Gorgonian Corals

Abstract

Mesophotic gorgonian corals comprise a polyphyletic group of octocorals mostly with a proteinaceous branching axial skeleton. Dense assemblages of gorgonian corals usually dominate the seascape in mesophotic coral ecosystems (MCEs). In this chapter, we review the mesophotic gorgonian coral biodiversity, followed by a synthesis of the ecological implications of inhabiting this environment, as well as the threats that these communities face. MCEs include ~87 gorgonian corals genera distributed worldwide, where the Indo-Pacific (65) is almost twice as diverse as the Caribbean and Gulf of Mexico (37) and Brazil (23), whereas the Tropical Eastern Pacific has only eight genera. We discuss several predictions on the nature of mesophotic gorgonian corals in areas such as microbial endosymbiosis to understand the health, ecology, and evolution of these assemblages. A notable colonization of shallow-water species into MCEs in the Caribbean suggests that colonizing deeper environments promotes ecological divergence. MCEs are not immune to the influence of natural events such as tropical storms and/or anthropogenic encroachment from coastal development, pollution, global climate change, ocean acidification, and overfishing; yet, gorgonian corals, in general, appear resilient to many of these threats.

Full text

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© Springer Nature Switzerland AG 2019
Y. Loya et al. (eds.), Mesophotic Coral Ecosystems, Coral Reefs of the World 12,
https://doi.org/10.1007/978-3-319-92735-0_39
Gorgonian Corals
JuanA.Sánchez, LuisaF.Dueñas, SoniaJ.Rowley,
FannyL.Gonzalez-Zapata, DianaCarolinaVergara,
SandraM.Montaño-Salazar, IvánCalixto-Botía,
CarlosEdwinGómez, RosalindaAbeytia, PatrickL.Colin,
RalfT.S.Cordeiro, andCarlosD.Pérez
Abstract
Mesophotic gorgonian corals comprise a polyphyletic
group of octocorals mostly with a proteinaceous branch-
ing axial skeleton. Dense assemblages of gorgonian cor-
als usually dominate the seascape in mesophotic coral
ecosystems (MCEs). In this chapter, we review the meso-
photic gorgonian coral biodiversity, followed by a synthe-
sis of the ecological implications of inhabiting this
environment, as well as the threats that these communities
face. MCEs include ~87 gorgonian corals genera distrib-
uted worldwide, where the Indo- Pacic (65) is almost
twice as diverse as the Caribbean and Gulf of Mexico (37)
and Brazil (23), whereas the Tropical Eastern Pacic has
only eight genera. We discuss several predictions on the
nature of mesophotic gorgonian corals in areas such as
microbial endosymbiosis to understand the health, ecol-
ogy, and evolution of these assemblages. A notable colo-
nization of shallow-water species into MCEs in the
Caribbean suggests that colonizing deeper environments
promotes ecological divergence. MCEs are not immune
to the inuence of natural events such as tropical storms
and/or anthropogenic encroachment from coastal devel-
opment, pollution, global climate change, ocean acidica-
tion, and overshing; yet, gorgonian corals in general
appear resilient to many of these threats.
Keywords
Mesophotic coral ecosystems · Biodiversity · Octocorals
· Biogeography · Holobiont
39.1 Introduction
Gorgonian corals comprise a polyphyletic group of branch-
ing octocorals (McFadden etal. 2010). Gorgonian refers to
the proteinaceous material (gorgonin) found in the axial
skeleton of some octocoral families such as the Plexauridae,
Gorgoniidae, Acanthogorgiidae, Ellisellidae, Primnoidae,
Keroeididae, Isididae, Chrysogorgiidae, Ifalukellidae,
Parisididae, Melithaeidae, and Subergorgiidae. Yet, scleraxo-
nian families, also colloquially referred to as gorgonian cor-
als, possess an axis that is made from loose or conglomerated
sclerites, such as the Coralliidae, Paragorgiidae, and
Anthothelidae (see additional systematic remarks on
Octocorallia in Benayahu etal. 2019). Common features of
J. A. Sánchez (*) · F. L. González-Zapata · D. C. Vergara
I. Calixto-Botía
Departamento de Ciencias Biológicas-Facultad de Ciencias,
Laboratorio de Biología Molecular Marina-Biommar, Universidad
de los Andes, Bogotá, Colombia
e-mail: juansanc@uniandes.edu.co
L. F. Dueñas · S. M. Montaño-Salazar
Departamento de Biología, Facultad de Ciencias, Universidad
Nacional de Colombia-Sede Bogotá, Bogotá, Colombia
S. J. Rowley
Department of Earth Sciences, University of Hawaiʻi at Mānoa,
Honolulu, HI, USA
C. E. Gómez
Temple University, Philadelphia, PA, USA
R. Abeytia
Unidad Académica Puerto Morelos, Instituto de Ciencias del Mar y
Limnología, Universidad Nacional Autónoma de México,
Cancún, QR, Mexico
P. L. Colin
Coral Reef Research Foundation, Koror, Palau
e-mail: crrfpalau@gmail.com
R. T. S. Cordeiro
Programa de Pós-graduação em Biologia Animal, Universidade
Federal de Pernambuco, Recife, Pernambuco, Brazil
C. D. Pérez
Centro Acadêmico de Vitoria, Universidade Federal de
Pernambuco, Vitoria de Santo Antão, Pernambuco, Brazil
39

