonad is
largely diffused within the digestive gland and both surround the
pericardium and part of the adductor muscle. A representative of
organs and tissues of C. corteziensis is in Figure 8.
Life Cycle
Few studies have been conducted on the biology of oysters in
the West American tropics and the subtropical eastern Pacific,
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 191

and knowledge of the life cycle, particularly the early stages, is
still to be determined in most native species. Embryogenesis,
larval, and postlarval development of Crassostrea corteziensis
and Striostrea prismatica have been described during spat
production in controlled conditions (Maz ´
on-Su ´
astegui 2014,
Lodeiros et al. 2017).
In these species, fertilization is external, and once the eggs
are fertilized, the embryonic development begins quickly,
reaching the trochophore larval stage in a few hours (12–18 h),
resulting in ‘‘D’’ veliger mixotrophic larvae before 24 h. The
planktonic life stage lasts 2 to 3 wk, during which time the larva
grows and modifies its shape and behavior with very active
swimming; then, the eye spot appears and the foot is developed
(pediveliger larvae). At this time, the larval swimming stops,
passing into a benthic phase, and then the larvae acquire a
creeping displacement until obtaining an adequate substrate
where they are cemented and metamorphosed to acquire their
sessile lifestyle.
From this point forward (18–22 days for Crassostrea corte-
ziensis and 22 days for Striostrea prismatica), the larva un-
dergoes metamorphosis, loses the velum, and develops its gills.
During this process, the larva chooses the substratum and ce-
ments with the left valve, thus initiating its sessile life, similar to
that of the adult. Figure 9 schematically shows the life cycle
details of S. prismatica from its fertilization, production of ju-
veniles (in laboratory conditions), and adult stages. Although
the embryonic and larval development in these oysters is similar
to that of many oysters, and shows the characteristics of many
bivalves, the umboned larva in S. prismatica has a reddish distal
spot, which can be used for identification in zooplankton
samples (Lodeiros et al. 2017).
Population and Ecological Interactions
Population studies and the ecological interactions of these
species are intended for the evaluation of the resource for fishery
and exploitation purposes, and oyster reef conservations and
restoration, so they are aimed at determining their abundance
and distribution by size, sex proportion, associated species,
water quality, and other parameters in the systems where they
are distributed naturally (Coen & Luckenbach 2000).
The rock oyster is one of the most abundant species in the
coastal and subcoastal rock systems, and, in most cases, it
provides the greatest fishing biomass. R ´
ıos-Gonz ´
alez et al.
(2018) performed a revision of Striostrea prismatica describing
its biology, exploitation, and conservation. On the coasts of
Mexico, the species S. prismatica, together with Chama cor-
alloides,Choromytilus palliopunctatus, and Saccostrea palmula,
accounts for almost 60% of the total abundance of species, with
S. prismatica representing the largest biomass (Flores-Garza
et al. 2014).
Unexploited stocks can have up to 175 individuals/m
2
,as
Campos and Fournier (1989) have reported for Cur ´
u Bay in
Costa Rica; however, the stocks are generally overexploited or
depleted by artisanal or commercial fisheries because of the lack
of extraction regulations. Additional anthropogenic factors,
such as pollution, have contributed to the decline of the fishery
(Grijalva-Chon et al. 2015).
In 2000, the stocks of Striostrea prismatica in the rocky coast
of Tumbes, Peru, showed a size range of 7–228 mm, with den-
sities of 6–77 oysters/m
2
, but in subsequent surveys (2007), the
densities dropped to 0.2 oysters/m
2
(Ordinola et al. 2010, 2013).
Gonzabay (2014) recorded a density of 0.7 oysters/m
2
in a rocky
system of the town of Ayangue, in the El Pelao nature reserve,
Santa Elena Province, Ecuador, with dominant sizes of
80.0–120.0 mm (77.1%), and Garc´
ıa and Leones (2016) report
sizes of 32.5–178.0 mm with a greater predominance of
60–90 mm, in the intertidal zone of Punta Napo and Punta
Gorda, province of Manab ´
ı, Ecuador. This difference in sizes,
with a shortage of large specimens, is a product of the fishing
pressure that exists on the species in rocky systems.
In other species such as Crassostrea columbiensis, population
studies are even fewer. In Punta Morales, Puntarenas, Costa
Rica (years 1984–1985), individuals showed a growth that
conforms to the model of Von Bertalanffy [Lt ¼8.9 (1 –e
–0.1567
(t¼–0.123))], with a relationship length-weight of P¼1.04 3
10
–3
Lt ¼2.3487 (Caballero et al. 1997), and for Magdalena
Bay, Baja California Sur, Mexico, it is estimated they had a
longevity of 6–7 y (Garc ´
ıa-Dom ´
ınguez, unpublished data).
The palmate oyster Saccostrea palmula and the Arcid cockle
Anadara tuberculosa are the most important components and
the most conspicuous species in mangrove swamps of southern
Baja California peninsula, Mexico. The palmate oyster can
reach 50 mm in shell length at 1 y (F ´
elix-Pico et al. 2015).
Nonetheless, in the estuary Morales, Punta Morales, Puntar-
enas, and Costa Rica, the maximum total weight length found
was 66.7 mm and 24.7 g, respectively, and the growth conforms
to the von Bertalanffy model, with a relation Lt ¼68.2 (1 –
e
–0.1577t
). In this locality, the length–weight population ratio is
allometric and fits the equation Pt ¼2.13 310
–2
Lt 1.6602
(Cabrera et al. 2001b); however, records of the biological-
fishing status of their populations are not yet available.
In addition to growth studies of Crassostrea corteziensis
during cultivation (Ch ´
avez-Villalba et al. 2005), growth in this
species has been examined from two perspectives, scope of
growth and use of growth models. In the first case, values of the
scope for growth revealed that C. corteziensis is able to grow in
different combinations of temperatures and salinities, but with
superior results at 26°C and salinity of 20 (Guzm ´
an-Ag ¨
uero
et al. 2013). In terms of growth modeling, growth data have
been analyzed using the model of von Bertalanffy with the
following relation: Lt ¼114 (1 –e
–1.1t
) (Ch ´
avez-Villalba et al.
2005). Another approach, implemented by Ch ´
avez-Villalba and
Arag ´
on-Noriega (2015), was multimodel inference by testing
five different asymptotic growth models (von Bertalanffy,
Figure 8. Main internal anatomical features of the Cortez oyster
Crassostrea corteziensis.
LODEIROS ET AL.192

logistic, Gompertz, Schnute, and Schnute–Richards). They
found that the best model to describe the individual growth of
the species is that of Schnute–Richards.
