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The biology and ecology of the ocean sunfish Mola mola: A review of current knowledge and future research perspectives

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Abstract Relatively little is known about the biology and ecology of the world’s largest (heaviest) bony fish, the ocean sunfish Mola mola, despite its worldwide occurrence in temperate and tropical seas. Studies are now emerging that require many common perceptions about sunfish behaviour and ecology to be re-examined. Indeed, the long-held view that ocean sunfish are an inactive, passively drifting species seems to be entirely misplaced. Technological advances in marine telemetry are revealing distinct behavioural patterns and protracted seasonal movements. Extensive forays by ocean sunfish into the deep ocean have been documented and broad-scale surveys, together with molecular and laboratory based techniques, are addressing the connectivity and trophic role of these animals. These emerging molecular and movement studies suggest that local distinct populations may be prone to depletion through bycatch in commercial fisheries. Rising interest in ocean sunfish, highlighted by the increase in recent publications, warrants a thorough review of the biology and ecology of this species. Here we review the taxonomy, morphology, geography, diet, locomotion, vision, movements, foraging ecology, reproduction and species interactions of M. mola. We present a summary of current conservation issues and suggest methods for addressing fundamental gaps in our knowledge.
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1 23
Reviews in Fish Biology and
Fisheries
ISSN 0960-3166
Volume 20
Number 4
Rev Fish Biol Fisheries (2010)
20:471-487
DOI 10.1007/
s11160-009-9155-9
The biology and ecology of the ocean
sunfish Mola mola: a review of
current knowledge and future research
perspectives
1 23
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The biology and ecology of the ocean sunfish Mola mola:
a review of current knowledge and future research
perspectives
Edward C. Pope
Graeme C. Hays
Tierney M. Thys
Thomas K. Doyle
David W. Sims
Nuno Queiroz
Victoria J. Hobson
Lukas Kubicek
Jonathan D. R. Houghton
Received: 12 June 2009 / Accepted: 16 December 2009 / Published online: 19 January 2010
Ó Springer Science+Business Media B.V. 2010
Abstract Relatively little is known about the biol-
ogy and ecology of the world’s largest (heaviest)
bony fish, the ocean sunfish Mola mola, despite its
worldwide occurrence in temperate and tropical seas.
Studies are now emerging that require many common
perceptions about sunfish behaviour and ecology to
be re-examined. Indeed, the long-held view that
ocean sunfish are an inactive, passively drifting
species seems to be entirely misplaced. Technolog-
ical advances in marine telemetry are revealing
distinct behavioural patterns and protracted seasonal
movements. Extensive forays by ocean sunfish into
the deep ocean have been documented and
N. Queiroz
e-mail: nuno.queiroz@gmail.com
D. W. Sims
School of Biological Sciences, University of Plymouth,
Drake Circus, Plymouth PL4 8AA, UK
N. Queiroz
CIBIO-UP, Centro de Investigac¸a
˜
o em Biodiversidade e
Recursos Gene
´
ticos, Campus Agra
´
rio de Vaira
˜
o, Rua
Padre Armando Quintas, 4485-668 Vaira
˜
o, Portugal
L. Kubicek
Whale Observation Project, W.O.P. Centre, Aussermatt,
3532 Za
¨
ziwil, Switzerland
e-mail: wop@vtxmail.ch
J. D. R. Houghton
School of Biological Sciences, Queen’s University
Belfast, Medical Biology Centre, 97 Lisburn Road,
Belfast BT9 7BL, UK
J. D. R. Houghton (&)
Queen’s University Belfast Marine Laboratory, 12-13 The
Strand, Portaferry, Newtownards BT22 1PF, UK
e-mail: j.houghton@qub.ac.uk
E. C. Pope
Centre for Sustainable Aquaculture Research, Swansea
University, Swansea SA2 8PP, UK
e-mail: e.c.pope@swansea.ac.uk
G. C. Hays V. J. Hobson
Institute of Environmental Sustainability, Swansea
University, Swansea SA2 8PP, UK
e-mail: g.hays@swansea.ac.uk
V. J. Hobson
e-mail: v.j.hobson@swansea.ac.uk
T. M. Thys
Ocean Sunfish Research and Tagging Program, 25517
Hacienda Place, Suite C, Carmel, CA 93923, USA
e-mail: tierneythys@gmail.com
T. K. Doyle
Coastal and Marine Resources Centre, Lewis Glucksman
Marine Facility, University College Cork, Haulbowline,
Cork, Ireland
e-mail: t.doyle@ucc.ie
D. W. Sims N. Queiroz
Marine Biological Association of the United Kingdom,
The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK
e-mail: dws@MBA.ac.uk
123
Rev Fish Biol Fisheries (2010) 20:471–487
DOI 10.1007/s11160-009-9155-9
Author's personal copy
broad-scale surveys, together with molecular and
laboratory based techniques, are addressing the
connectivity and trophic role of these animals. These
emerging molecular and movement studies suggest
that local distinct populations may be prone to
depletion through bycatch in commercial fisheries.
Rising interest in ocean sunfish, highlighted by the
increase in recent publications, warrants a thorough
review of the biology and ecology of this species.
Here we review the taxonomy, morphology, geogra-
phy, diet, locomotion, vision, movements, foraging
ecology, reproduction and species interactions of M.
mola. We present a summary of current conservation
issues and suggest methods for addressing funda-
mental gaps in our knowledge.
Keywords Teleost Telemetry
Foraging ecology Locomotion Diet
Range Phylogeny
Introduction
One of the long-standing ironies within marine
science remains the paucity of knowledge of many
iconic and charismatic species. Gaps in our under-
standing may reflect a lack of commercial interest in
a particular species or an exclusion from the regional
and international conservation strategies that focus
research efforts upon certain target groups (Sims
et al. 2009a). These issues are compounded for many
large vertebrates by the inherent difficulties of
gathering data in remote locations or when animals
are present in seemingly low abundances (Nelson
et al. 1997; Doyle et al. 2008). A combination of
these factors has contributed to our poor understand-
ing of the world’s heaviest bony fish, the ocean
sunfish (Mola mola L), with outdated or loosely
documented facts persisting in the literature. For
example, with the possible exception of its notable
size (the heaviest recorded M. mola measured 2.7 m
in length and weighed 2.3 tonnes; Roach 2003;
Fig. 1) the most obvious physical characteristic of the
ocean sunfish is the degeneration of the vertebral
column resulting in the replacement of the caudal fin
by a broad, stiff lobe, the clavus (Latin: rudder;
Fraser-Brunner 1951). This bizarre morphology,
likened to a ‘swimming head’ (Thys 1994), led to
a perception of these animals as sluggish, inefficient
swimmers that is now ceding to one of an active
predator capable of protracted oceanic movements
independent of prevailing current regimes (Cartamil
and Lowe 2004; Sims et al. 2009a). Moreover, we
now know that the unusual ‘basking’ behaviour that
so often typifies M. mola is only a fragment of a
complex behavioural repertoire (Watanabe and Sato
2008; Houghton et al. 2009) and recent studies have
shown regular deep forays into the open-ocean (Sims
et al. 2009a; Hays et al. 2009). Taken together, these
findings signpost the need to consolidate our under-
standing of this species, especially considering recent
reports of substantial aggregations of ocean sunfish in
coastal waters and the extremely high level of
incidental bycatch in regional fisheries in Mediterra-
nean, Californian and South African waters (Silvani
et al. 1999; Cartamil and Lowe 2004; Petersen and
McDonell 2007). This review attempts to identify
unanswered fundamental research questions and
suggest techniques for addressing them.