730
gorgonian corals involve their habitat-forming
three- dimensional structure, with individuals growing in
dense communities on reefs and hard substratum (Sánchez
2016). Their tree-like architecture provides them with access
to resources in the water column, as well as the allometric
advantages of changing shape by forming a branching struc-
ture (Lasker and Sánchez 2002; Sánchez 2004). The habitats
formed by gorgonian corals are common in mesophotic coral
ecosystems (MCEs; 30 to >150m; Hinderstein etal. 2010)
worldwide (Fig.39.1) although our knowledge about their
biodiversity, ecology, and threats is nascent.
Indo-Pacic coral reefs, particularly in the Coral Triangle
(including Indonesia, Malaysia, the Philippines, Papua New
Guinea, Timor-Leste, and the Solomon Islands), are at the
center of shallow-water marine biodiversity. The origin of
such diversity remains the source of much intrigue and
intense research with a variety of models proposed suggest-
ing a center of origin, overlap, and accumulation. All models
likely contribute in part to the current high biodiversity, as
well as biodiversity feedback between the various hypothe-
sized processes (Bowen etal. 2013). Such models are almost
solely based on shallow-water reef shes and scleractinian
corals (Bowen etal. 2013; Sanciangco etal. 2013) and do not
consider shifting centers of biodiversity (Renema et al.
2008). How these hypotheses relate to mesophotic gorgonian
corals worldwide is unknown due to a lack of eld sampling
and taxonomic uncertainty, with concomitant difculties in
identication (Bayer 1981; Fabricius and Alderslade 2001;
Sánchez and Wirshing 2005).
Gorgonian corals in shallow waters exhibit clear local and
regional inuence over their community structure, and com-
petition does not seem to have an important role in structur-
ing their communities as with other marine invertebrates
(Velásquez and Sánchez 2015). How gorgonians change in
mesophotic environments may be related to morphological
plasticity to overcome these conditions by means of adjust-
ing polyp and branch structure (Lasker and Coffroth 1983;
Yoshioka and Yoshioka 1989; Sánchez etal. 1997). These
adjustments may be the cause of the ecological divergence
observed at mesophotic depths (Sánchez etal. 2007; Prada
etal. 2008; Prada and Hellberg 2013). Gorgonian fauna colo-
nizing MCEs differ from their shallow-water counterparts.
Thus, the “deep reef refugia” hypothesis (Bongaerts etal.
2010; Bongaerts and Smith 2019), which purports that MCE
communities are not as impacted as shallower reefs and
therefore may serve as a source of species and propagules to
replenish shallow reef populations, may not be relevant for
gorgonians.
Herein, we summarize the mesophotic gorgonian coral bio-
diversity from selected areas in the Atlantic and Pacic Oceans
from 30 to ~200m, with a synthesis of the ecological implica-
tions of inhabiting this environment and the threats that these
communities face. The material presented includes a review of
the published literature, as well as results from recent surveys.
Additionally, we have laid out several predictions about the
nature of mesophotic gorgonian corals in relation to microbial
endosymbiosis and gorgonian ecology and evolution. Finally,
we consider the variable responses of gorgonian phenotypic
traits (e.g., polyp and branching characters) to environmental
change, particularly across bathymetry.
39.2 Biodiversity
The systematics of mesophotic gorgonian corals is in a state
of disarray, with many of the genera needing revisions (e.g.,
Ellisella, Thesea, and Swiftia). In Table39.1, we summarize
the distribution of 87 known gorgonian genera, of which 13
are zooxanthellate and 1 is aposymbiotic (Muricea). Based
on published information and our recent surveys, the Indo-
Pacic (65) is almost twice as diverse as the Caribbean (37)
and Brazil (23), whereas the Tropical Eastern Pacic (TEP)
had only eight genera (Table39.1).
39.2.1 Caribbean Sea, Colombia,
andVenezuela
The Caribbean Sea has the second largest concentration of
coral reefs in the world with contrasting differences at its
eastern and western anks and great variation in environmen-
tal conditions such as storm intensity and rainfall (Spalding
etal. 2001; Foster etal. 2013; Velásquez and Sánchez 2015).
Gorgonians have been considered dominant members of the
wider Caribbean MCEs (30–200m) since the seminal work
by Bayer (1961). SCUBA diving surveys have found an
assemblage replacement in the reef slope below 30m, which
means that most mesophotic species and genera are azooxan-
thellate and overall different from their shallow-water coun-
terparts (Kinzie 1973; Sánchez etal. 1998; Sánchez 1999).
Recently, gorgonian corals of the Caribbean MCEs have
been explored using closed-circuit rebreather (CCR) diving
at three sites, which included the fore-reef and leeward
slopes of San Andrés Island barrier reef complex, the lee-
ward slope of Courtown Cays (Bolivar) atoll, and the deep
banks off the coast of Cartagena, Colombia (JA Sánchez,
pers. obs.). These deep banks comprised the oldest slope of
these barrier reef complexes (Geister 1975; Diaz et al.
1996a). The National Natural Deep-Sea Corals Park (Parque
Nacional Natural Corales de Profundidad, Colombia or
PNNCRP) also possesses rhodolith beds along its slopes
(Vernette etal. 1992) and siliciclastic reefs and banks on the
J. A. Sánchez et al.

731
Fig. 39.1 Gorgonian corals from Papua New Guinea, Palau, and Pohnpei, Micronesia, at lower mesophotic depths. (a) Nicella at 110m and (b)
Acanthogorgia at 125m, Papua New Guinea; (c) Primnoid at 125m and (d) Parisis at 152m, Palau; (e) Paracis at 110m and (f) Nicella and
Paracis at 140m, Pohnpei. (Photo credits: (a, b, e, f) S.J.Rowley and (c, d) P.L.Colin)
39 Gorgonian Corals

732
Table 39.1 Mesophotic gorgonian corals in four biogeographic regions: Indo-Pacic, TEP, Caribbean and the Gulf of Mexico, and Brazil
Genera
Depth range (m)
Symbiotic
relationship
Indo-
Pacic
Tropical Eastern
Pacic
Caribbean and Gulf of
Mexico Brazil
Acanthogorgia Gray, 1857 3–1215 56–729 60–417 AZ
Acanthomuricea Hentschel, 1903 12–600 73–200 AZ
Adelogorgia Bayer, 1958 50–200
Alertigorgia Kükenthal, 1908 3–40 AZ
Annella Gray, 1858 3–278 AZ
Anthogorgia Verrill, 1868 12–130 AZ
Anthothela Verrill, 1879 165–823 AZ
Antillogorgia Bayer, 1951 1–65 Z
Astrogorgia Verrill, 1868 4–139 AZ
Astromuricea Germanos, 1895 0–92 AZ
Bebryce Philippi, 1841 4–388 40–514 60–619 AZ
Briareum Blainville, 1830 1.5–75 1–55 Z
Caliacis Deichmann, 1936 37–188 AZ
Callogorgia Gray, 1858 106–960 82–732 137–965 AZ
Candidella Bayer, 1954 85–1801 514–2063 AZ
Chrysogorgia Duchassaing &
Michelotti, 1864
85–1050 128–2265 128–
1716
AZ
Convexella Bayer, 1996 107 AZ
Corallium Cuvier, 1798 198–463 AZ
Ctenocella Valenciennes, 1855 13–40 20–479 AZ
Cyclomuricea Nutting, 1908 61–335 AZ
Dichotella Gray, 1870 15–127 AZ
Diodogorgia Kükenthal, 1919 30–183 60–180 AZ
Echinogorgia Kölliker, 1865 4–142 AZ
Echinomuricea Verrill, 1869 4–100 AZ
Ellisella Gray, 1858 3–184 30–70 24–219 60–706 AZ
Eugorgia Verrill, 1868 10–70 AZ
Eunicea Lamouroux, 1816 1–45 Z
Euplexaura Verrill, 1869 4–110 AZ
Fanellia Gray, 1870 190–419 AZ
Guaiagorgia Grasshoff & Alderslade,
1997
11–40 Z
Heliania Gray, 1860 25–150 AZ
Heterogorgia Verrill, 1868 20–40 60–200 AZ
Hicksonella Nutting, 1910 10–35 Z
Hypnogorgia Duchassaing & Michelotti,
1864
60–86 AZ
Iciligorgia Duchassaing, 1870 13–56 5–366 AZ
Ifalukella Bayer, 1955 12–35 Z
Jasminisis Alderslade, 1998 10–75 AZ
Junceella Valenciennes, 1855 5–70 AZ
Keratoisis Wright, 1869 193–465 AZ
Keroeides Studer (& Wright), 1887 15–388 170–592 AZ
Lapidogorgia cf.a4–35 AZ
Lepidisis Verrill, 1883 175–665 611–1160 AZ
Leptogorgia Milne Edwards, 1857 1–70 2–77 60–180 AZ
Lytreia Bayer, 1981 18–77 AZ
Melithaea Milne Edwards & Haime,
1857
4–429 AZ
Menella Gray, 1870 6–51 AZ
Muricea Lamouroux, 1821 2–70 1–128 60–324 AP
Muriceopsis Aurivillius, 1931 2–50 60–91 Z
(continued)
J. A. Sánchez et al.