Considering that the Japanese oyster Crassostrea gigas is an
exotic species on the shores of the American Pacific, it is spec-
ulated that it is highly probable that the various causative
agents of epizootics may have arrived as dispersal vehicles as
larvae and C. gigas spat from hatcheries in the United States.
Many lots of oysters (spat, juveniles, and adults) were illegally
introduced to Mexico, without strict sanitary controls, includ-
ing the minimum required by international standards (Flores-
Higuera 2011).
Although there are no reports of predators in the oyster
species stocks of the tropical and subtropical American Pacific,
Maz ´
on-Su ´
astegui (2014) points out that the main predators of
Crassostrea corteziensis are crabs (Callinectes spp.), the sea
urchin Echinometra vanbrunti, sea stars (Heliaster kubiniji and
Pentaceraster cuminigi), the puffer fish Sphoeroides annulatus,
rays, and carnivorous molluscs such as Thais spp.
In the oyster stocks of Tumbes, Peru, Striostrea prismatica
serves as a substrate for a variety of benthic communities (35
species) that are predominantly molluscs, within which are the
genera Diodora and Crepidula, and the drilling mussels in
the genus Lithophaga were the most abundant. These were
followed by crustaceans, echinoderms, cnidarians, annelids,
and poriferans, as well as several types of macroalgae (Ordinola
et al. 2013). Similarly, Maz ´
on-Su ´
astegui (2014) reports that the
shells of Crassostrea corteziensis house a great diversity of
species of molluscs, sponges, tunicates, bryozoans, crustaceans,
scales, and some macroalgae. This high diversity of species and
invertebrate epibionts that use oyster shells as habitats confers
the character of bioengineers to these two species.
Oysters can harbor other organisms within their mantle
cavities such as the tiny soft-bodied pea crabs of the family
Pinnotheridae that live commensally in the mantle of certain
bivalve molluscs. A series of adaptations have evolved to cope
with their symbiotic lifestyle (Peir ´
o et al. 2011), which makes
them an interesting model to study the evolution of associations
between decapods and other invertebrates. Some studies
(Cabrera et al. 2001a, Salas-Moya et al. 2014) of the pea crab
Austinotheres angelicus, living in the palmate oyster Saccostrea
palmula, have included biometric and reproductive data, adding
important information about both species (commensal: A.
angelicus and host: S. palmula); however, more data are needed
to get a better understanding of the interactions between pop-
ulation dynamics of the host and adaptive responses of the
symbiotic pea crab (Salas Moya et al. 2014).
In the case of Crassostrea gigas, as a cultured resource in
many parts of the world, particularly in temperate zones, the
species can create large hard-substrate biogenic reefs, making
Figure 9. Life cycle of the rock oyster Striostrea prismatica, showing its planktonic (embryonic and larval) and benthonic (pediveliger and postlarval)
development, from spat to adult stage. (A) Unfertilized oocyte; (B) First polar body indicating oocyte fertilization 15 min; (C) First cell cleavage 1h30
min; (D) Second cell cleavage 2 h; (E) Immobile morula 4 h; (F) Blastula 5 h; (G) Late gastrula 7 h 30 min; (H) Trochophore larvae 10 h; (I) Calcified
early D-shaped larvae with prodissoconch I 20 h; (J) Veliger larvae 2 days; (K) Umboned larvae with a distal reddish-orange toward the ventral zone 9
days; (L) Umboned larvae with a more marked distal reddish-orange toward the ventral zone 12 days; (M) Large umboned larvae showed spot eye 18
days; (N) Pediveliger larvae 22 days; (O) Early postlarvae 3 days postmetamorphosis; (P) Visually observable postlarvae 9 days postmetamorphosis; (Q)
Spat juvenile 90 days postmetamorphosis; and (R) adult. The images have different magnification scales delineated as horizontal bars.
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 193

them one of the most transformative invaders of marine eco-
systems (Crooks et al. 2015). Oyster reefs may harbor inverte-
brate groups, support other bivalves (Geukensia spp., Macoma
spp., Ensis spp., Mya arenaria, and Mercenaria mercenaria),
and are used by many fishes as a recruitment substrate, nursery
habitat, and foraging ground (Coen & Luckenbach 2000, Beck
et al. 2011). These reefs may influence water quality, sediment
erosion rates, and hydrodynamic patterns of water in estuaries
and coastal lagoons. All of this affects the distribution and
abundance of other biogenic habitats, such as seagrass beds, salt
marshes, algal beds, and also influences the dynamics of disease
dispersal. Despite the ecological importance of oyster reefs, the
large original populations in native areas had been severely
affected because of overfishing, contamination, and the con-
struction or expansion of seaports (Beck et al. 2011). Some of
the efforts aimed at reestablishing natural oyster populations
have focused on finding suitable alternative substrates. In this
sense, Goelz et al. (2020) make an excellent review of the use of
various alternative substrates used for oyster reef restoration.
Aquaculture activities pose a risk of species introductions
when oysters are transplanted or introduced into different
zoogeographic regions, including the introduction of new dis-
eases. In addition, exotic oyster introduction produces compe-
tition and displacement of congener species with the same or
greater capacities of negative effects on the ecosystems to which
they are introduced. Although the movements of introduced
oysters as in the case of Crassostrea gigas along the American
coasts are almost exclusively related to aquaculture practices,
this activity is recognized as one of the largest potential vectors
for the transport of marine invasive species (Ruesink et al. 2005,
2006). For example, the snail Batillaria attramentaria was in-
troduced to the west coast of the United States with C. gigas,
where it outcompetes and is displacing the native snail Cer-
ithidea californica in several estuaries (Byers 2000).
Ecological effects of nonindigenous species when introduced
in native habitats are due to not only ineffective control strat-
egies to prevent parasite and pathogen dispersion around the
world but also as a natural consequence of globalization of
world commerce. Every day, substantial amounts of live fishery
or aquacultural products are distributed from one country to
many other countries around the world. In this overall context,
the species Crassostrea gigas introduced by aquaculture prac-
tices may have substantial effects in terms of biodeposits, al-
tered flow regimes, and disturbance of the substrate (see Everett
et al. 1995).
Oysters remove particles from the water column during
suspension feeding and produce feces and pseudo-feces, which
lead to reduced particle size and increased organic content in
sediments (Ruesink et al. 2005). Although this process facili-
tates the action of the bacterial flora of the environment and of
marine floors, Villarreal (1995) argue that in Bah ´
ıa San Quint ´
ın
(Baja California, Mexico), biodeposits coming from oyster
cultures have been associated with eutrophication.