Phylogeny and morphology of the Molidae
Ocean sunfish belong to the family Molidae within
the highly derived order Tetraodontiformes (e.g.
pufferfish, triggerfish, boxfish), although Molidae’s
location within Tetraodontiformes is subject to con-
troversy. It is agreed that Molidae is monophyletic
(Bass et al. 2005; Santini and Tyler 2004; Yamanoue
Fig. 1 An ocean sunfish caught off the coast of Kamogawa,
Japan in 1996 measuring 2.7 m in length and 2.3 tonnes in
weight. Photograph courtesy of Kamogawa Seaworld
472 Rev Fish Biol Fisheries (2010) 20:471–487
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et al. 2004, 2008) but the molids have been variously
placed as a sister group to Diodontidae (porcupine-
fish), Tetraodontidae (pufferfish) ? Diodontidae,
Ostraciidae (trunkfish) or Diodontidae ? Ostraciidae
(see Bass et al. 2005; Britz and Johnson 2005; Alfaro
et al. 2007). Recent molecular work has suggested a
basal split in Tetraodontiformes into Tetraodontoidei
(Triacanthidae ? Balistidae ? Monacanthidae ?
Tetraodontidae ? Diodontidae ? Molidae) and Tri-
acanthodoidei (Ostraciidae ? Triodontidae ? Tri-
acanthodidae) attributable to ecological radiation
into shallow (Tetraodontoidei) and deep (Triacantho-
doidei) water habitats (Yamanoue et al. 2008).
The position of M. mola within Molidae is less
contentious. In his taxonomic revision of Molidae,
Fraser-Brunner (1951) identified five species divided
into three genera: Ranzania, Masturus and Mola. The
number of recognised species subsequently dwindled
until each genus was considered monotypic (Santini
and Tyler 2003). Fraser-Brunner’s view that Mastu-
rus and Mola are more closely related to each other
than to Ranzania is in agreement with earlier workers
such as Bonaparte (1841) and Gill (1897) and has
received considerable corroboration, both morpho-
logically (Santini and Tyler 2002) and through
molecular studies (Alfaro et al. 2007; Bass et al.
2005; Yamanoue et al. 2004).
Recent investigations into M. mola mitochondrial
sequences have supplied compelling evidence that the
genus Mola contains two species: M. mola and the
resurrected southern sunfish, M. ramsayi (Giglioli
1883; Bass et al. 2005). Bass et al. (2005) took
samples (fin clips or muscle biopsies) from 13 animals
identified as M. mola
, four as M. lanceolatus, and a
single R. laevis specimen from multiple locations in
the Atlantic and Pacific Oceans. The sequences of the
mitochondrial DNA control region (D-loop) were
analysed as well as any relevant GenBank sequences.
The workers then assayed a subset of the individuals,
chosen to represent the major clades identified by the
D-loop analysis, for variation in the more slowly
evolving cytochrome b gene sequence. All individual
and group analyses showed high genetic divergence
among M. mola specimens indicating two clades; one
consisting of animals exclusively from the southern
hemisphere and the other containing individuals from
both hemispheres. This first group was suggested to be
the sister species, M. ramsayi, previously described by
Giglioli (1883). In addition, both Mola species appear
to be subdivided into Atlantic-Mediterranean and
Indo-Pacific Ocean clades. The estimated times of
divergence were of relatively recent origin
(0.05–0.32 mya for M. mola and 1.55–4.10 mya for
M. ramsayi) and so the isolating mechanism is
unclear, although these estimates are based upon only
two individuals and may be subject to further revision
(Bass et al. 2005).
Another report by Yoshita et al. (2009) analysed
the complete nucleotide sequences of D-loop in 101
sunfish collected from Japanese waters and published
sequences in EMBL/GenBank/DDBJ from other
studies. All fish were determined to belong to one
of two genetically isolated clades (Groups A and B)
that occurred sympatrically in Japanese waters.
Group A specimens were found only on the Pacific
coast of Southern Japan and other southern locations
(Okinawa Island, Taiwan and Australia). Group B, on
the other hand, contained specimens from various
locations around the Japanese coast as well as from
farther afield, including the Atlantic Ocean. When the
study was expanded to use the sequences from Bass
et al. (2005) all individuals identified as M. mola in
the earlier study nested within Group B. Those
identified as M. Ramsayi nested either within Group
A or a new group (Group C) which exclusively
contained animals collected in the southern hemi-
sphere. They concluded that the genus Mola con-
tained three species.
Interestingly, unlike Bass et al. (2005), who did
not record any concurrent physical data, Yoshita et al.
(2009) also recorded morphological differences
between Groups A and B. The 15 animals comprising
Group A were all large animals, with total lengths
(TL) ranging from 175 to 332 cm, whilst Group B
contained fish of much more varied sizes (TL range
29–277 cm) so morphometic analyses were limited to
animals [180 cm TL. Specimens from Group A
possessed significantly larger head bumps and deeper
bodies than Group B animals. Group A sunfish also
possessed a greater number of clavus fin rays
(mean ± SD = 15.5 ± 1.3, range = 14–17, N = 4)
than Group B (11.5 ± 1.1, 10–13, 10). This agrees
with Fraser-Brunner (1951) who stated that the clavus
had 16 fin rays in M. ramsayi and 12 in M. mola.In
addition, all larger (TL 193.7–277.0 cm) sunfish from
Group B showed a wavy edge to the clavus absent
from all animals in group A and smaller (TL 28.4–
155.3 cm) Group B fish.
Rev Fish Biol Fisheries (2010) 20:471–487 473
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Genetic and morphological evidence strongly
support the concept that Mola is not a monotypic
genus (Bass et al. 2005; Yoshita et al. 2009; Fig. 2).
More work is required, however, to clarify the
interspecific relationships (Yoshita et al. 2009)so
this review will concentrate on M. mola only,
although misidentifications are possible in some of
the studies cited.
Molids are distinguished from other tetraodonti-
forms by several distinct morphological characters,
including reduced/fused caudal elements, presence of
a clavus, absence of a swim bladder and a degenerate,
cartilaginous skeleton. The origin of the clavus has
been controversial since the earliest studies (see
Johnson and Britz 2005). Fraser-Brunner (1951)
argued strongly that the clavus consisted entirely of
fin rays and musculature derived from the anal and
dorsal fins and that the caudal fin was lost. Recently
this argument has received considerable support
(Johnson and Britz 2005; Nakae and Sasaki 2006).
The vertebral column is severely reduced, with no
ribs or pelvic fins (Cleland 1862) and the lateralis
muscles, unable to perform their primary function of
flexing the body, insert upon the deep muscles of the
anal and dorsal fins (Fraser-Brunner 1951). The result
is a rigid body capable of reduced lateral flexion but
with much more powerful dorsal and anal fins that
become the primary means of locomotion (see
Fig. 2a). Indeed, recent biotelemetry studies have
shown that despite this lack of caudal propulsion, M.
mola remains a powerful swimmer capable of highly
directional horizontal movements, deep-water dives
and even breaching (e.g. Cartamil and Lowe 2004;
Konow et al. 2006; Watanabe and Sato 2008; Sims
et al. 2009a; Hays et al. 2009). Thus the unusual
morphology of M. mola may represent the evolution-
ary constraint of an already highly derived, reef
dwelling tetraodontiform adopting a pelagic lifestyle.