733
Colombian shelf, where mesophotic conditions are shal-
lower due to low light penetration (Sánchez 1995, 1999;
Diaz etal. 1996b; Velásquez and Sánchez 2015).
Octocorals are the most diverse group of corals on the
Caribbean MCEs below 60m depth. The deepest zooxan-
thellate gorgonian coral observed was Antillogorgia hystrix
(60m), followed by Muricea laxa, Muriceopsis petila, and
Eunicea pinta (usually above 50m). Occasionally, E. knighti,
A. bipinnata, and A. americana can reach about 45m depth.
These zooxanthellate octocorals share habitat with some
Table 39.1 (continued)
Genera
Depth range (m)
Symbiotic
relationship
Indo-
Pacic
Tropical Eastern
Pacic
Caribbean and Gulf of
Mexico Brazil
Muriceides Wright & Studer, 1889 56–232 53–592 AZ
Muricella Verrill, 1869 2–134 AZ
Nicella Gray, 1870 27–135 62–592 60–481 AZ
Olindagorgia Bayer, 1981 60–100 Z
Pacigorgia Bayer, 1951 1–70 AZ
Paracis Kükenthal, 1919 10–469 AZ
Paramuricea Kölliker, 1865 55–200 527 146 AZ
Paraplexaura Kükenthal, 1909 6–63 AZ
Parisis Verrill, 1864 17–256 AZ
Perissogorgia Bayer & Stefani, 1989 55–750 AZ
Phyllogorgia Milne Edwards & Haime,
1850
47 Z
Placogorgia Wright & Studer, 1889 142–421 51–479 143 AZ
Pleurocorallium Gray, 1867 116–463 AZ
Plexaura Lamouroux, 1812 1–65 Z
Plumarella Gray, 1870 9–130 AZ
Plumigorgia Nutting, 1910 6–48 Z
Primnoella Gray, 1858 60–160 AZ
Psammogorgia Verrill, 1868 72 30–183 AZ
Pseudoplumarella Kükenthal, 1915 55–115 AZ
Pseudopterogorgia Kükenthal, 1919 14–68 AZ
Pteronisis Alderslade, 1998 40–60 AZ
Pterostenella Versluys, 1906 61–77 AZ
Riisea Duchassaing & Michelotti, 1860 93–188 110–704 AZ
Rumphella Bayer, 1955 2–61 Z
Scleracis Kükenthal, 1919 51–1604 60–390 AZ
Solenocaulon Gray, 1862 14–84 AZ
Stephanogorgia Bayer & Muzik, 1976 10–56 AZ
Subergorgia Gray, 1857 3–115 AZ
Swiftia Duchassaing & Michelotti, 1864 115–500 21–985 60–93 AZ
Thelogorgia Bayer, 1991 80 45–90 83–117 AZ
Thesea Duchassaing & Michelotti, 1860 44–90 65–837 60–180 AZ
Thouarella Gray, 1870 73–608 AZ
Tobagogorgia Sánchez, 2007 27–74 AZ
Trichogorgia Hickson, 1904 50–120 12–32 AZ
Trimuricea Gordon, 1926 12–183 AZ
Verrucella Milne Ewards & Haime, 1857 6–130 100–200 AZ
Villogorgia Duchassaing & Michelotti,
1860
2–521 101–478 100–520 AZ
Viminella Gray, 1870 17–78 20–481 AZ
Zignisis Alderslade, 1998 35–146 AZ
Total 65 8 37 23 87
Total zooxanthellate: azooxanthellate 7:59 0:8 6:31 4:19 13:74
Biogeographic region and depth distribution record sources: Indo-Pacic from museum and expedition records (n=8749 records summarized in
Rowley et al. (2019) and SJ Rowley unpubl. data), TEP (Breedy and Cortés 2008; Abeytia etal. 2013; Breedy and Guzman 2013), Caribbean, and
the Gulf of Mexico (Cairns and Bayer 2002, 2009; Sánchez 1999, 2007; Sánchez and Wirshing 2005; Etnoyer etal. 2010; Chacón-Gómez etal.
2012; Velásquez and Sánchez 2015), and Brazil (Castro etal. 2010; Pérez etal. 2011; Cordeiro etal. 2015)
Symbiotic relationship: Z Zooxanthellate, AZ Azooxanthellate, or AP aposymbiotic
aDenotes taxonomic uncertainty
39 Gorgonian Corals

734
azooxanthellate species such as Iciligorgia schrammi,
Diodogorgia nodulifera, and diverse ellisellids, which can be
found as shallow as 25m depth (Sánchez 1999). Ellisellids
were the most abundant group in the upper mesophotic zone
(30–60 m), comprised of mostly Ellisella barbadensis, E.
elongata, and E. schmitti. These species were progressively
replaced by large sea fan species such as Nicella goreaui, N.
toeplitzae, Ellisella nivea, Verrucella sp., and numerous
Ellisella spp. (Fig.39.2).
The family Plexauridae was the most abundant and diverse
group of gorgonian corals in the lower mesophotic zone (60–
115m; Fig.39.3). The most abundant members observed in
the San Andrés area and PNNCRP were Caliacis nutans, fol-
lowed by Hypnogorgia pendula, Scleracis guadalupensis,
Thesea spp., and other unidentied species of the subfamily
Paramuriceinae. Other abundant gorgonian corals in this zone
were Swiftia exserta, Leptogorgia hebes, Thelogorgia stud-
eri, Lytreia plana, and Trichogorgia lyra. One of the main
contributions of these rst exploratory surveys was to obtain
the rst detailed photographic account of most gorgonian cor-
als in this area, where the coral species were observed to be
noticeably different underwater. For instance, H. pendula and
C. nutans are both black in color when dry, but when alive are
bright yellow and beige in color, respectively (Fig. 39.3e;
Alonso etal. 2015). Additionally, Villogorgia nigrescens and
Acanthogorgia aspera were once collected off Isla Tesoro,
near Cartagena (JA Sánchez, pers. obs.), but were not
observed in the most recent surveys.
Off the Caribbean coast of Venezuela, Ruiz and Rada
(2006) assessed mesophotic octocorals from 40 to ~150m
and reported species from the families Ellisellidae (Ellisella
barbadensis 50–60m, Ellisella elongata 50m, and Nicella
guadalupensis 130m), Plexauridae (H. pendula 50–130m,
S. exserta 53m, Thesea guadalupensis 50m, and Plexaura
fusifera 40m), Gorgoniidae (Leptogorgia virgulata 50m),
and Anthothelidae (Diodogorgia nodulifera 130m) rendering
similar observations to the sites in Colombia. Some of the
species recorded in Caribbean MCEs can also be found in
shallower depths at temperate latitudes in the Gulf of Mexico
and Atlantic coasts (Devictor and Morton 2010) as suggested
by Bayer (1961).
39.2.2 Gulf ofMexico, USA
The Gulf of Mexico encompasses a tropical-subtropical latitu-
dinal gradient along over 5000km of Mexican and US coast-
lines. Its biodiversity has been extensively studied in the last
50–60years (Felder and Camp 2009), although mesophotic
gorgonian corals in the Gulf of Mexico have received little
attention. In the northern Gulf of Mexico, the banks and reefs
support a diverse community of mesophotic octocorals (Rezak
etal. 1990). In some parts of the outer continental shelf, hard
substratum between 53 and 110 m depth off the coast of
Mississippi, Alabama, and eastern Louisiana (Gittings etal.
1992) provide favorable habitat for diverse communities of
gorgonian corals. In the northwestern Gulf of Mexico (off the
coasts of Texas and Louisiana), the Flower Garden Banks rep-
resents different mesophotic gorgonian genera such as
Callogorgia, Thelogorgia, Diodogorgia, Scleracis, Swiftia,
Thesea, Muricea, and Placogorgia (Rezak et al. 1990;
Schmahl etal. 2008). In the northeastern Gulf of Mexico (the
Mississippi-Alabama Delta and De Soto Canyon), reef-like
structures or “pinnacles” near the continental edge have also
been documented as harboring mesophotic gorgonians at
depths between 50 and 110m (Gittings etal. 1992; Etnoyer
etal. 2016). At least 31 gorgonian species have been identied
on these hard substrata with the most common genera includ-
ing Bebryce, Nicella, Swiftia, Hypnogorgia, Thesea,
Paramuricea, Placogorgia, and Muriceides (Etnoyer et al.
2016). Modeling studies have identied more than 400km2 of
suitable habitat for MCEs in this part of the Gulf of Mexico
that occur along carbonate mounds and paleo-shoreline ridges
(Silva and MacDonald 2017).
Other studies have characterized communities deeper than
250m, with a series of surveys focused on species delimita-
tion, community composition, community assembly, and phy-
logenetic analyses (Quattrini etal. 2013, 2014, 2015). From
the reported 52 nominal species, at least 7 species span the
range from the mesophotic to deep sea (Quattrini etal. 2014;
Etnoyer and Cairns 2017) such as Acanthogorgia aspera,
Callogorgia americana, Callogorgia gracilis, Scleracis gua-
dalupensis, Nicella sp., Anthothela sp., and S. exserta. In some
areas of the Gulf of Mexico, there are no specic boundaries
that divide mesophotic and deep-sea gorgonian communities,
and there is evidence of species composition overlap in the
layers from 60 to 500m (Etnoyer and Cairns 2017). Within the
genus Callogorgia, Quattrini etal. (2013) studied its ecologi-
cal and evolutionary aspects and found that there is a segrega-
tion by depth of the three species found in the Gulf of Mexico,
where C. gracilis is distributed in the shallower range (82m),
followed by C. americana and C. americana delta in the
deeper parts of the continental shelf. They suggested a separa-
tion between C. americana and C. delta as distinct species
based on their ecological niche and phylogeny, which was
later conrmed by Bayer etal. (2015).
39.2.3 Brazil
Brazil has one of the most extensive coastlines in the world,
approximately 7500km long, and a continental shelf up to
320km wide (Campos etal. 1974). These physical and geo-
logical conditions create an excellent setting for the develop-
ment of MCEs, but studies on these environments are still
incipient. Octocoral fauna, mostly gorgonians, is at the
moment represented by approximately 100 species (Pérez
et al. 2016), with 17 shallow water (<30 m) being mostly
J. A. Sánchez et al.