There are two species in the genus Dendostrea that have been
reported sporadically in the tropics and subtropics of the
American Pacific (Taxonomy, Identification, and Distribution),
and very little information is available for these small and re-
gionally rare species. The few reports show Dendostrea folium
and Dendostrea sandvichensis from the Indo-Pacific, as species
recorded in lists of Pacific islands, such as Isla Cocos in Costa
Rica and the Galapagos Islands in Ecuador (Magan
˜a-Cubillo &
Espinosa 2009, Coan & Valentich-Scott 2012). It is important to
mention that D. folium shows a quantitatively different karyo-
type (2n¼18; Ieyama 1990) the species of the family Ostreidae
(n¼20), and that D. sandvichensis is called the Hawaiian oyster.
There are only a few studies on Ostrea angelica (formerly
Myrakeena angelica), dealing with taxonomy (McLean & Coan
1996, Holguin-Quin
˜ones et al. 2008, Bastida-Zavala et al. 2013)
as well as paleontological and archaeological records (Laynder
et al. 2013, Rugh 2014). Marcus and Harry (1982) described the
planarian Zygantroplana ups associated with the mantle cavity
of the Ostrea angelica,and recently, molecular phylogenetic
studies confirm its integration into the genus Ostrea (Raith et al.
2016).
Reproduction
Most of the reproduction studies on tropical and subtropical
oysters have been carried out in natural populations (Table 1).
All species whose reproductive cycle has been studied so far,
including Striostrea prismatica,Crassostrea columbiensis,
Crassostrea corteziensis, and Saccostrea palmula, are gon-
ochoric organisms, and in their populations, most of the males
are smaller in size (Fournier 1992, Caballero et al. 1997,
Cabrera et al. 2001b, Romo-Pin
˜era 2005, Ch ´
avez-Villalba et al.
2008, Loor & Sonnenholzner 2014). This suggests a common
characteristic of protramental hermaphroditism.
In these species, the gonad is distributed throughout the
visceral mass (Fig. 9), which contains large amounts of oocytes.
Spawning Striostrea prismatica individuals of 120 mm (dorso-
ventral axis) induced at CENAIM-ESPOL/Ecuador hatchery
indicate a fecundity of 60 million oocytes/individual. This
number could be higher considering that oysters of this species
may reach twice that size (Lodeiros et al. 2017). Although fe-
cundity of Crassostrea corteziensis is lower (50 million oocytes/
individual) than that of S. prismatica according to several
studies at CIBNOR Mexico (Ch ´
avez-Villalba et al. 2008,
Rodr ´
ıguez-Jaramillo et al. 2008), both species are considered
great broadcast spawners. While C. corteziensis has the ability
to attain sexual maturity and carry out several partial spawn-
ings during a breeding season (Maz ´
on-Su ´
astegui et al. 2011).
The size at the sexual maturity in 50% of Striostrea pris-
matica populations on the coasts of Nayarit, Mexico, is
87–91 mm of the dorsoventral axis (Hern ´
andez-Covarrubias
et al. 2013), whereas in Crassostrea corteziensis in Las Guasi-
mas, Sonora, and in Ceuta Bay, Sinaloa (Mexico), it is
50–60 mm (Ch ´
avez-Villalba et al. 2008, Maz ´
on-Su ´
astegui et al.
2011). For other species, such as Saccostrea palmula and
Crassostrea columbiensis of Punta Morales, Puntarenas (Costa
Rica), the size at sexual maturity is 10–11 mm (Cabrera et al.
2001b).
R´
ıos-Gonz ´
alez et al. (2018) described phases of reproduction
of Striostrea prismatica observed in a latitudinal gradient from
north to south in tropical eastern Pacific. As observed in
Table 1, studies on the reproduction of S. prismatica cover a
wide latitudinal range, from 03°S (Costa de Tumbes, Peru) to
27.5°N (Guaymas, Sonora, Mexico). With the exception of
studies conducted in Cur ´
u, Gulf of Nicoya, Costa Rica, where
temperatures are always high (29.0°C–32.7°C), significant
changes related to high temperatures were always associated
with reproductive processes, particularly the maturation and
spawning of S. prismatica.
LODEIROS ET AL.194

TABLE 1.
Studies of the reproductive cycle of populations of native Ostreidae species of the tropical and subtropical American Pacific in north-south latitudinal order.
Place
Latitude/
longitude (°C) Reproductive activity Comments Source
Striostrea prismatica
Guaymas, Sonora, Mexico 27°55#N Gametogenesis maturation: mid-May–mid-June Ram ´
ırez and Sevilla (1965)
110°55#W Spawning: mid-June–August
– Rest: December–February
Mazatl ´
an Bay, Sinaloa, Mexico 23°10#–20#N 23.5–31 Gametogenesis maturation: Macro and microscopic observation (gonadal
smear)/larval abundance and spat settlement
Silva (1984)
106°29#–30#W April–September
– Spawning: June, and October–November
– Rest: December–March
Del Pozo and del Rey estuaries, San Blas,
Nayarit, Mexico
21°35#N 24.4–31.3 Gametogenesis: February–June Histology. Spawn at high temperatures above
27.5°C
Cuevas-Guevara and Mart ´
ınez-
Guerrero (1979)105°20#W Maturation: June–August
– Rest: October–May
– Spawning: August–September
San Crist ´
obal estuary, San Blas, Nayarit,
Mexico
21°31#N 20.1–30.9 Gametogenesis: January–May Index; gravimetric methods. Fr ´
ıas-Espericueta et al. (1997)
105°17#W Maturation: May Gametogenesis at 23.8°C–27.3°C
– Spawning: July–September Spawning at 30°C–31°C
L´
azaro C ´
ardenas port and Zapote de Huahua,
Michoac ´
an, Mexico
17°57#N Gametogenesis: May–June Histology. Maturation and spawning in association
with warm months
Mel ´
endez-Galicia et al. (2015)
102°12#W Maturation: August
– Spawning: August–December
– Rest: January–April
De la Ventosa bay, Salina Cruz, Oaxaca,
Mexico
16°09#N Gametogenesis maturation: February–July Histology, and spawning at high temperature Ru ´
ız-Dur ´
a (1974)
95°09#W Spawning: August–September
– Rest: December–February
Cur ´
u, Gulf of Nicoya, Costa Rica 09°47#N 29.0–32.7 Continuous reproductive activity throughout the year Histology. Low temperature variation. Spawning
with the decrease in salinity
Fournier (1992)
84°54#W
Ayangue Bay, Province Santa Elena, Ecuador 01°58#S 23.4–28.5 Gametogenesis: Histology Loor and Sonnenholzner (2014)
80°45#W – October–December. Maturation: December–March High temperatures induce maturation and spawning
– Spawning: January–March –
Rest: April–September
General Villasmil, province of Guayas, Ecuador 02°38#S 24.1–29.6 Gametogenesis: September–December High temperatures induce maturation and spawning Loor and Sonnenholzner (2014)
80°26#W Maturation: December–February
– Spawning: January–March
Rest: August–September
Punta Sal Grande and Nueva Esperanza, Coast
of Tumbes, Per ´
u
03°43#–44#S 21.3–26.3 Gametogenesis: all year Visual scale/recruitment. Ordinola et al. (2013)
80°45#–47#W Maturation: December–March mostly ripe High temperatures associated with the greatest
spawning
– Spawning: all year, maximum February–Mar and May. Recruit all year, maximum March–May
Saccostrea palmula
Santo Domingo, Magdalena Bay, Mexico 24°48#N 18.7–29.4 Gametogenesis maturation: all year (except February) Histology. Romo-Pin
˜era et al. (2015)
112°07#W Spawning: January, and June–October (maximum
activity)
Maturation correlated with temperature
– Rest: February Opportunistic species
Magdalena Bay, Mexico 24°48#N 17–29.8 Gametogenesis maturation: September–mid-January. Garc ´
ıa-Pamanes (1978)
111°58#WSpawning: November–December.