Geographical range
Records from the Mediterranean (Silvani et al. 1999;
Dulc
ˇ
ic
´
et al. 2007), North Atlantic (Sims and Southall
2002; Houghton et al. 2006a), South Atlantic (Peter-
sen 2005), Gulf of Mexico (Fulling et al. 2007), East
Pacific (Cartamil and Lowe 2004) and West Pacific
(Sagara and Ozawa 2002; Liu et al. 2009
) indicate a
wide-ranging distribution for M. mola. Figure 3
illustrates ocean sunfish sightings, redrawn from
OBIS data with additional sightings from literature
Fig. 2 The phylogeny of Molidae with illustrations of each member species. Each photograph is a lateral view. Photographs courtesy
of a Mike Johnson, b Tao-Jen Kuo and c Wolfgang Sterrer. Redrawn with modification from Santini and Tyler (2002)
474 Rev Fish Biol Fisheries (2010) 20:471–487
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and personal communications. It should be noted,
however, that such maps based on sighting and catch
data, whilst a best estimate, suffer from various
reporting biases. First, the map is constructed from
recorded occurrences and may include aberrant
observations of individuals outside of a species’
typical range (as discussed in McMahon and Hays
2006). Second, a lack of records may not reflect a
lack of occurrence of M. mola in particular areas but
rather a lack of rigorous sampling. More accurate
estimates of range are obtained for many fish species
from commercial catch data. Such anecdotal data are
not readily available for ocean sunfish, however,
because there are few dedicated fisheries.
Diet
Ocean sunfish are often referred to as obligate or
primary feeders on gelatinous zooplankton (Fraser-
Brunner 1951; MacGinitie and MacGinitie 1968;
Hooper et al. 1973; Bass et al. 2005). The importance
of this group of animals in the diet of M. mola is far
from clear, however, and most fish that eat gelatinous
prey are known to have broad diets (Purcell and Arai
2001). There are several eyewitness accounts of
M. mola surface predation on gelatinous zooplankton
(MacGinitie and MacGinitie 1968;Thys1994) but
caution is necessary when interpreting near-surface
feeding events, especially in a species now known to
show deep diving behaviour (Hays et al. 2009; Sims
et al. 2009a; see Foraging ecology’ section) and the
relatively small gape of M. mola does not appear well
adapted for capturing and ingesting large scyphozoan
jellyfish (cf. the large gape of the leatherback turtle).
The diet of M. mola has not been investigated by
any dedicated study to our knowledge and the
literature on this subject is sparse. To illustrate this,
a recent review that included a list of fish known to
predate pelagic coelenterates based upon stomach
contents data did not include M. mola, one of the few
fish frequently cited as an obligate gelativore,
because no quantitative data were available for this
species (Arai 2005). Field guides and reference books
mention varied stomach contents for ocean sunfish
including algae, crustaceans, ophiuroids, molluscs,
hydroids and fish (Norman and Fraser 1949; Clemens
Fig. 3 World map showing locations of Mola mola sightings
(red squares) redrawn from OBIS data (www.fishbase.org,
accessed 18th February 2009) with additional sightings from
literature and personal communications to www.oceansunfish.
org. Commercial fisheries targeting M. mola are highlighted in
gold, those known to experience considerable levels of M. mola
bycatch are highlighted in blue. Inset shows M. mola bycatch
on a horse mackerel trawler off South Africa (photograph
courtesy of Birdlife International).
1
Rand Rasmussen, personal
communication, Southwest Fisheries Science Center (based
upon observed drift net fishery catch for 1990–1998).
2
Petersen
(2005).
3
Petersen and McDonell (2007).
4
Crude estimate from
Tudela et al. (2005).
5
Estimate from Liu et al. (2009).
6
Sagara
and Ozawa (2002)
Rev Fish Biol Fisheries (2010) 20:471–487 475
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and Wilby 1961; Hart 1973), although how these
observations were obtained is unknown. Schmidt
(1921) stated that the ocean sunfish was known to
feed on small forms of pelagic life, including large
numbers of leptocephalus eel larvae. Fraser-Brunner
(1951) made reference to a 60 cm ling (Molva
macrophthalma) found in an individual’s stomach,
although whether this represented active predation or
opportunistic scavenging is unknown. The general
impression of an omnivore is supported by the
capture of ocean sunfish on pelagic long-line hooks
(Petersen 2005) baited with either squid, or a
combination of squid and fish (pilchard, mackerel).
Alternatively, it is possible that M. mola is attracted
to the highly visible light sticks used in conjunction
with the bait (Petersen 2005) which may mimic the
appearance of bioluminescent gelatinous zooplank-
ton, or they may simply be foul-hooked rather than
actively taking the bait.
Hooper et al. (1973) used thin layer chromatogra-
phy and gas liquid chromatography to analyse the
lipids extracted from various tissues (connective
tissues, white muscle, dark muscle, liver and
intestinal contents) from four ocean sunfish. They
identified two unusual component fatty acids: trans-
6-hexadecenoic acid and 7-methyl-7-hexadecenoic
acid. Trans-6-hexadecenoic acid was known to
comprise 2% of the total fatty acids in the moon
jellyfish Aurelia aurita (Hooper and Ackman 1972).
In addition, both of these fatty acids had previously
been isolated from tissues of the leatherback turtle,
Dermochelys coriacea (Hooper and Ackman 1970),
an animal known to be an obligate gelatinous
zooplanktivore (Houghton et al. 2006c). Thus the
presence of these fatty acids in both leatherback
turtles and ocean sunfish was taken as an indicator of
a diet containing jellyfish (Ackman 1997). The role of
jellyfish in the diet of M. mola remains unclear,
however, since the study that identified trans-6-
hexadecenoic acid in D. coriacea also identified it in
two other turtle species: the loggerhead Caretta
caretta and Kemp’s Ridley Lepidochelys kempii
(Hooper and Ackman 1970), both of which are
omnivorous, not obligate feeders on jellyfish (Bjorn-
dal 1997). A proposed link between these fatty acids
and predation on jellyfish becomes less certain when
subsequent studies also identified both fatty acids in
the liver oil of the reef-dwelling Atlantic spadefish
Chaetodipterus faber (Pearce and Stillway 1976) and
7-methyl-7-hexadecenoic acid as a constituent of
sperm whale oil (Pascal and Ackman 1975).
Locomotion
Cartamil and Lowe (2004) showed conclusively that
M. mola is an active swimmer capable of highly
directional movements. Eight individuals equipped
with acoustic transmitters off southern California
were shown to move greater distances (mean
26.8 km day
-1
) than they could possibly accomplish
through simply planktonic passive drifting. It was
also demonstrated that these horizontal movements
were highly directional and independent of the
prevailing current regime.