735
Fig. 39.2 Gorgonian corals (family Ellisellidae) in the lower Caribbean MCEs (60–115m) from San Andrés Island. (a, b) Nicella goreaui,
(c) N. toeplitzae, (d, e) Ellisella sp., (f) Ellisella nivea, (g, h) Ellisella elongata (45m), and (i) Ellisella sp. (Photo credits: J.A.Sánchez)
39 Gorgonian Corals

736
endemic species (Castro etal. 2010), and around 80% deep-
water/azooxanthellate species (Cordeiro etal. 2015). At least
40 (~40%) gorgonian species of Brazil inhabit MCEs
(Table39.1). The three most representative octocoral families
in MCEs throughout Brazil are the Plexauridae (19 species),
Gorgoniidae (8 species), and Elliselliidae (7 species). The
rst two are common on the continental shelf, whereas the
latter is usually found at the shelf break to upper slope
(Cordeiro etal. 2015). In recent trawling surveys performed
by the R/V Seward Johnson in May 2011, during the cam-
Fig. 39.3 Gorgonian corals (family Plexauridae) in the lower Caribbean MCEs (60–115m) from San Andrés Island. (a) Scleracis guadaloupen-
sis, (b) Hypnogorgia pendula (c, d) Caliacis nutans, (e) Swiftia exserta, and (f) Thesea sp. (Photo credits: J.A.Sánchez)
J. A. Sánchez et al.

737
paigns of the “Potiguar Basin Continental Slope Environmental
Characterization Project,” off the coast of Rio Grande do
Norte and Ceará states, Brazil (04°21′–04°48′ S, 36°03′–
37°53′ W), Nicella was one of the most ubiquitous genera in
such environments, with dense multi-specic aggregations
from around 80–200 m deep. Representatives of deep-sea
genera are often found in MCEs, such as Callogorgia (Bayer
et al. 2015), Primnoella (Cairns 2006), and Chrysogorgia
(Cairns 2001). All mesophotic gorgonian records in Brazil
were, at least initially, from museum specimens obtained on
oceanographic cruises up to the 2000s. Overall, there have
been few efforts to describe mesophotic communities, either
by SCUBA or remotely operated vehicle (ROV) throughout
Brazil (Brasileiro etal. 2015; Meirelles etal. 2015).
Brazilian corals are distributed in four main regions: north,
northeast, east, and southeast. The major reef zones are found
in the northeast and east regions (Leão etal. 2016), but the
northeast is the most poorly studied, and, yet it is likely to
harbor extensive MCEs as indicated by recent reports
(Cordeiro etal. 2015; de Oliveira Soares etal. 2016; Amado-
Filho etal. 2017). The eastern region (i.e., Abrolhos Bank)
has the only “true” coral reefs in the southwestern Atlantic
and was extensively surveyed in the last 10years. Abrolhos
Bank had the highest species diversity of Brazil’s exclusive
economic zone so far, with 29 gorgonian species recorded
(Castro et al. 2010; Leão et al. 2016; Mazzei et al. 2017).
Brazil’s northern and southern regions are still poorly known.
In the northern region, the two largest reef systems, below
35m, are the Amazon and the Parcel do Manuel Luís reefs,
with 25 (Cordeiro etal. 2015) and 6 (Amaral etal. 2007) zoo-
xanthellate gorgonian species, respectively. Nevertheless,
these regions still require further assessments of deeper com-
munities. Reports of gorgonian communities in the southern
region are rare in the literature, with only four species cur-
rently documented (Cairns 2006; Castro etal. 2010).
There is a clear isolation of shallow reef exclusive octo-
corals, corresponding to the Brazilian biogeographic prov-
ince, as 59% are endemic species (Castro et al. 2010).
However, biogeographic patterns are still unclear for the
mesophotic gorgonians within the western tropical Atlantic
MCEs. The Amazon River runoff, previously assumed to be
a barrier between the Caribbean and Brazilian faunas, has
little direct inuence on the dispersion of reef organisms
below 30m in depth (Moura etal. 2016). In particular, octo-
coral richness (27 species) in front of the mouth of Amazon
River is high, with 74% of the species shared on both sides
of the river mouth (Cordeiro etal. 2015).
39.2.4 Tropical Eastern Pacic Ocean, Mexico
The TEP includes the coasts and associated oceanic archi-
pelagos and islands between the Gulf of California and
Ecuador (Allen and Robertson 1994). Coral reefs are rather
unusual in this region, yet gorgonian corals (all azooxanthel-
late) are a dominant seascape community on rocky environ-
ments (Sánchez et al. 2014; Sánchez 2016). Direct
exploration of gorgonian corals in this region below 30m is
scarce. Species diversity of gorgonians from 30 to 70m have
been surveyed in Mexico along the Oaxaca coast (Abeytia
etal. 2013). In this area, the composition of gorgonian corals
revealed a clear difference between the mesophotic (>30m)
and shallow-water zone (<30m). Of the 34 species that have
been registered, 18 were found in <30m depth and 17in
>30m depth (Table39.1). Leptogorgia alba was the only
species observed from 0 to 70m in the Oaxaca coast, giving
L. alba the widest depth range of any gorgonian in the TEP
(Breedy and Cortés 2008; Abeytia etal. 2013). This species
is distributed all along the TEP coast and around oceanic
islands, e.g., Isla del Coco, Costa Rica and the Galapagos
Islands, and Ecuador (Breedy and Guzman 2007).
Comparing mesophotic and shallow-water habitats along
the Oaxaca coast, a larger number of species were identied
in MCEs in comparison with shallower environments (17 vs.
11 species, respectively), which suggests that deeper areas
harbor more gorgonian species than shallow ones at this
locality. All species belonging to the genera Eugorgia,
Psammogorgia, and Ellisella were found only in mesophotic
environments. In addition, mesophotic gorgonian species in
the Oaxaca coast seems to be site specic, indicating limited
connectivity between sites, in comparison to shallow-water
gorgonians, which were distributed more homogeneously
along the Oaxacan coast (Abeytia etal. 2013). The TEP gor-
gonian coral fauna is endemic and diverse and need further
exploration of its MCEs. Of particular interest is the diverse
specialist fauna found on gorgonian corals in this region
including associated mollusks and crustaceans (Sánchez
2016; Sánchez etal. 2016).
39.2.5 Indo-Pacic Ocean, Federated States
ofMicronesia, andtheCoral Triangle
Gorgonian corals typically dominate MCEs throughout the
Indo-Pacic Ocean (Colin etal. 1986; Rowley etal. 2019).
Recent advances in autonomous underwater vehicles and
ROVs, as well as CCR diving technologies, have increas-
ingly facilitated research and exploration at depths below
30m. Until recently, however, most exploration throughout
the region has been conducted on shes with observations
suggesting that less than 50% of species overlap exists
between shallow and mesophotic reef taxa (Pyle 2000).
Gorgonian taxa also show a similar bathymetric distribution,
but due to the current lack of taxonomic resolution, it is
unclear how taxonomically distinct the communities are
(Table39.1). Of the 65 Indo-Pacic genera currently recog-
nized to occur at mesophotic depths down to 200m, 14 were
specic to the upper mesophotic zone (30–59m), 15 to the
39 Gorgonian Corals