El Chisguete estuary, Magdalena Bay, Baja
California Sur, Mexico
24°48#N 17–29.8 Reproductive activity throughout the year Study of spat collection Ch ´
avez-Villalba and C ´
aceres-
Mart ´
ınez (1994)111°58#W Highest recruitment in March to June (maximum May)
continued on next page
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 195

TABLE 1.
continued
Place
Latitude/
longitude (°C) Reproductive activity Comments Source
San Carlos port, Magdalena Bay, Baja
California Sur, Mexico
24°46#N 18.5–31.5 Gametogenesis maturation: all year Spawning influenced by temperature, tides, and
photoperiod
Romo-Pin
˜era (2005)
112°06#W Spawning: June–October, July, and September
maximums
– Rest: November–March (low reproductive activity)
Falsa Bay, Ensenada La Paz, Baja California
Sur, Mexico
24°16#N 18–32 Reproductive activity throughout the year Spat collection study Ch ´
avez-Villalba and C ´
aceres-
Mart ´
ınez (1991)110°19#W Recruitment December–January (maximum) and
April–May
El Conchalito, Ensenada La Paz, Mexico 24°08#N 16.5–28 Gametogenesis: February–August Histology Romo-Pin
˜era et al. (2015)
110°20#W Maturation: April–September Maturation correlated with temperature and
chlorophyll a.
– Spawning: September–December Conservative species
Del Pozo and del Rey estuaries, San Blas,
Nayarit, Mexico
21°35#N 24.5–31.3 Gametogenesis: February Histology Cuevas-Guevara and Mart ´
ınez-
Guerrero (1979)105°20#W Maturation: March–May Spawning related to high temperatures
– Spawning: June–November Gametogenesis with low temperatures
Morales estuary, Punta Morales, Puntarenas,
Costa Rica
10°04#N Reproductive activity throughout the year Gonad smear macro and microscopically Cabrera et al. (2001b)
84°57#W High spawning between November and January
Marina Papagayo (Culebra bay), Costa Rica 10°38#N 26.3–30.7 Gametogenesis maturation: all year (except February) Cytology (microscopic observation) Alvarado (2018)
85°39#W Maturation Maturation positively correlated with
phytoplanktonic biomass and low salinity.
Spawning possibly related to high temperature
and low salinity.
– High maturity proportion from August to November –
Crassostrea corteziensis
Guaymas, Sonora, Mexico 27°55#N – Gametogenesis maturation: mid-February–July – Ram ´
ırez and Sevilla 1965
110°55#WSpawning: August–September
– Rest: mid-November–mid-February
Las Gu ´
asimas, Sonora, Mexico 27°53#N 15–33.5 Gametogenesis maturation: May–November Histology Ch ´
avez-Villalba et al. (2008)
110°38#W Spawning: April–November Culture
– Rest: December High association with temperature changes
Agiabampo Bay, Sonora, Mexico 26°22#N 22.3–32 Gametogenesis: March–April Histology Maz ´
on-Su ´
astegui et al. (2011)
10°912#W Maturation: April–November December–January not sampled
– Spawning: August–September –
– Rest: February–half of March –
System lagoons of Ceuta, Gulf of California,
Sinaloa, Mexico
24°05#N 19.5–31.5 Gametogenesis, maturation, and spawning during most
of the year
Histology Rodr ´
ıguez-Jaramillo et al.
(2008)
107°09#W Rest: December–February Maturation is activated >20°C
– – Spawning >27°C
San Crist ´
obal estuary, San Blas, Nayarit,
Mexico
21°35#N 20.1–30.9 Gametogenesis: January–November Histology and presence of larvae Stuardo and Marti `
ınez (1975)
105°20#W Spawning: June–October Spawning when temperatures are high (>25.5°C)
– Rest: November–April
Del Pozo and del Rey estuaries, San Blas,
Nayarit, Mexico
21°35#N 24.5–31.3 Gametogenesis: February Histology Cuevas-Guevara and Mart ´
ınez-
Guerrero (1979)105°20#W Maturation: March–May Spawning when high temperatures
– Spawning: May–November Gametogenesis at low temperatures
– Rest: January –
continued on next page
LODEIROS ET AL.196

TABLE 1.
continued
Place
Latitude/
longitude (°C) Reproductive activity Comments Source
San Crist ´
obal estuary, San Blas, Nayarit,
M´
exico
21°32#N 20.1–30.9 Gametogenesis maturation: February–April and
May–July
Indexes by tissue weights Fr ´
ıas-Espericueta et al. (1997)
105°17#W Spawning: April–May and July–September (less
intense)
Spawning when temperature rises 4°C
– – Rest: November–January –
La Palicienta lagoon, Boca de Camich ´
ın,
Nayarit, M ´
exico
21°43#–45#N 25.5–33–0 Gametogenesis maturation: Histology Mena-Alc ´
antar et al. (2017)
105°29#–30#W March, April, and July Temperature positively related to gametogenic
development and particularly to spawning
– Spawning: May, August, and October –
– Rest: December–February –
De la Ventosa Bay, Salina Cruz, Oaxaca,
Mexico
16°09#N Gametogenesis maturation: mid-February–July Histology Ru´
ız-Dur ´
a (1974)
95°09#W Spawning: August–September Interruption of reproductive activity with lower
temperatures
– Rest: December–February –
Crassostrea columbiensis
Los Praditos, Santo Domingo Bay, Baja
California Sur, M ´
exico
25°20#N 14.5–30 Reproduction occurs throughout the year Histology F ´
elix-Pico et al. (2011)
Temperatures >22°C favor gametogenesis,
26°C–30°C maturation
Temperature decrease favors spawning112°04#W
–
Morales estuary, Punta Morales, Puntarenas,
Costa Rica
10°04#NReproductive activity throughout the year. Maximum
spawning in May and October
Gonad smear and macro and microscopic
observation
Caballero et al. (1997)
84°57#W
Rest$undifferentiated and postrecovery reabsorption (no reproductive activity).