Further insights into M. mola locomotion were
achieved when Watanabe and Sato (2008) used high-
resolution multi-channel data loggers to record the
movements of three individuals in Otsuchi Bay,
Japan under natural conditions. The fish undertook
frequent vertical movements and swam at continuous
speeds of 0.4–0.7 m s
-1
(similar to records of other
large fishes such as salmon, marlins and pelagic
sharks). It was also confirmed that ocean sunfish
stroke their dorsal and anal fins synchronously
(dominant frequency 0.3–0.6 Hz) to generate a lift-
based thrust that was likened to the symmetrical
flipper beats of penguins (Watanabe and Sato 2008).
Essentially, M. mola uses its anal and dorsal fins as a
pair of wings. It is worth noting that this is the only
animal known to use two fins for this purpose that are
not originally bilaterally symmetrical (Watanabe and
Sato 2008). Watanabe and Sato (2008) argued that
the symmetrical pair of dorsal and anal fins are well
suited for cruising because they are mechanically
more efficient (Walker and Westneat 2000) and give
more thrust per stroke at high speed (Vogel 1994)
compared to the drag-based swimming of other
Tetraodontiform fishes (e.g. pufferfish, Gordon et al.
1996; box fish, Hove et al. 2001 and burrfish, Arreola
and Westneat 1996).
The same study also demonstrated that the aspect
ratio (length to mean width) of the anal and dorsal
fins dramatically decreases with age, with an
expected concomitant decrease in swimming effi-
ciency. Watanabe and Sato (2008) hypothesised that
mechanical strength was of more importance than
efficiency in larger fish. This decrease in expected
476 Rev Fish Biol Fisheries (2010) 20:471–487
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efficiency with increasing size may explain why a
similar study observed that the allometry of fin-beat
frequency is reversed in ocean sunfish (Houghton
et al. 2009). In other words, large ocean sunfish
have a faster fin beat frequency than smaller sunfish.
This is in contrast to most fish with a lateral
propulsive system where larger individuals show
slower fin-beat frequencies than smaller ones (Bain-
bridge 1958; Gleiss et al. 2009). This later study by
Houghton et al. (2009) demonstrated the use of a
new low-impact harness with an automated release
mechanism to attach multi-channel data loggers
(‘daily diaries’; see Wilson et al. 2008; Shepard
et al. 2008 for full descriptions) to three ocean
sunfish off the coast of County Kerry, Ireland. It
was discovered that ocean sunfish roll far more
extensively than would be expected for a teleost
during typical locomotion and body roll was intrin-
sically linked with vertical velocity, with highest
roll angles recorded at relatively slow vertical
velocities (Houghton et al. 2009). M. mola also
showed an unusual decrease in the amplitude of
body sway with increasing frequency (mean fre-
quency and amplitude of body sway were taken as a
proxy for timing and magnitude of fins strokes) as
higher frequencies would be expected to have larger
amplitudes (Gleiss et al. 2009).
The study by Watanabe and Sato (2008) also
examined the internal and external morphologies of
49 ocean sunfish (body mass ranging from 2 to
959 kg) collected from local fish markets. It was
demonstrated that despite the missing swim-bladder,
ocean sunfish are neutrally buoyant (mean body
density 1,027 ± 4kgm
-3
, N = 20) in seawater
(density ca. 1,026 kg m
-3
) and that a thick layer of
low-density, subcutaneous, gelatinous tissue plays a
major role providing this buoyancy (Watanabe and
Sato 2008). The degenerate, cartilaginous skeleton of
M. mola (Cleland 1862) also likely contributes to
buoyancy (Davenport and Kjørsvik 1986). Impor-
tantly, the gelatinous tissue is incompressible,
enabling rapid depth changes without the changes
in buoyancy that would be experienced by fish
possessing swim-bladders (Yancey et al. 1989). This
combination of a lift-based swimming mode and
neutral buoyancy from incompressible, gelatinous
tissue appears to allow M. mola to move over
considerable distances, despite its unusual
morphology.
Vision
A recent study by Kino et al. (2009) investigated the
eyes of three immature M. mola from Japanese
waters. They found a region of high retinal ganglion
cell density that implied a main visual axis directed
towards the lower frontal portion of the visual field
(10–20° below horizontal; Kino et al. 2009). High-
density areas are indicative of a well-developed
visual system and also require the ability to move the
eye to direct this region of more acute vision to
objects of interest (Fritsches et al. 2003). Such eye
movements, especially in the ventral plane, were
documented in live specimens by Kino et al. (2009).
Visual acuity, calculated from peak ganglion cell
densities, was between 3.51 and 4.33 cycles per
degree, comparable to adult sharks (2.8–3.7 cycles
per degree; Kino et al. 2009) but much lower than
adult blue marlin, Makaira nigricans (8.5 cycles per
degree; Fritsches et al. 2003). It should be noted,
however, that these were very small ocean sunfish
and visual acuity typically increases as fish grow
(Fritsches et al. 2003). The study also suggested that
this angled visual axis indicated that M. mola
detected its prey items mainly whilst descending or
foraging near the sea bottom. The recent work
showing that M. mola rolls extensively when swim-
ming (i.e. it swims at an angle; Houghton et al. 2009)
and its observed ability to direct its vision weaken
this view.
Published eyeball size data for M. mola are all
from juvenile animals less than 1 m TL but ocean
sunfish appear to possess large eyes; a 97 cm TL
specimen possessed an eye [38 mm in diameter
(Cleland 1862) and the three fish described by Kino
et al. (2009; TL 42.3–45.5 cm, 3.3–3.5 kg) had
eyeballs with diameters ranging from 33.3 to
35.4 mm. Eye size usually reflects the importance
of vision in animals (Walls 1942) and many deep
ranging species simply possess large eyes able to
accommodate more retinal photoreceptive cells and
acquire more light per solid angle of image space
(Motani et al. 1999). For example, billfish are known
to possess very large eyes (Fritsches et al. 2003) with
brain and eye heaters for maintenance of function in
deep, cold waters (Carey
1982). Whilst eyeball size is
a crude estimate of visual acuity, it is worth noting
that ocean sunfish have been tracked targeting depths
of greater than 200 m for protracted periods (Hays
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et al. 2009) and undertake deep dives to nearly 500 m
depth (Sims et al. 2009a).
Horizontal movements
Sightings of M. mola at temperate latitudes are more
common in summer months (Sims and Southall 2002;
Houghton et al. 2006a), a phenomenon seen with
other large marine vertebrates, such as the leather-
back turtle (Brongersma 1972; Houghton et al.
2006a) and the basking shark, Cetorhinus maximus
(Sims et al. 2003). Leatherback turtles migrate
seasonally into higher latitudes once water tempera-
tures are warm enough (McMahon and Hays 2006),
presumably to take advantage of the greater plankton
productivity in spring and summer at these latitudes
(Parsons et al. 1984). Conversely, basking sharks, in
the North East Atlantic at least, remain in temperate
waters during the winter months with increased
sightings during the summer simply reflecting
increased visibility of the species at this time of year
(Sims et al. 2003).
Recent tracking studies have provided valuable
insights into the seasonal horizontal movements of
ocean sunfish. Sims et al. (2009a) attached satellite-
linked archival transmitters to ocean sunfish in the
North East Atlantic in 2007; two fish in the waters of
southern Portugal in February and a third off south-
west Ireland in August. The animals were tracked for
between 42 and 94 days. M. mola tagged in the
waters off southern Portugal in February moved in
northerly and westerly directions whilst the animal
equipped in Irish waters in August moved southerly
to the Bay of Biscay, North Spain, 959 km south of
the tagging location (Fig. 4). This pattern of
increased latitude in late winter to summer and
decreasing latitude from late summer to autumn is
consistent with a model of northerly travel at the end
of winter and movement south at the end of summer.