738
lower mesophotic zone (60–200m), and 36 spanned the full
depth range (30–200m). Only seven zooxanthellate genera
were present at mesophotic depths. Notably, Rumphella (up
to 61m) and Briareum (up to 75m) were present at the lower
mesophotic depths of Pakin Atoll in the Federated States of
Micronesia likely due to the remarkable water clarity
(Ko =0.038 ± 0.002 m−1 and a 1% subsurface irradiance of
23μE m−2 s−1 at 110m; Rowley etal. 2019). The most com-
mon genera conned to the upper mesophotic depths were
within the family Plexauridae (n=5), none of which are zoo-
xanthellate. Lower MCEs were characterized by genera
(n=7) primarily within Primnoidae, e.g., Callogorgia Gray,
1858. Furthermore, taxa spanning the full MCE depth range
(30–200m) were dominated by genera within the Plexauridae
(n=11), Elliselliidae (n=7), and Acanthogorgiidae (n=3).
Current distribution records suggest that a shift or transition
zone from upper to lower MCE communities occurs at ~55
to 75m depth (SJ Rowley, pers. obs.; Table39.1), which has
also been observed on the MCEs of the Great Barrier Reef,
Australia (Bridge etal. 2012).
By combining records from recent and historical expedi-
tions, two clear patterns begin to emerge (Fig.39.4). First, a
Fig. 39.4 Distribution of tropical gorgonian taxa. (a) the Indo-Pacic, (b) within the Philippine Archipelago, (c) Hoga Island and the Northwestern
tip of Kaledupa Island within the Wakatobi Marine National Park, Southeast Sulawesi, (d) Enewetak Atoll, and (e) Bikini (Pikinni) Atoll in the
Marshall Islands. Green-colored circles represent shallow [morpho-]species records from 0 to 39m depth, and blue circles for mesophotic depths
from 40 to 200m depth. (Data sources summarized from Rowley etal. 2019; SJ Rowley, unpubl. data). Note: records of uncertain bathymetry, i.e.,
sampled by trawl or dredge were omitted from this analysis
J. A. Sánchez et al.

739
decrease in shallow-water taxa occurs from the eastern Coral
Triangle across the Pacic Ocean, a pattern similarly
observed for other shallow reef corals and shes (Hoeksema
2007; Veron et al. 2009; Carpenter etal. 2010). Shallow-
water taxa likely populate habitats horizontally outward
from the Coral Triangle and thus are largely unrelated to
their MCE counterparts. This may well explain, at least in
part, the eastward attenuation in shallow-water gorgonian
diversity. Second, gorgonian diversity at mesophotic depths
persists across the Pacic (Fig.39.4a–e). This pattern may
possibly be due to taxa at lower mesophotic depths being
older lineages, as well as the persistence of deep-reef habi-
tats over geological time. It is clear, therefore, that the shal-
low and mesophotic gorgonians (Rowley et al. 2019) and
shes (Pyle etal. 2016) of the Indo-Pacic have contrasting
biogeographic distributions. Moreover, an increase in [mor-
pho-]species1 richness with increased depth particularly in
azooxanthellate taxa has been shown within the Indo-Pacic
(e.g., Bridge etal. 2012; Rowley etal. 2019). Evidence has
yet to reveal if the eastward attenuation in shallow-water
diversity from the Coral Triangle can be explained by dif-
ferential dispersal abilities and the reduction in habitat avail-
ability (lack of land mass to populate) and similarly to
explain the increased and sustained diversity of azooxanthel-
late gorgonian taxa at depth across the Indo-Pacic (Fig.39.4;
e.g., Fig.39.1).
Gorgonian ecological and biogeographic distributions
vary relative to habitat availability, local species pool (i.e.,
the presence of conspecics and environmental conditions
conducive for successful reproduction), the local and regional
environment, and habitat disturbance history (Fabricius etal.
2007). For example, high densities of the fragile gorgonian
Stephanogorgia faulkneri populate the slopes of certain reefs
of Palau (Colin 2009). This unusual genus, however, is only
sparsely populated in other regions (Rowley etal. 2019).
Remote locations such as the Hawaiian Archipelago possess
unique benthic communities and biodiversity (Kosaki etal.
2017), where only a single shallow-water gorgonian species
is present, the rarely encountered endemic Melithaea
(Acabaria) bicolor (Nutting, 1908) whose full bathymetric
and biogeographic distribution is unknown. However, a sig-
nicant abundance of gorgonian corals occurs at greater
depths (≥130m) in Hawaiʻi and consists of taxa often typical
of Indo-Pacic deep-sea environments (e.g., Coralliidae;
Ardila etal. 2012).
Unique gorgonian taxa and assemblages may be relatively
consistent throughout the Indo-Pacic; this is because islands
and atolls provide stepping-stones for dispersal within and
between other regions. Overlapping gorgonian taxa across
bathymetry (as seen in Table39.1) are widely dispersed and
1 Morphospecies is the morphological similarity of a group of organ-
isms with respect to all others within the genus.
thus are ideal targets for tests of resilience and comparative
divergence through local adaptation. Interestingly, azooxan-
thellate gorgonian genera such as Acanthogorgia, Annella,
Bebryce, Ellisella, and Villogorgia span remarkable depths
(e.g., 5 to >1000m) across their distributional ranges. Yet
within-group polyphyly across bathymetry (e.g., deepwater
monophyly that is typically disrupted by shallow-water taxa;
McFadden etal. 2006; Pante etal. 2012) calls for further sys-
tematic assessment with polyphyletic groups either as a con-
sequence of convergent evolution or deep divergence.
39.3 Threats
Gorgonians in MCEs are not immune to the impacts of natu-
ral and anthropogenic events; however, they may be better
adapted to survive climatic anomalies compared to their
scleractinian counterparts. MCEs are inuenced by natural
events such as typhoons (Baker et al. 2016), as well as
anthropogenic encroachment from coastal development, pol-
lution, global climate change, and overshing (Rowley etal.
2019). Irrespective of the detrimental accelerations in global
climate change, MCE benthic communities can be severely
compromised by smothering and, often, suspended surface
sediments and dislodged coral rocks.
Fouling of colonies with cyanobacteria in the upper meso-
photic zone corresponded with high chlorophyll pulses and
lower water temperatures during the 2010 El Niño event in
Palau (P Colin, pers. obs.). Such colony fouling by cyano-
bacteria and other biota has been observed in other regions
due to excess eutrophication (e.g., Indonesia and Pohnpei:
SJRowley, pers. obs.). However, in the case of Palau anoma-
lous high chlorophyll levels that are negatively correlated
with water temperature has been shown to occur during the
warm pool El Niño decay period (Lee etal. 2014; Hou etal.
2016). Thereafter, and with the onset of the La Niña, the
chlorophyll maximum sinks to the lower mesophotic and
reduces in concentration (SJ Rowley, pers. obs.). No notice-
able effect was recorded on the gorgonian colonies at depth,
but shallow and upper mesophotic scleractinians incurred
bleaching (Baker etal. 2016). High nutrients leading to such
biological responses during El Niño events are likely short-
term impacts that lead to recovery, yet increased eutrophica-
tion and shing impacts (e.g., herbivores such as shes and
echinoderms) are likely to impact coral reefs into upper
mesophotic depths.
Overshing particularly from international factory
shing vessels disrupt the marine food chain. The exacer-
bating effects of targeting pelagic shes and an increase
inlocal community reliance on reef shes lead to a lack of
herbivores and subsequent increases in algal coverage on
MCEs. For example, coral reef communities up to 68m
depth in Pohnpei have been smothered by a succession of
39 Gorgonian Corals