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 197

Loor and Sonnenholzner (2014) reported that there is no
correlation between phytoplankton biomass (estimated by
chlorophyll a) and the reproduction of Striostrea prismatica in
the two places studied on the Santa Elena peninsula, Ecuador
(General Villasmil and in Ayangue). In these locations, the
chlorophyll aconcentrations are greater than 2 mg/mL, sug-
gesting that food availability is not a limiting factor. Similarly,
other studies conducted in tropical areas such as the Gulf of
Nicoya, Costa Rica, show that food availability does not appear
to be a limiting factor for the reproductive processes of the
species. Although Fournier (1992) did not record data on food
availability in his study at the Gulf of Nicoya, it is known that
the area is highly productive (Le ´
on et al. 1998). In these high-
productivity tropical systems, where the temperature is stable
(<4°C annual variability in our analysis, Table 1), other envi-
ronmental factors could be the main modulators of reproduc-
tion (Giese & Pearse 1974). Accordingly, Fournier (1992)
found a negative association between spawning periods and
salinity periods. In view of this, everything seems to indicate
that temperature variability is the main factor in the modulation
of S. prismatica reproduction, and, when its anual variability is
minimal, salinity appears to be the main modulation factor.
This agrees with the protocols established by Arg ¨
uello et al.
(2013) for gonadic conditioning of S. prismatica at high tem-
peratures, consequently causing induction to spawning tem-
perature changes from 28°Cto33°C accompanied by the
reduction of salinity from 32 to 15.
In the southern Gulf of California, at El Verde, Sinaloa, it
has been observed that Striostrea prismatica accumulates re-
serves during most of the year, which it mobilizes according to
reproductive demand. Lipids decrease from October to March
during the resting phase, and carbohydrates have an inverse
trend in the months of April to May and June to July, whereas
proteins increase in May when gametogenesis is activated and,
with it, sexual maturation (P ´
aez-Osuna et al. 1993). The
aforementioned suggests that the reproductive strategy in the
species is conservative.
Unlike Striostrea prismatica, reproduction studies in Cras-
sostrea corteziensis,Saccostrea palmula, and Crassostrea
columbiensis (Table 1) show in general, and regardless of
latitude, a year-round reproductive activity with few periods of
sexual rest. Temperature is the predominant modulating factor
both for maturation (average temperatures) and spawning (high
temperatures). An exception, and at the same time an interest-
ing case, is the behavior of S. palmula indicated by Romo-
Pin
˜era (2005) and Romo-Pin
˜era et al. (2015) in Santo Domingo,
Magdalena Bay, and El Conchalito in La Paz Bay, Baja
California Sur, Mexico. Even though these sites have similar
temperature ranges and latitudes, natural populations of this
species behave reproductively different, being opportunistic at
the first site and conservative in the second. This difference is
possibly associated with food availability, because at Santo
Domingo, chlorophyll ahas always been greater than 1 mg/L,
with periods of higher production (>3mg/L and up to 9 mg/L),
whereas at El Conchalito, it was always less than 1 mg/L.
Other studies on the effect of environmental variables on
gonad maturation, immunology, and genetic diversity have
been carried out on Crassostrea corteziensis. Rodr ´
ıguez-
Jaramillo et al. (2008) and Ch ´
avez-Villalba et al. (2008)
described the quantitative and qualitative histology and histo-
chemistry of C. corteziensis. Mature organisms were found
during most of the year, and there were at least two strong
spawning periods, first in summer and then in autumn.
The two reproductive studies reported for Crassostrea
columbiensis in the Bay of Santo Domingo, Mexico (F ´
elix-Pico
et al. 2011), and at Estero Morales, Costa Rica (Caballero et al.
1997), are contrasting in terms of the distance in a latitudinal
order (24°and 10°, respectively); however, both populations
show reproductive activity throughout the year (Table 1). In the
case of Santo Domingo, where there is a greater annual tem-
perature range, average temperatures greater than 22°C favor
gametogenesis, and high temperatures (26°C–30°C) favor
maturation, with a temperature decrease inducing spawning
(F ´
elix-Pico et al. 2011).
In general, the natural or harvest habitats, where the tropical
and subtropical Pacific North American oysters develop, seem
to provide enough phytoplankton biomass to support their
reproductive processes so that the environmental modulation of
reproduction is governed mainly by temperature, particularly in
the maturation and spawning processes. As was indicated, in
those tropical ecosystems where the temperature variability is
not high, salinity also seems to be an important factor for
spawning.
There are no published data to indicate the establishment of
naturally reproducing populations of the Japanese or Pacific
oyster Crassostrea gigas or the Kumamoto oyster Crassostrea
sikamea beyond southern California, United States, or in the
subtropical and tropical countries of the American Pacific
Southern Hemisphere. Recently, however, the presence of the
Japanese oyster has been reported outside culture sites in the
Ojo de Liebre lagoon, which corresponds to the El Vizca ´
ıno
Biosphere Reserve in the Gulf of California (Carrasco & Bar ´
on
2010). Studies detailing the reproductive processes of cultivated
C. gigas have been performed in Mexico. The species behaves
as a protandrous hermaphrodite (Paniagua-Ch ´
avez & Acosta-
Ru ´
ız 1995), and depending on the region (Baja California or
Sonora), the gonads are in early gametogenesis from January
until June, in vitellogenesis from March to July, and mature
from June to October, and partial spawning occurs from
August to October (C ´
aceres-Mart ´
ınez et al. 2004, Rodr ´
ıguez-
Jaramillo et al. 2017). Nevertheless, Ch ´
avez-Villalba et al.
(2007) indicated that the species did not spawn in Sonoran
waters, and there was evidence that oocytes were reabsorbed
within the gonad. It seems that temperature regimes are the
main limiting factors for the normal processes of spawning,
fertilization, and larval settlement, thus preventing the estab-
lishment of feral populations. Recently, histological studies to
determine the reproductive condition of C. gigas culture in a
tropical estuary at Bah ´
ıa de Car ´
aquez, Manab ´
ı, Ecuador (0°
36#3$S, 80°25#23$O), show that oysters greater than 40 mm
maintain a protrandic condition with a female to male ratio of
1:2.6 with partial spawning (Lodeiros, unpublished data). This
indicates limited processes for the development of C. gigas
larvae or postlarvae in subtropical and tropical regions,
restricting the establishment of feral populations.