Sims et al. (2009b) equipped three ocean sunfish
with fast-acquisition Global Positioning System
(Fastloc GPS) tags off southern Portugal in the first
long-term GPS tracking study of a large pelagic fish.
These tags produce very low spatial errors for
geolocations (between ca. 26–64 m) and are thus
able to resolve behaviour on a much finer scale. Two
animals were tagged in May 2008 for 5 and 15 days
whilst a third, larger (1.0 m TL) animal was tracked
for 92 days between early November 2008 and early
February 2008. The larger animal travelled an
estimated 1,819 km, moving at a mean speed of
19.8 km day
-1
, proceeding south-westerly and then
south-easterly into the warmer waters of the gulf of
Cadiz as winter developed (Sims et al. 2009b). The
rates of travel were comparable to those of pelagic
sharks and GPS track integration showed that this fish
often swam into and across prevailing currents and
showed intermittent periods of reduced movement in
localised areas and faster, directional movements. It
was hypothesised that these ‘stopovers’ in localised
areas represented encounters with patches of pre-
ferred prey (Sims et al. 2009b).
Fig. 4 Movements of three ocean sunfish, Mola mola,
equipped with satellite tags in the North-East Atlantic in
2007. The first two fish (filled circle and open circle) were
tagged off southern Portugal at the end of February and
travelled in a predominantly northerly direction whilst the third
animal (filled triangle) was tagged off west Ireland in August
2007 and travelled southward. These movements are consistent
with the seasonal migration of ocean sunfish to higher latitudes
and their subsequent return south. Start and end points are the
tagging site and the pop-off location of the tag determined by
multiple good quality Argos locations. Intermediate locations
are based on light-based geolocation and are restricted to
period prior to inferred detachment of the tags (see Sims et al.
2009a for details). From Sims et al. (2009a)
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The classic migratory paradigm illustrated by Sims
et al. (2009a, b) may not be geographically uniform,
however. Hays et al. (2009) attached satellite-linked
archival transmitters to four M. mola in the waters of
South Africa, obtaining data spanning January to
August 2004 and March to August 2006. Interest-
ingly, the tracks of all these ocean sunfish showed
prolonged residence off South Africa from the austral
summer to winter. Presumably, the waters off South
Africa must represent a suitable year-round habitat
for these fish (Hays et al. 2009), although it would be
useful to determine whether migratory behaviour
differs between M. mola and the putative southern
hemisphere species, M. ramsayi (see Phylogeny and
morphology of the Molidae’).
Foraging ecology
Ocean sunfish occupy a broad range of depths and
move extensively throughout the water column (Sims
et al. 2009a; Hays et al. 2009). Cartamil and Lowe
(2004) observed a diel pattern in depth utilisation,
with fish residing in the warmer mixed layer above or
within the thermocline at night and repeatedly diving
beneath the thermocline into cooler water during the
day (Californian waters, July–September 2001). Sims
et al. (2009a) tracked sunfish in the North-East
Atlantic in late winter to early spring and late summer
to early autumn and observed all three study animals
occupying significantly (P \ 0.003) greater depths
during the day (mean diurnal depths of 250.7, 112.1
and 50.7 m versus mean nocturnal depths of 104.7,
63.18 and 29.9 m for each fish, respectively). This
behaviour matches the strategy of normal diel vertical
migration (DVM) and has been attributed to a
strategy of near continual feeding on vertically
migrating prey (Sims et al. 2009a; Hays et al. 2009)
although concurrent data on the distribution of
potential prey have not yet been analysed. Hays
et al. (2009) again saw DVM with sunfish in South
African waters, although this was not an invariant
behaviour with individuals sometimes not displaying
vertical movements.
Despite showing normal DVM when it occurs,
ocean sunfish repeatedly return to the surface during
the day and do not simply remain at depth (Cartamil
and Lowe 2004; Hays et al. 2009). Cartamil and
Lowe (2004) observed that daytime periods were
characterised by short (mean dive duration
11.2 ± 9.5 min) repeated dives below the thermo-
cline (40–150 m). Hays et al. (2009) also noted that
although individual sunfish showed a range of
preferred depth that altered over time, they were still
moving up and down the water column and regularly
returning to close to the surface. Similar behaviour
has been observed in a variety of fish species such as
swordfish Xiphias gladius (Carey and Robison 1981),
blue shark Prionace glauca (Carey and Scharold
1990) and bluefin ((Thunnus thynnus) and bigeye
(T. obesus) tuna (Dagorn et al. 2000
; Kitagawa et al.
2007). Although its exact function has yet to be
resolved, Cartamil and Lowe (2004) proposed that
periodic returns to the surface in M. mola may imply
some form of behavioural thermo-regulation i.e.
prolonged time in cold waters necessitates time spent
at the surface to re-warm. They recorded M. mola
diving into water with a temperature of 6.8°C, well
below the thermal range recorded for ocean sunfish in
the North East Atlantic of 10–19°C([99% of time;
Sims et al. 2009a) and identified a significant
relationship between time spent in the cold water
and the ensuing post-dive surface period. This may
Fig. 5 The ocean sunfish, Mola mola, showing characteristic
basking behaviour. Photograph courtesy of Mike Johnson
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explain the frequent observations of ocean sunfish
observed ‘basking or swimming on their sides at the
surface during the day (Cartamil and Lowe 2004;
Fig. 5). Conversely, Sims et al. (2009a) argued that
extensive vertical movements may simply reflect a
crepuscular searching strategy when prey are
descending or ascending, with sunfish attempting to
locate maximum prey abundances at this time. It has
been also been suggested more generally that large
amplitude dives or ascents of large pelagic fish and
other vertebrates during prey tracking may represent
prey searching behaviour (Shepard et al. 2006)
characterised by extensive movements to new loca-
tions (Sims et al. 2008).
Hays et al. (2009) subsequently proposed that
individual ocean sunfish respond to patchily distrib-
uted plankton prey with differing levels of DVM.
This pattern may mirror the behavioural plasticity
noted in leatherback turtles, whereby individuals are
thought to shift predation from seasonally abundant
surface medusae in summer months towards verti-
cally migrating deep-water gelatinous species during
the winter (Houghton et al. 2008).
Reproductive biology
Very few studies have been conducted on the
reproductive biology of ocean sunfish. The spawning
period of M. mola, in Japanese waters at least, is
estimated as between August and October and the
same study also identified asynchronous egg devel-
opment, suggesting they are multiple spawners
(Nakatsubo et al. 2007). Average sea-surface temper-
ature (SST) around this study region of Japan between
August and October 1982–2006 was 25.52 ± 0.58°C
(Mean ± 1 SD, using NOAA NCDC ERSST version
2, 32–36E, 136–142N, http://www.cdc.noaa.gov/).