740
cyanobacteria followed by invasive algal overgrowth lead-
ing to mortality in 2016–2017. Interestingly, the azooxan-
thellate gorgonians present such as Astrogorgia, Annella,
Acanthogorgia, Melithaea, Subergorgia, and taxa within
the Elliselliidae were mostly unaffected although were
sparsely distributed to ~70m depth (Rowley etal. 2019).
Local and international shing impacts, as well as
increases in land- based pollution runoff, have led to an
increasingly detrimental combination of human impacts
with natural phenomena (e.g., El Niño to La Niña) on
coral reefs which may in turn tip natural resilience to algal
dominance.
MCEs may be far more sensitive to surface activities than
previously thought. Diurnal and seasonal temperature oscil-
lations at mesophotic depths (>50m) on the oceanic islands
and atolls of the Indo-Pacic show remarkable variability
(e.g., Wolanski et al. 2004; Rowley et al. 2019). Ongoing
research shows that while bleaching of scleractinian corals
does occur at mesophotic depths (Baker etal. 2016), azoo-
xanthellate gorgonians within the same environment can still
thrive suggesting the development of adaptive tolerance due
to considerable exposure over geological time.
Oil spills are another threat that have been recognized to
be harmful for gorgonian octocorals where both mesophotic
and deep-sea species have been found to be negatively
affected (White etal. 2012; Fisher etal. 2014; Etnoyer etal.
2016). The Deepwater Horizon oil spill occurred in 2010in
the Gulf of Mexico and released an estimated 4.1 million
gallons of oil and gas for over 80days (Reddy etal. 2012),
directly affecting the MCEs located in the northwest, more
specically the rocky reefs called the “Pinnacle Trend”
located from 60 to 90m (Etnoyer etal. 2016; Silva etal.
2016). Two of the reefs most impacted by the oil spill were
located between 60 and 80m below the oating oil for more
than 19days and were used to assess the before and after
effects of the oil spill (Etnoyer etal. 2016). Following the oil
spill, signicant damage on mesophotic gorgonians was doc-
umented, which included species such as S. exserta, Thesea
sp., H. pendula, and Placogorgia sp. that showed a tenfold
increase in injuries compared to before the spill. The most
common injuries were overgrowth by hydroids and smother-
ing by sedimentary material (47%), broken branches (26%),
and bare branches with loss of coenenchyme and polyps
(14%). One possible mechanism by which these corals were
affected by the oil spill was by the sinking of contaminated
material in the form of marine snow that formed on the colo-
ny’s surface (Passow etal. 2012).
Pathogenic bacteria can lead to diseases, population
decreases, and mortality in mesophotic gorgonian corals. In
the Mediterranean Sea, bacteria caused disease outbreaks,
and fungal and bacterial agents led to mass mortalities in
Brazil, with both being linked to high thermal anomalies
associated with climate change (Weil etal. 2017). A Vibrio
strain isolated from diseased colonies of Paramuricea clavata
suggests that pathogenic bacteria play a role in gorgonian
coral mortality events (Bally and Garrabou 2007). Vibrio-
TAV24 strain, belonging to the species Vibrio coralliilyticus,
has been involved in mass mortality events of the purple gor-
gonian P. clavata in the northwest Mediterranean Sea
(Rosenberg etal. 2007; Vezzulli etal. 2013). In the same gor-
gonian species, Weil et al. (2017) found fungal diseases
resulting from Aspergillosis produced the highest mortalities
in Mediterranean colonies (Weil etal. 2017). Similarly, in the
Caribbean, disease affecting Gorgonia ventalina was caused
by Aspergillus sydowii, with a similar disease also found in
Tropical Eastern Pacic sea fans (Rosenberg et al. 2007;
Barrero-Canosa etal. 2012; Thompson et al. 2015; van de
Water etal. 2017; but see Quintanilla et al. 2018). In the gor-
gonian corals Muricea fruticosa and M. elongata, Mycoplasma
was found associated with the microbiome of bleached colo-
nies, which is another kind of bacterial-driven bleaching
(Holm and Heidelberg 2016). Another bacteria associated
with bleached gorgonian corals (present in 30% of all bacteria
isolated from bleached corals) were Vibrio spp., which
includes the species V. shiloi and V. coralliilyticus (Kushmaro
etal. 2001; Rosenberg etal. 2007; van de Water etal. 2017).
Given the diverse microbial community associated with gor-
gonian corals, it is important to understand the microbiome
associated with its healthy and diseased states.
The effects of changes in seawater chemistry and subse-
quent changes in seawater pH (ocean acidication) have
been investigated in a mesophotic gorgonian Corallium
rubrum collected from the Mediterranean Sea at 40 m
(Bramanti etal. 2013). Detrimental effects occur at low pH
(7.8), where a 59% decrease in calcication was observed
compared to control conditions. This study contrasts with
other experimental evidence in gorgonian corals from shal-
lower populations, where there is no observable negative
effect at the same pH (Gómez etal. 2015). There is, however,
a lack of information for other mesophotic and deep-sea gor-
gonians, which could be an artifact of the difculties and
challenges for maintaining these gorgonian corals alive in
articial conditions simulating global climate change. As
calcifying organisms, mesophotic gorgonians rely heavily on
different physical parameters such as the concentration of
Ca2+ and the alkalinity of the seawater in order to grow and
build their calcium carbonate (CaCO3) structures. Therefore,
one of the main concerns for mesophotic and deep-sea corals
is the saturation state of different CaCO3 polymorphs (Orr
et al. 2005). Aragonite and calcite are two different poly-
morphs of CaCO3 with different physical properties and
equilibrium constants (Feely etal. 2004). Calcite is thermo-
dynamically the more stable with a dissolution constant
higher than aragonite (Feely etal. 2004); however, when it is
enriched with magnesium such as high-magnesium calcite
(MgCO3), it becomes more soluble than either calcite or ara-
J. A. Sánchez et al.