There are no reproductive studies of the Kumamoto oyster
Crassostrea sikamea in the tropical and South American Pacific.
Nevertheless, C ´
aceres-Mart ´
ınez et al. (2012) reported that
Kumamoto oysters start gametogenesis as young as 71 days
from spawning (35 days from postsettlement), with a mean shell
height of 3.0 mm. This constitutes a record in age size for ga-
metogenesis in oysters of commercial importance and adds
LODEIROS ET AL.198

another biological difference comparing this species with the
Pacific oyster Crassostrea gigas.
Bioacumulators
Because of the filtering characteristics of oysters and their
commercial importance, various studies have been undertaken
to determine whether they bioaccumulate unwanted substances,
compounds, or organisms. For instance, Striostrea prismatica
has been used to detect organochlorine pesticides (Ponce-
V´
elez & Botello 2018) and heavy metals (P ´
aez-Osuna et al.
1995, Fr ´
ıas-Espericueta et al. 1999a, 1999b, Soto-Jim ´
enez et al.
2001, Osuna-L ´
opez et al. 2007). Oysters also accumulate bac-
terial species that can cause epizootics and pathogenic agents
that affect their consumption safety at harvest. In this sense, it is
necessary to remember that the microbiota associated with the
intestinal digestive tract of the ostreids contributes various
beneficial substances to the host; it presents antibiosis or com-
petition for space and nutrients against pathogens and their
composition, and diversity varies according to the age of the
mollusc and the culture site (Trabal-Fern ´
andez et al. 2014).
The distribution of S. prismatica along the rocky coasts of the
tropical and subtropical eastern Pacific suggests that the species
could be used as a model in ecotoxicological studies for rocky
systems. Similarly, evidence of heavy metal accumulation in
conspicuous oyster species (Crassostrea corteziensis,Saccostrea
palmula, and Crassostrea columbiensis) in the mangrove systems
of the region has allowed them to be considered (particularly C.
corteziensis) as biomarker organisms (P ´
aez-Osuna et al. 2002,
Jara-Marini et al. 2008, Bernal-Hern ´
andez et al. 2010, Garc´
ıa-
Rico et al. 2010, P ´
aez-Osuna & Osuna-Mart ´
ınez 2015). Given
the direct human consumption of various molluscs, including S.
prismatica, some studies have been carried out to advise man-
agers on the advisability of establishing continuous monitoring
programs for toxicity because of harmful algal blooms (Barraza
et al. 2004, Band et al. 2010, Alonso-Rodr ´
ıguez et al. 2015).
Diseases
Few diseases have been described for tropical and subtrop-
ical oysters of the American Pacific. Most reports have been for
oysters (Crassostrea gigas and Crassostrea sikamea) introduced
from temperate zones (Elston & Wilkinson 1985, Bower et al.
1997, Friedman et al. 1998, Burreson & Ford 2004, Friedman
et al. 2005, Burge et al. 2011, Moore et al. 2011).
In a subtropical region (Baja California, M ´
exico), a virus
belonging to the Herpesviridae was detected after a Crassostrea
gigas mortality event in 2000, although the role of the pre-
sumable pathogens has not been clearly defined (V ´
asquez-
Yeomans & C ´
aceres-Mart ´
ınez 2004, V ´
asquez-Yeomans et al.
2004). The OsHV-1 herpesvirus was later found in the gills of
the species (V ´
asquez-Yeomans et al. 2010). Recently, nine ge-
notype variants of OsHV-1 were identified in the Gulf of Cal-
ifornia, and genetic analysis suggested that linear genotypic
evolution had occurred from strain JF894308 present in lagoon
La Cruz (Sonora, Mexico) in 2011 (Mart ´
ınez-Garc ´
ıa et al.
2020). The prevalent pathogen of oysters, Perkinsus marinus,
causing the dermo disease or perkinsosis, has been associated
with massive mortalites of C. gigas in culture conditions along
Sonoran coasts, Mexico (Enr ´
ıquez-Espinoza et al. 2010). Sim-
ilarly, the protozoan Marteilia refringens was detected for the
first time in the Gulf of California in samples of C. gigas
(Grijalva-Chon et al. 2015). The presence of parasites including
ciliates (Ancistrocoma and Trichodina), the annelid Polydora
sp., and trematode worms has also been detected in Pacific
oysters in northwestern Mexico, but they have not been asso-
ciated with mortalities (see C ´
aceres-Mart ´
ınez & V ´
asquez-
Yeomans 2008 for more details).
In the native oyster Crassostrea corteziensis, a series of epi-
zootics diseases associated with parasites, bacteria, or viruses
have been recorded (Grijalva-Chon et al. 2015). This problem
has been associated with the presence of Perkinsus marinus,
Nematopsis sp., and some protozoa such as Halteria grandi-
nella,Hexamita spp., Bodo spp., and Marteilia refringens during
the larval and juvenile stages of the species. Reports of high
mortality in larval culture associated with the presence of Vibrio
spp. are also common and increasingly frequent (Campa-
C´
ordova et al. 2009, 2011). Mart ´
ınez-Garc ´
ıa et al. (2017)
studied the possible presence of a temporary coupling of in-
fectious events in distant cultures of C. corteziensis in the Gulf of
California considering seasonal variation, presence, and the
prevalence of P. marinus,M. refringens,and OsHV-1. Results of
the molecular analysis showed a higher prevalence of P. marinus
in the north area and M. refringens in the southern area. The
OsHV-1 herpesvirus was only present during summer and au-
tumn with low prevalence in the two areas. The histological
analysis of the PCR-positive organisms presented alteration
characteristic of infections. The presence of M. refringens in a
new location in the Gulf of California suggests that this path-
ogen is already well established in the area, and the dual pres-
ence of pathogens is reported for the first time in C. corteziensis.
Guti ´
errez-Rivera (2014) compared the immune response of
the Cortez oyster Crassostrea corteziensis and the eastern oyster
Crassostrea virginica after their experimental infection with the
protozoan Perkinsus marinus. Results indicated that the pro-
tozoan caused greater mortality in the eastern oyster. This
possible resistance to Perkinsus adds consideration of C. cor-
teziensis as a good candidate for aquaculture, in addition to its
better flavor and texture (Maz ´
on-Su ´
astegui 2014). Conversely,
Bravo-Guerra (2018) showed that the parasite could be treated
therapeutically because it shows a high susceptibility to silver
nanoparticles at a concentration of 0.0927 mM Ag, an order of
magnitude less than the concentration known to impart nega-
tive effects on C. corteziensis.