Reproductive seasonality for M. mola in other loca-
tions is less well documented. Although the central
gyres of the North and South Atlantic, North and
South Pacific and Indian Oceans have been suggested
as spawning areas (Bass et al. 2005) evidence in
support of this concept is sparse. The means by which
adult fish aggregate for spawning also remain
unclear, as does the link between open ocean habitats
and the large groupings of M. mola observed in many
coastal areas worldwide (Silvani et al. 1999; Cartamil
and Lowe 2004). A recent study did not identify any
chromosomal or morphometric sexual dimorphism in
M. mola other than in TL (Nakatsubo 2008). Females
were larger than males, with all animals larger than
250 cm TL being female (Nakatsubo 2008).
The lifespan or reproductive age of M. mola under
natural conditions is unknown, although captive
animals have been maintained for more than 8 years
(Nakatsubo et al. 2007). A growth curve derived from
repeated measurements of captive specimens esti-
mated an individual with a total length of 3 m would
be about 20 years old (Nakatsubo 2008). Neverthe-
less, caution is prudent in comparisons between wild
animals and those in captivity. Recent work used
growth band pairs in vertebrae to age the closely
related sharptail mola M. lanceolatus caught in the
eastern Taiwan fishery (Liu et al.
2009). The age
estimates ranged between [2 and [23 years for
females and [1 and [16 years for males (Liu et al.
2009). A similar approach may be more suitable for
ageing M. mola than using their anatomically prim-
itive otoliths (Gauldie 1990).
M. mola is famously the most fecund of all
vertebrates (Carwardine 1995) with a 137 cm female
containing an estimated 300 million eggs (Schmidt
1921). By necessity, these eggs are very small (mean
diameter 0.13 cm; Gudger 1936) and so M. mola
growth is staggering. For the 0.25 cm larva to grow to
a 3 m adult requires an increase in mass of 60 million
times (Gudger 1936). An individual in captivity at the
Monterey Bay Aquarium gained 373 kg in just
15 months (454 days, 0.82 kg day
-1
; Powell 2001)
although more typical captive growth rates have been
found to be between 0.02 and 0.49 kg day
-1
in
weight (M. Howard, personal communications. Mon-
terey Bay aquarium, 800 Cannery Row, Monterey
CA 93490; T. Nakatsubo, personal communications.
International Marine Biological Institute, Kamogawa
Sea World, Kamogawa, Chiba 296-0041, Japan) and
on average 0.1 cm day
-1
TL (Nakatsubo and Hirose
2007). These rates are also highly dependent on the
starting TL of the captive animal (Nakatsubo and
Hirose 2007).
Predators and parasites
There are several documented accounts of predation
on M. mola by sealions (T. Thys, unpublished
observations), orcas (Gladstone 1988) and large
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sharks (Fergusson et al. 2000). For example, ocean
sunfish have repeatedly been found in the stomachs of
blue sharks (N. Queiroz, unpublished observation;
Fig. 6) and a white shark, Carcharodon carcharias
measuring over 5 m (TL) that was harpooned near the
Italian port of Messina in Sicily was found to include
the fresh remains of a large adult M. mola (Fergusson
et al. 2000). The fish had been bitten into three
sections that when combined indicated a TL of
around 2 m (Fergusson et al. 2000). It should be
noted, however, that the accuracy of this account by
Fergusson et al. (2000) has been contested (Celona
et al. 2001). Californian sea lions, Zalophus califor-
nianus, have been observed biting the fins off ocean
sunfish during the autumn months in Monterey,
California and beating the dismembered bodies against
the sea surface, presumably to tear through M. mola’s
tough skin (T. Thys, unpublished observations).
Another possible explanation for the basking
behaviour of ocean sunfish may relate to the
substantial infestations of parasites observed in some
individuals (Fraser-Brunner 1951; Konow et al.
2006). A report by the National Marine Fisheries
Service (National Oceanic and Atmospheric Admin-
istration) lists 54 species of parasitic organisms found
on M. mola (Love and Moser 1983). More than half
of these species are members of Platyhelminthes,
with cestodes (7 species), monogeans (8 species) and
especially digeneans (16 species) found in the
intestines, liver, gills, muscle connective tissue and
skin (Love and Moser 1983). Crustaceans, especially
copepods (14 species) but also isopods, cirripedes and
branchiurans have been found on the skin, gills,
buccal cavity and operculum (Love and Moser 1983).
Other parasites are nematodes, protozoans and mem-
bers of Acanthodephala (Love and Moser 1983).
There is growing evidence to support the long-held
view that M. mola may reside at the surface to solicit
cleaning by seabirds or fish (Thys 1994; Konow, et al.
2006). This is supported by the association of
M. mola with kelp beds that has been linked with
parasite elimination through the action of cleaner fish
(Cartamil and Lowe 2004; Hixon 1979). An Indone-
sian study observed ocean sunfish being cleaned by
five different species of fish and also witnessed
breaching events that were suggested as a possible
means of decreasing parasite load (Konow et al.
2006).
The heavy parasite loads observed in M. mola may
support the hypothesis that gelatinous zooplankton
are a preferred prey group as such animals are known
to be intermediate hosts for trematode, cestode and
nematode larvae (Purcell and Arai 2001). There is
also evidence that hydromedusae and ctenophores
show greater incidence of trematode infection than
other zooplankton (Purcell and Arai 2001). Many of
these parasites are larval forms of species where the
definitive hosts are fish (Arai 2005). Unfortunately,
we know very little about interactions between fish
and gelatinous zooplankton (Purcell and Arai 2001)
but a better understanding of ocean sunfish parasite
associations will also provide insights into potential
prey types. Molecular analyses of the parasites may
also reveal connectivity between M. mola populations
and overall movements.
Conservation issues: Mola mola in a changing
ocean environment
It has been contended that 90% of all large predatory
fish have disappeared from the oceans (Myers and
Worm 2003) since the arrival of industrialised fishing
and there is substantial evidence that sharks, one of
the known predators of M. mola, are declining
worldwide at an alarming rate (Baum et al. 2003;
Myers et al. 2007; Heithaus et al. 2008). Whilst
trophic cascades are hard to predict, any reduction in
Fig. 6 Mola mola remains found in the stomach of a sub-adult
blue shark Prionace glauca (168 cm TL) captured on a
longline near the Azores in 2006. Photograph from Nuno
Queiroz
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top–down control may have significant effects on
M. mola populations globally (Heithaus et al. 2008).
Insightful data have already emerged from the North
West Atlantic where the decline of 11 species of large
sharks has resulted in an increase in 12 out of 14
investigated prey species (Myers et al. 2007),
although it should be noted that many of these
predatory species have not been associated with
M. mola. In addition, there is evidence that over-
fishing can be correlated with a concomitant increase
in gelatinous zooplankton numbers (Mills 2001;
Lynam et al. 2006) as the removal of large numbers
of fishes makes food resources available to jellyfish
(Mills 2001). The increased water temperatures and
water column stratification associated with climate
warming may also favour outbreaks of gelatinous
fauna, although attributing such instances to climate
change alone is highly problematic (Richardson et al.
2009). Nonetheless, a merger of reduced top–down
control and increased prey availability in certain
locations may provide conditions favourable for
M. mola population growth.
Then again, any such amelioration of conditions
for ocean sunfish may be offset by anthropogenic
mortality. M. mola is not a commercially important
fish (Fulling et al. 2007; Silvani et al. 1999) although
there is some market for it in Japan (Sagara and
Ozawa 2002; Watanabe and Sato 2008) and Taiwan
(20.8–49.4 tonnes per annum, Liu et al. 2009). Ocean
sunfish populations may still be vulnerable to fishing
activity nonetheless because of the high levels of
bycatch observed in many fisheries (Silvani et al.