741
gonite (Chave etal. 1962; Morse etal. 2006) and is a higher
concern, since it is the main polymorph used by gorgonians
(Thresher etal. 2010; Lebrato etal. 2016). As the increase in
atmospheric carbon dioxide continues, the saturation states
of these polymorphs are expected to shoal, causing under-
saturation in the whole water column by the end of the cen-
tury according to different climate change scenarios (Orr
etal. 2005). This poses a threat to several mesophotic corals,
especially to octocorals since they might not be able to cal-
cify at the same rate that they dissolve, creating an unbal-
anced state between accretion and erosion, as evidenced by
the study of C. rubrum (Bramanti etal. 2013). Mineral com-
position of CaCO3 is highly inuenced by phylogeny, but
other external factors such as temperature, pressure, carbon-
ate concentration, and food supply also are believed to be
important in marine biomineralization (Mackenzie et al.
1983; Ries 2011). Octocorals in general produce sclerites in
the form of MgCO3 with a range between 4 and 16mole per-
centages, including species common in the mesophotic and
deep sea (Thresher etal. 2010, 2011; Lebrato etal. 2016).
The gorgonian octocoral axis is an important taxonomic
trait and is also a key factor to consider when discussing
ocean acidication in these communities. For example, the
suborder Calcaxonia is a group of octocorals with a highly
calcied axis and numerous external scale-type sclerites,
which are distributed in most cases from 60 to 3000 m
(Cairns and Bayer 2002). In naturally low pH ocean basins,
such as the Central and South Pacic, there is a rich and
abundant community of octocorals, and calcaxonians are the
most abundant in places where there is undersaturation for
CaCO3 (Thresher etal. 2011). This suggests that Calcaxonia
has developed special adaptations to resist corrosive waters
and thrive in places where scleractinian corals will dissolve.
A good understanding about the physiology and ecology
regarding mesophotic octocorals in the context of ocean
acidication is lacking, and very limited experimental work
has been done.
39.4 Mesophotic vs. Shallow Gorgonian
Corals
39.4.1 Are Gorgonian Communities
Dependent onEndosymbionts?
There is a missing piece of information regarding the trophic
ecology, metabolism, and energetics of gorgonian corals,
which is needed to fully understand differences between
shallow and mesophotic species. Gorgonian corals are lter-
feeding organisms, but as in most octocorals have low diver-
sity and density of nematocysts (Fabricius etal. 1995a, b;
Ribes etal. 1998), which means that they rely on passive
feeding or mucus to capture particles. In addition, most zoo-
xanthellate gorgonian corals associated with Symbiodinium
do not rely entirely on photosynthesis for their energy needs,
and only the species with the thinnest branches seem to rely
entirely on their endosymbionts for energy (Baker et al.
2015). Since mesophotic gorgonian corals are usually thin
(Figs.39.1, 39.2, and 39.3) and azooxanthellate (Table39.1),
several important questions arise. Why are their branches
thin even if they do not have photosynthetic symbionts?
What other strategies have mesophotic gorgonians for main-
taining their metabolic demands? Are the microbial endo-
symbiont communities responsible for maintaining different
metabolic processes of the coral holobiont and coral
energetics?
Several studies have shown that microbial communities
are key endosymbionts and perform important roles within
the coral holobiont including biogeochemical cycling (Azam
and Malfatti 2007; Falkowski et al. 2008), production of
antibiotics (Rosenberg etal. 2007; Rypien etal. 2010), pro-
tection against pathogens (Rohwer etal. 2002; Rypien etal.
2010) and help the host adapt and acclimate to different
environmental conditions and build up immunity (Webster
and Reusch 2017). These relationships are complex and
fundamental for host tness (Bourne etal. 2016). We suggest
that bacterial endosymbionts comprise a major metabolic
strategy for nutrient cycling in gorgonian corals (Fig.39.5).
Some bacteria associated with mesophotic gorgonian cor-
als have multiple roles in biogeochemical cycles (e.g., the
Alphaproteobacterial genus Bradyrhizobium; Kellogg etal.
2016). Bacteria are also capable of degrading xenobiotics
dealing with potentially toxic compounds. Oceanospirillales,
Rhodobacterales, and Flavobacteriales are known for their
ability to produce hydrolytic enzymes, helping the host
acquire nutrients in otherwise oligotrophic environments
(Goffredi etal. 2005) such as many MCEs. In addition, these
abundant symbionts may also play a probiotic role through
the production of antibiotics in the Caribbean zooxanthellate
gorgonian coral Antillogorgia elisabethae (Jeong etal. 2005)
or through competitive exclusion of pathogens (Robertson
etal. 2016). The potential functions of Rhodobacterales may
include the production of antimicrobial agents, as well as
nutrient cycling (Brinkhoff et al. 2008; Robertson et al.
2016). Bacterial isolates from Paramuricea arborea grouped
closely with Colwellia psychrerythraea, a psychrophilic het-
erotrophic bacteria associated with carbon cycling and other
nutrient cycling functions (Methé et al. 2005; Gray et al.
2011), which highlights the key role of bacterial communi-
ties on nutrient cycling (Fig.39.5).
The high diversity of symbiotic microorganisms corre-
sponds to coral health and resilience (Rosenberg etal. 2007;
Ainsworth et al. 2010). The genus Endozoicomonas
(Proteobacteria) is the dominant bacterial associate in sev-
eral temperate and tropical gorgonians (Rosenberg et al.
2007; Ainsworth etal. 2010, 2015; Bayer etal. 2013; Rivière
39 Gorgonian Corals