The parasite Bonamia spp. is commonly associated with
oysters from temperate zones, but Hill et al. (2014) detected a
Bonamia sp. in Dendostrea sandvicensis from Hawaii. This re-
cord, together with others detected in Ostrea edulis in Australia
(Morton et al. 2003) and South Africa (Haupt et al. 2010),
confirms that Bonamia parasites have extended their presence in
tropical climates.
Bacterial infections have been little studied in the subtropical
and tropical oysters of the American Pacific, and, although
there is a relationship of bacterial infection when oysters are
infected with the OsHV-1 virus (C ´
aceres-Mart ´
ınez & V ´
azquez-
Yeomans 2013), there are no specific studies that can determine
the type of these infections. Luis-Villasen
˜or et al. (2018)
conducted a study on the communities of the oysters Crassos-
trea corteziensis and Crassostrea sikamea from commercial
oyster farms in Mexico. They found that the Kumamoto oyster
C. sikamea presented a greater diversity of species and the
highest number of pathogenic bacteria for aquatic organisms,
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 199

which is a probable reflection of their poor health status in
tropical culture.
POPULATION GENETICS AND PHYLOGENETIC STUDIES
Despite the fact that the Ostreidae are a conspicuous com-
ponent of the coastal marine biodiversity of the tropical
American Pacific, the inherent complications associated with an
easy and secure identification are reflected in the paucity of
studies on population genetic structure for ostreids from this
region. Given the diversity of species in the family and their
extended geographic ranges, relatively little has been published
on the population genetics of the species in the Northern
Hemisphere of the tropical American Pacific. In this sense, there
are no data published on Dendostrea folium,Dendostrea sand-
vichensis,Undulostrea megodon,Crassostrea aequatorialis,or
Crassostrea sikamea.
Saccostrea palmula
There are no population genetic studies for this species,
but in the contribution by Cruz et al. (2007) on the
microsatellite loci for Crassostrea corteziensis,theymention
that of the 11 loci characterized for the Cortez oyster, seven
can be amplified in Saccostrea palmula, which makes them
suitable for population studies. Maz ´
on-Su ´
astegui et al.
(2016) reported sequences of mitochondrial cytochrome c
oxidase subunit 1 (COI) gene, the nuclear intertranscribed
spacer 1 (ITS-1), and the nuclear 28S ribosomal gene. By
means of a phylogenetic analysis with sequences of other
species of ostreids, these authors concluded that the 28S gene
was more reliable in correctly identifying species; thus, it can
be used as an alternative marker. Raith et al. (2016) included
S. palmula in a phylogenetic analysis with ostreids from the
Gulf of California using COI and 16S ribosomal gene se-
quences and mentioned a lack of genetic variation in S. pal-
mula, and validated the Saccostrea genus.
Ostrea angelica
This species also lacks studies of population genetic diver-
sity. The phylogenetic analysis carried out by Raith et al. (2016)
confirms the closeness of this species with Ostrea conchaphila
and Ostrea lurida, and placed the species within the genus
Ostrea while invalidating the genus Myrakeena.
Striostrea prismatica
The levels of genetic diversity and population structure are
also unknown in this species. For this species, Maz ´
on-Su ´
astegui
et al. (2016) report sequences of COI,ITS-1, and 28S that can be
used in phylogenetic studies. In the phylogenetic study carried
out by Raith et al. (2016), it is proposed that Striostrea is the
only member of the new subfamily Striostreinae.
Ostrea conchaphila
The level of genetic diversity in this species is unknown.
When analyzing 16S and COIII mitochondrial markers, Polson
et al. (2009) determined that this species is not a synonym for
Ostrea lurida. Subsequently, the phylogenetic study of Raith
et al. (2016) confirmed the genetic closeness with O. lurida and
determined that there is no sympatry between them. With this
molecular identification, these authors also extended its distri-
bution range to Laguna San Ignacio, south of Punta Eugenia,
Baja California Sur (Mexico).
Crassostrea columbiensis
For this species, there is only a report of the sequences of
COI,ITS-1, and 28S by Maz ´
on-Su ´
astegui et al. (2016).
Crassostrea corteziensis
In the tropical and subtropical eastern Pacific, Crassostrea
corteziensis is the native ostreid species with the greatest aqua-
culture and fishing exploitation. Because of this, more attention
has been paid to it than to the oyster species in the region.
Thirty-one years ago, Rodr ´
ıguez-Romero et al. (1979) de-
scribed the karyotype of the species, determining that it has 20
chromosomes. Hedgecock and Okazaki (1984) carried out the
first population genetics study with allozymes in organisms
from Estero el Pozo, Nayarit (Mexico). These authors analyzed
17 loci, of which 58.8% were polymorphic, with an average of
2.12 alleles per locus and observed heterozygosity (Ho) of 13.8
and expected heterozygosity (He) of 19.6, outside the equilib-
rium of Hardy–Weinberg (HW). In the same state of Nayarit,
Rodr ´
ıguez-Romero et al. (1988) compared the electrophoretic
profile of general proteins in organisms from two localities
(Estero el Pozo and Estero Teacapan), finding statistically sig-
nificant differences between the areas.
Cruz et al. (2007) reported 11 microsatellite loci for Cras-
sostrea corteziensis, of which 10 proved to be polymorphic.
These authors performed a test analysis with 30 wild oysters
from Sinaloa and found four loci that were out of HW equi-
librium, and concluded that these loci were useful for paternity
tests and for population genetics studies. Later, in a study with
samples from five locations within the Gulf of California, P ´
erez-
Enr ´
ıquez et al. (2008) carried out a population genetic study
with allozymes and mitochondrial DNA markers. The analysis
of six allozymic loci (four polymorphic) showed that Ho was in
the range of 0.058–0.117 and the He was 0.089–0.129, with HW
equilibrium and without evidence of a population structure
because of high genetic flow. Considering the levels of diversity
reported by Hedgecock and Okazaki (1984), it was concluded
that diversity did not change substantially in 20 y; on the other
hand, an atypical result in the genotypic frequencies of the
samples taken in Topolobampo, Sinaloa (Mexico), motivated
these authors to perform a sequence analysis of the 16S mito-
chondrial gene, which demonstrated the presence of an un-
known species, and the authors considered it to be Crassostrea
columbensis.
Enr ´
ıquez-Espinoza and Grijalva-Chon (2010) carried out an
analysis of allozymes in a sample of the F1 generation descen-
dent from a stock of breeders of wild origin from Sinaloa,
Mexico. That F1 generation was cultivated in the state of
Sonora. The 13 allozymic loci studied included eight
polymorphic loci that were out of HW equilibrium, with an
average Ho of 0.065 and He of 0.294, and with high inbreeding
values.