1999; Cartamil and Lowe 2004; Fulling et al. 2007,
Petersen and McDonell 2007). For example, the
South African longline fishery for tuna and swordfish
was estimated to have annually caught between 0.08
and 0.29 sunfish for every 1,000 hooks set (1.6–2.7
million hooks per year) in the 4 years between 2000
and 2003 (Petersen 2005). Furthermore, M. mola was
by far the most common bycatch species in the Cape
horse mackerel (Trachurus trachurus capensis) mid-
water trawl fishery in the same region (Petersen and
McDonell 2007; see Fig. 3) representing 51% of the
total bycatch between 2002 and 2005 (Petersen and
McDonell 2007). This fishery caught predominantly
large individuals (172 ± 1.66 cm, mean ± SD,
N = 581) and observed a significant decline in ocean
sunfish catch rates during the study period, with other
bycatch species remaining comparatively stable
(Petersen and McDonell 2007). Similarly, M. mola
comprised between 70 and 93% of the total fish catch
in Spanish drift gillnet fisheries within the Mediter-
ranean between 1992 and 1994 (Silvani et al. 1999)
and bycatch estimates from the Californian swordfish
fishery suggest ocean sunfish make up 29% of all
bycatch; far outnumbering the target species (Carta-
mil and Lowe 2004). Whilst the Spanish fishery was
closed in 1994 (Silvani et al. 1999; Tudela et al.
2005), an illegal Moroccan driftnet fleet continued to
operate until the Moroccan government passed a law
in 2007 intended to phase out the use of driftnets. A
crude calculation using values from a recent study of
the Moroccan fleet (Tudela et al. 2005) suggests an
annual bycatch of 36,450 ocean sunfish. Whilst the
majority of M. mola are returned to the water alive
(Silvani et al. 1999), they often show varying levels
of trauma (Cartamil and Lowe 2004) and post-catch
survival data are lacking. If M. mola exist as discrete
populations, even within ocean basins, as has been
proposed (Bass et al. 2005) then these isolated
populations may be more prone to local depletion,
or even extinction, than the previously envisioned
pan-global population. The broader ecological impli-
cation of removing such large numbers of predatory
fish with a poorly resolved diet is even less clear.
Future research perspectives
In order to make absolute comment on possible
effects of climate change in this species, it is essential
that we gather more robust data on seasonal abun-
dances in specific locations. One suggestion is to
incorporate ocean sunfish into existing surveys
already looking for marine megafauna (Houghton
et al. 2006a) or gelatinous zooplankton (Houghton
et al. 2006b). The advent of chemical and molecular
techniques is also helping unravel long-distance
movements and ontogenetic habitat shifts in fish
populations and may prove useful for molid studies.
For example, the use of stable isotopes has answered
key questions about the relationships between Atlan-
tic bluefin tuna populations (Rooker et al. 2008)
whereby the otoliths in yearlings from regional
nurseries were used as natural tags to assess natal
homing and mixing. Significant trans-Atlantic move-
ment (East to West) was detected, with individuals of
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Mediterranean origin mixing with the western pop-
ulation in the U.S. Atlantic (Rooker et al. 2008).
Additionally, genetic markers have been used to
unravel protracted movements in fish such as the
coastal migration of juvenile Coho (Oncorhynchus
kisutch) and Chinook Salmon (O. tshawytscha;
Trudel et al. 2004). The combination of these
techniques with additional satellite tracking efforts
should further our knowledge of sunfish population
ecology, although structures such as fin rays (Arai
et al. 2002) may be substituted for otoliths for reasons
stated previously.
It is clear that more work is required to resolve the
fundamental question of diet. Future studies are
warranted that complement analytical techniques
with studies simultaneously tracking M. mola and
gelatinous zooplankton in the field (Hays et al. 2008).
Stomach content investigations using appropriate
fixation for gelatinous animals (Arai 2005) may yield
valuable data and putative food groups for stable
isotope and fatty acids analyses. The use of stable
isotope ratios will prove more useful in identifying
predator trophic levels rather than the species com-
position of a diet (Iverson et al. 2004) but may help to
pinpoint the relative importance of gelatinous zoo-
plankton in M. mola diet. Although not directly
relevant to studies of gelatinous zooplankton, such a
combination of stable isotope analysis and stomach
contents has recently been used to identify a marked
ontogenetic shift in prey types amongst yellowfin
tuna, Thunnus albacares (Graham et al. 2007).
Alternatively, techniques such as quantitative fatty
acid signature analysis (QFASA; see Iverson et al.
2004) can provide more detailed information on
particular prey items and their relative importance.
More detailed work on the overall visual acuity of
ocean sunfish at depth may shed further light on their
basic foraging strategies. Recent detailed retinal
studies of blue marlin have shown that their eyes
are specifically adapted to cope with low light levels,
such as those experienced when diving (Fritsches
et al. 2003). Conversely, similar studies of leather-
back turtles revealed that they have small eyes in
comparison with their biomass that are not well
adapted to dim light (Brudenall et al. 2008), even
though this species is known to engage in deep diving
behaviour (Houghton et al. 2008). It was concluded
that marlin are able to hunt visually at depths greater
than 200 m (Fritsches et al. 2003) whilst leatherbacks
predominantly feed at the surface, or during shallow
dives below 150 m (Brudenall et al. 2008).
Further deployments of multi-channel data-loggers
will provide greater understanding of specific behav-
iours such as basking. Further knowledge of the
thermal conditions and depths sunfish experience
prior to this behaviour may prove illuminating. In
addition, no studies so far have tracked larger animals
([2 m TL) which may display markedly different
behaviour to smaller fish, which appear to be coastal
in nature. Finally, the likelihood of sympatric sister
species to M. mola, especially in the southern
hemisphere, needs to be acknowledged. Future stud-
ies will benefit greatly from careful identification in
the field, the recording of morphometric data for each
individual used, including detailed descriptions or
photographs of the clavus and the acquisition of
blood or tissue samples.
In conclusion, a number of key foci are emerging
for future research, namely deep-diving behaviour
and long-distance migrations, population structure,
foraging ecology and the effects of incidental
bycatch. The technology and techniques required to
address these questions are now largely in place as
described above, allowing us to move away from an
incidental, slow accumulation of data towards col-
laborative, international efforts targeted to understand
the ecological importance of this charismatic, yet
poorly understood species.
Acknowledgments We are indebted to Francesco Santini for
his comments on molid phylogeny and to Toshiyuki Nakatsubo
and Rand Rasmussen for their kind contribution of fisheries and
bycatch data. We also gratefully acknowledge Mike Johnson,
Wolfgang Sterrer, Tao-Jen Kuo, Kamogawa Seaworld and
Birdlife International for the photographs used in this review.
We would like to thank an anonymous reviewer for their
constructive advice and suggestions. ECP was supported by the
Welsh Institute for Sustainable Environments (WISE) Network.