742
etal. 2013; Robertson etal. 2016). For example, Eunicella
cavolini and P. clavata, two azooxanthellate gorgonians
from the Mediterranean Sea (Rivière etal. 2013; Ransome
etal. 2014), and A. elisabethae a zooxanthellate species from
the Western Atlantic (Robertson et al. 2016) have
Endozoicomonas in healthy colonies, forming host-specic
symbiotic relationships (Rivière etal. 2015; Quintanilla et al.
2018). Another function attributed to Endozoicomonas
includes the transport of molecules, glycogenesis, synthesis
of different amino acids (Ding etal. 2016; Neave etal. 2017a,
b), and the production of hydrolytic enzymes (Goffredi etal.
2005). In addition, it is involved in antimicrobial and antibi-
otic compounds that limit the growth of other bacteria and
protect the gorgonian coral host against diseases (Long and
Azam 2001; Ritchie 2006; Bourne etal. 2007, 2009), which
prevents the proliferation of competing or invading microor-
ganisms. Despite the recent research on Endozoicomonas,
more studies are needed to elucidate the role and metabolic
functions in the holobiont with focus on contrasting shallow
vs. mesophotic species.
39.4.2 Shallow-Deep Ecological Divergence
Depth is a key ecological variable in the generation of con-
spicuous adaptations in marine organisms, and gorgonians
are no exception. In marine ecosystems, variation related to
environmental heterogeneity along the depth gradient has
been studied using reciprocal transplant experiments of the
gorgonian corals Eunicea exuosa (Lamouroux, 1821) and
Antillogorgia bipinnata (Verrill, 1864), both of which have
shallow and deep morphotypes (Prada et al. 2008, 2014;
Prada and Hellberg 2013, 2014).
The shallow and deep morphotypes of E. exuosa range
from 0 to ~30 m deep. Using nuclear and mitochondrial
markers across several populations in the Caribbean
(Panamá, Puerto Rico, and the Bahamas), two independent
lineages segregated by depth were depicted with a low cor-
relation to geography, as expected for a broadcast spawner
(Prada etal. 2008, 2014; Prada and Hellberg 2013, 2014). In
natural populations of E. exuosa, the reduction of gene ow
was enhanced by the length of the juvenile phase (~15years),
Fig. 39.5 Bacterial communities are key gorgonian coral endo symbionts. (a) Gorgonian corals with photosynthetic symbionts, Symbiodinium,
typical from shallow-water reefs. (b) Bacterial communities in gorgonian corals without zooxanthellae, mesophotic, and deep-sea species. In both
(a, b) gorgonian corals obtain nutrients and other metabolites from bacterial communities. OM organic matter
Zooxanthellate
ab
Azooxanthellate
Nutrient dependency
interaction
Coral host
Adaptation
and
Aclimatization
process
Patogenesis
Coral host
Bacterial
communities
Bacterial
communities
Symbiodinium
Provide C
Diazotrofic bacteria
provide fixed N
-Growth surface
-Protection
-Metabolites
and nutrients
-CO2, inorganic
nutrients
-Growth surface
-Protection
-Metabolites
and nutrients
-CO2
-Filtered OM
-Nutrients supply
-Antimicrobial
activity
-Nutrient cycling
-Vitamins
-Trace minerals
-Cofactors
-Oxygen
-Glucose
-Glycerol
-Amino acids
Nutrient
cycling
J. A. Sánchez et al.

743
promoting selection and divergent adaptation between shal-
low and deep habitats (Prada etal. 2008; Prada and Hellberg
2013). In addition, each morphotype was associated with a
particular Symbiodinium type; thus, high specicity found in
holobiont clusters could play a key role in pre-zygotic isola-
tion by the strong effect of coral-symbiont inmigrant invia-
bility (Prada and Hellberg 2014; Prada et al. 2014).
One study using transplant experiments on the feather-
like gorgonian coral A. bipinnata and A. kallos (considered
as deep and shallow morphotypes; Sánchez et al. 2007) was
carried out to test the impact of genotype and environmental
interactions on colonial traits (i.e., branch pattern; Calixto-
Botía and Sánchez 2017), which seems to be a product of
adaptive plasticity. A. bipinnata is distributed along shallow
waters to depths of 45m in the Caribbean but is absent from
the eastern side of the basin. Along the bathymetric gradient,
the species exhibits signicant variation in its morphological
traits such as colony size, colony coloration, sclerite form,
and branching pattern. Accordingly, the shallow and deep
morphotypes of A. bipinnata are conspicuously different at
the extremes of the depth cline with intermediate forms in
between (Sánchez etal. 2007). For both E. exuosa and A.
bipinnata, ecological divergence of the two morphotypes
could be the result of shallow-water species colonizing
MCEs (Sánchez 2016). There are several potential species
undergoing similar ecological divergences in the Caribbean
Sea such as Eunicea knighti, Muriceopsis petila, Antillogorgia
hystrix, A. americana, and Plexaura nina.
39.5 Conclusions
Gorgonian corals are widely distributed throughout the
MCEs of the Indo-Pacic and Atlantic Oceans. There is still
a paucity of information on gorgonians throughout the Indo-
Pacic despite their high regional abundance and diversity
(Van Ofwegen 1997). Thus, the origin and distribution of
both shallow- and deepwater gorgonian taxa across the Indo-
Pacic remain a source of intrigue, providing a natural labo-
ratory to investigate patterns of ecology, population genetics,
biogeography, and phylogenetic hypotheses relative to other
tropical reef regions. In the Caribbean, we found an abrupt
lineage/community replacement where zooxanthellate spe-
cies disappear and azooxanthellate species dominate occur-
ring at around 60m.
Mesophotic gorgonian coral microbiology is in its infancy
and promises to be a fruitful area of future research.
Differentiating core symbiotic bacteria from the diverse host-
associated consortium is essential for characterizing funda-
mental interactions between microorganisms and their coral
hosts, in determining the functional and ecological contribu-
tions of bacteria, and whether they have different roles in zoo-
xanthellate and azooxanthellate corals. It is necessary to
elucidate the specic functions of these symbiotic partner-
ships and the response of the mesophotic gorgonian coral
microbiome to climate change, ocean acidication, and other
anthropogenic factors. Environmental conditions inuencing
the quality and quantity of gorgonian microbiota are unknown.
The Caribbean and TEP have endemic gorgonian corals.
In these regions, shallow-water gorgonians also colonize
MCEs, which suggest that these environments can promote
ecological divergence and eventually speciation. With a few
seemingly allopatric-generated species, numerous closely
related species of Eunicea, Antillogorgia, Plexaura-
Pseudoplexaura, Muricea, and Pacigorgia exhibit signa-
tures of recent radiations (e.g., high morphological
differentiation and low genetic divergence), which require
further studies to test the shallow-deep ecological divergence
scenario.
Acknowledgments This chapter includes data and observations from
diverse grant sources from all authors. U. of Los Andes researchers
were partially funded by the agreement between Corporación para el
Desarrollo Sostenible del Archipiélago De San Andrés, Providencia y
Santa Catalina – Coralina and Universidad de los Andes-UniAndes
(Agreements 13–14 and 21–15), and COLCIENCIAS (grant No.
120465944147), Colombia. Additional funding was possible, thanks to
Vicerrectoría de Investigaciones, Programas de Investigación-
Especiación Ecológica (UniAndes). The support from Bluelife dive
shop (family Garcia) was fundamental to accomplishing this study. The
San Andres Hospital kindly supplied medical oxygen for CCR diving.
We are very grateful with Gregg Stanton, Wakulla Dive Center, for the
continuing support and advice for deep diving. We are thankful for
eldwork support from Fabian García, Santiago Herrera, Mariana
Gnecco, Manu Forero, Federico Botero, and Camilo Martinez. SJR is
thankful for the generous support of the Systematic Research Fund
(SRF); the Edmondson Trust of the Bernice P. Bishop Museum,
Honolulu; and Ocean First Education. Gratitude is also extended to
Prof. Steven Stanley for encouragement and logistical support; Wilbur
Walters, Ed Roberts, Richard Pyle, and Mae Dorricott for eldwork
support; and to Poseidon/Cis-Lunar technology for CCR equipment for
the Indo-Pacic research. CDP thanks CNPq, Conselho Nacional de
Desenvolvimento Cientíco e Tecnológico (Grant, CNPq/MCTI/
FACEPE/PROTAX N° 001/2015. Processos 440633/20150 and APQ–
0913–2.04/17). RTSC was supported by FACEPE (IBPG–0558–
2.04/13), Brazil. Acknowledgments are also extended to the US
National Cancer Institute (NCI) for extensive research support to PLC
in Palau, Micronesia. Comments from the book editors and reviewers
(K.Puglise, A.Quatrini, A.Shuler, and an anonymous reviewer) greatly
improved the manuscript.
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