Regarding the inclusion of this species in phylogenetic ana-
lyses, Raith et al. (2016) included it in their analyses of ostreids
from the Gulf of California, so they also reported 16S and COI
LODEIROS ET AL.200

sequences for this species. Maz ´
on-Su ´
astegui et al. (2016) also
report sequences for COI,ITS-1, and 28S.
Crassostrea gigas
Although some studies indicate the possibility of feral pop-
ulations of Crassostrea gigas in Ojo de Liebre lagoon, Gulf of
California, Mexico (Carrasco & Bar ´
on 2010), the population
genetic studies of the species are limited to cultivated pop-
ulations. Enr ´
ıquez-Espinoza and Grijalva-Chon (2010) carried
out an analysis with allozymes in a batch of triploid organisms
that arrived as spat at Sonora. Sixteen loci were analyzed, of
which 12 were polymorphic, with nine of them out of HW
equilibrium, and with Ho and He averages of 0.350 and 0.309,
respectively. The authors discussed the quality of these organ-
isms because the heterozygosity values were lower than those
reported for triploid oysters in the United States.
Grijalva-Chon et al. (2013) used microsatellites to analyze ge-
netic variability in a sample from a batch of breeders that were
purchased as spat from France. A second sample consisted of or-
ganisms from the F1 generation of that batch. Up to 68 alleles and
168 genotypes were detected with six microsatellite loci, with dif-
ferences in the averages of the heterozygosity observed and ex-
pected in both samples (Ho ¼0.65 and He ¼0.81 for the parental
lot; Ho ¼0.67 and He ¼0.84 for F1), and out of HW equilibrium
in almost all loci. Finally, in the phylogenetic study reported by
Raith et al. (2016), organisms of this species obtained from an
oyster farm in Puerto Pen
˜asco, Sonora, Mexico, were included.
FUTURE STUDIES
The morphological plasticity of oysters complicates their
identification and, therefore, classification and taxonomy.
Identification based on shell morphology has led to confusion,
which has skewed production statistics, leading to an uncertain
scenario for the administration of the resource, particularly in
Latin America.
In recent decades, in general, efforts have been made with
genetic-evolutionary studies to understand the diversity and tax-
onomy of oysters (Bayne 2017); however, taxonomic uncertainties
still exist, and many species have not been subjected to adequate
genetic analysis. There is a great need for a global survey of living
oysters and a comprehensive analysis of both genetic and mor-
phological data (Guo et al. 2018). This globally analytical need is
more critical for oysters in the tropical Pacific and South American
tropics, given the lack of taxonomic and genetic studies (Li et al.
2017). Therefore, it is recommended to develop studies for this
purpose. A suggested focus of study is the identification and
taxonomic-systematic evaluation of the endemic equatorial oyster
Crassostrea equatorialis, whose identification characteristics are
uncertain and unclear. Similarly, the taxonomic identification or
reformulation of oysters with major exploitable banks is necessary
even without genetic identification or clarification.
Oyster populations in the tropical and subtropical Pacific
have shown signs of overexploitation for a few decades
(MacKenzie & Buesa 2006), and organized research has not
been carried out for these species; thus, it is essential to establish
strategies for ecological protection and restoration of their
populations. By contrast, there are studies on the reproduction
of various commercial species that could be used to adminis-
trate their populations.
Oysters in general have a euryplastic physiology and could
easily be model species to study molluscan responses to stressors
as xenobiotic agents. Although there are proven studies on the
consideration of oysters as model species in ecotoxicology, these
studies of oysters from the American tropic and subtropical
Pacific are practically nonexistent. Studies that are adequately
designed to establish physiological responses, optimal ranges,
and tolerance to environmental factors are necessary in these
species not only to understand the physiology of tropical in-
vertebrates but also to establish ecophysiological knowledge
tool for reproduction under controlled conditions for the de-
velopment of production strategies and ecological restoration.
Oysters that are filter feeders thus become systems of mi-
crobial and toxin accumulation. Few studies have been con-
ducted on these topics. They are necessary for the development
of both aquaculture and the safety of human health.
Some studies have been conducted on oyster diseases in the
American Pacific subtropics, particularly in introduced oysters
(Crassostrea gigas and Crassostrea sikamea) and one native
oyster (Crassostrea corteziensis); however, it is necessary to
strengthen these studies and extend them to tropical oysters,
particularly those of greater commercial interest and potential
for aquacultural purposes (Striostrea prismatica and Crassos-
trea columbiensis).
Although the nutritional benefits of oysters are known in
general, mainly in temperate zones (see review of Venugopal &
Gopakumar 2017, Wright et al. 2018), it is necessary to initiate
nutritional studies that also include a profile of fatty acids,
amino acids, and other molecules of great benefit to man for
oyster species living in subtropic and tropical environments, to
nutritionally enhance their consumption. It is suggested that
this research should be carried out by separating cultivated
oysters from those in natural populations, as well as seasonally
sampling to differentiate their physiological response, particu-
larly in relation to their reproductive state. The results of these
investigations could generate information to benefit the mar-
ketable price at harvest and the overall economic profitability of
the developing oyster industry in subtropical and tropical
regions.
It would be useful to know the impacts of aquaculture ac-
tivities with Crassostrea corteziensis and Crassostrea gigas on
natural environments, especially in areas with large production
volumes. It is also necessary to identify places where C. gigas
has established feral populations, particularly in subtropics of
the American Pacific, but without excluding the possible long-
term adaptation of the species to tropical environments. This is
needed to determine the risks to endemic oysters and other
residents, and in general for the ecology of the region. In these
cases, it is essential to establish management strategies to con-
trol feral populations.
The paucity of information on population genetic structure
and genetic diversity of ostreids from the American Tropical
Pacific is a need that must be addressed in the short term;
however, this should be seen as a window of opportunity for
collaboration between the academic institutions of all countries
bordering the American Tropical Pacific.
ACKNOWLEDGMENTS
The study is part of the following projects: Conacyt Basic
Science 258282 and Proinnova Conacyt 241777 financed by the
OYSTERS OF THE TROPICAL AMERICAN PACIFIC 201

Sectoral Fund for Research and Education of M ´
exico, PIC-14-
CENAIM-002 financed by the Secretar ´
ıa T ´
ecnica de Educaci ´
on
Superior, Ciencia, Tecnolog´
ıa e Innovaci ´
on del Ecuador,
Factibilidad del cultivo de la ostra del pac ´
ıfico Crassostrea
gigas y la ostra perlera alada Pteria sterna en el estuario del r ´
ıo
Chone, provincia de Manab ´
ı, Ecuador, financed by Technical
University of Manab ´
ı, Ecuador. We appreciate assistance at the
Natural History Museum, London, for accommodating the
visit by P.V.-S. to study type specimens of many species
included herein.
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