GCH, TKD and VJH were supported by the EcoJel project
funded by INTERREG IVA, a component of the European
Regional Development Fund. TMT was supported by the Adopt
a Sunfish Project and a grant from the National Geographic
Society’s Committee for Research and Exploration #7369-02.
NQ was funded by a Fundac¸a
˜
o para a Cie
ˆ
ncia e a Tecnologia
(FCT) Ph.D. grant SFRH/BD/21354/2005. DWS was supported
by the UK Natural Environment Research Council Oceans 2025
Strategic Research Programme and an MBA Senior Research
Fellowship.
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... The mola or ocean sunfish (Mola mola) belongs to the Order Tetraodontiformes, fishes not known for their pharyngeal jaws (Berkovitz & Shellis, 2017). Ocean sunfish are renowned for their massive size and bizarre appearance, weighing up to 2000 kg, measuring up to three meters in length, all while being a laterally flattened disk shape with no true caudal fin and a primarily cartilaginous skeleton (Davenport et al., 2018;Pope et al., 2010;Sawai et al., 2018;Watanabe & Sato, 2008) ( Figure 1a). The mola oral jaw is a simplified shortened beak, similar to that of other tetraodontiform fishes, with the premaxilla and the dentary fused and the teeth four slab-like plates (Berkovitz & Shellis, 2017;Matsuura, 2015;Porgasi & Andrews, 2001). ...
... obs.; Supplement Videos S1 and S2). Whereas young mola share a similar cosmopolitan benthic diet with their tetraodontiform relatives (Frazer et al., 1991;Koide & Sakai, 2021;Matsuura, 2015), adult mola gradually shift to a highly specialized diet over ontogeny, becoming increasingly focused on soft prey (jellyfish and gelatinous zooplankton) (Nakamura & Sato, 2014;Pope et al., 2010;Sousa et al., 2016). ...
... The mass of skeletal tissue into which teeth are mounted is highly trabeculated, with mineralized tissue only loosely framing tooth bases (Figure 3). Whether the three tooth rows in each half-jaw represent individual gill arch segments is unclear: the pharyngeal jaws and associated musculature are virtually ignored in the limited mola anatomical literature (Chanet et al., 2012;Davenport et al., 2018;Gregory & Raven, 1934;Pope et al., 2010) and mounted museum skeletal specimens tend to omit gill arch elements entirely, perhaps due to the poor preservation of the mola's largely cartilaginous skeleton (Porgasi & Andrews, 2001). Ontogenetic investigations are necessary for understanding the development of mola pharyngeal jaw elements and their homology with other teleost pharyngeal jaws. ...
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Full-text available
Many fish use a set of pharyngeal jaws in their throat to aid in prey capture and processing, particularly of large or complex prey. In this study—combining dissection, CT scanning, histology, and performance testing—we demonstrate a novel use of pharyngeal teeth in the ocean sunfish (Mola mola), a species for which pharyngeal jaw anatomy had not been described. We show that sunfish possesses only dorsal pharyngeal jaws where, in contrast to their beaklike oral teeth, teeth are recurved spikes, arranged in three loosely connected rows. Fang‐like pharyngeal teeth were tightly socketed in the skeletal tissue, with shorter, incompletely‐formed teeth erupting between, suggesting tooth replacement. Trichrome staining revealed teeth anchored into their sockets via a combination of collagen bundles originating from the jaw connective tissue and mineralized trabeculae extending from the teeth bases. In resting position, teeth are nearly covered by soft tissue; however, manipulation of a straplike muscle, running transversely on the dorsal jaw face, everted teeth like a cat's claws. Adult sunfish suction feed almost exclusively on gelatinous prey (e.g., jellyfish) and have been observed to jet water during feeding and other activities; flume experiments simulating jetting behavior demonstrated adult teeth caught simulated gelatinous prey with 70%–100% success, with the teeth immobile in their sockets, even at 50x the jetting force, demonstrating high safety factor. We propose that sunfish pharyngeal teeth function as an efficient retention cage for mechanically challenging prey, a curious evolutionary convergence with the throat spikes of divergent taxa that employ spitting and jetting.
... They are distributed worldwide from tropical to temperate waters. Molids comprise a large percentage of bycatch in set-net, longline, gillnet, and trawling fisheries in the Mediterranean Sea and Atlantic Ocean (Pope et al. 2010;Nyegaard et al. 2020). Additionally, molids are targeted fishery species in some regions of the Pacific Ocean (Pope et al. 2010;Chang et al. 2018). ...
... Molids comprise a large percentage of bycatch in set-net, longline, gillnet, and trawling fisheries in the Mediterranean Sea and Atlantic Ocean (Pope et al. 2010;Nyegaard et al. 2020). Additionally, molids are targeted fishery species in some regions of the Pacific Ocean (Pope et al. 2010;Chang et al. 2018). Due to the impact of fishing pressure and their potentially slow growth rate (Liu et al. 2009), their populations are expected to decline globally (Liu et al. 2015;Phillips et al. 2023). ...
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Full-text available
Marine sunfishes (also known as molids) of the Family Molidae are widely distributed from tropical to temperate waters and are typically recognized as predators of gelatinous plankton almost exclusively. Despite similar morphological features and behaviors, the trophic ecology and potential interactions among species of molids remain largely unknown. We reviewed literature on the diets of each species and conducted diet analyses of three species sampled off the east coast of Taiwan. We examined diet separation among sympatric species—ocean sunfish Mola mola, bumphead sunfish Mola alexandrini, and sharptail sunfish Masturus lanceolatus—through stomach content and stable isotope analyses. A literature review revealed that the Family Molidae exhibits broader diets than previously characterized. Mola mola, M. alexandrini, and M. tecta consume prey from epi/mesopelagic environments, while M. lanceolatus and slender sunfish Ranzania laevis consume prey from both epi/mesopelagic environments and benthic habitats. No gelatinous prey was found in the stomachs of R. laevis. Off Taiwan, M. mola and M. alexandrini had similar and relatively narrow diet breadths, primarily feeding on scyphozoans, suggesting similar trophic niches. In contrast, M. lanceolatus displayed a broader diet, mainly consuming tunicates and augmenting their diet from epi- and mesopelagic, coastal, and benthic habitats. Dietary differences between M. lanceolatus and the other species might be linked to morphological differences such as gape size and eye length. Mola mola and M. alexandrini tend to have larger gapes and eyes and our diet analysis shows that they forage on larger sized prey and at greater depths.
... 距離的遷徙,且垂直移動行為頻繁 (Watanabe and Sato, 2008;Houghton et al., 2009)。標識放流結果 也顯示翻車魨的移動行為與水溫和餌料的季節 性變動相關 (Sims et al., 2009a, b;Sousa et al., 2016)。同時也有文獻指出,翻車魨的幼體攝食底 棲生物,而在成長至體長 80-100 cm 時發生食性 上的轉變,改成攝食浮游生物 (Pope et al., 2010;Nakamura and Sato, 2014)。 翻車魨科多在西太平洋、大西洋與地中海等 海域以混獲方式漁獲,包含延繩釣、定置網、流 刺 網 與 拖 網 等 漁 法 (Sagara and Ozawa, 2002;Petersen, 2005;Tudela et al., 2005;Petersen andMcDonell, 2007)。日本與紐西蘭皆有記載翻車魨漁 獲的相關資料,翻車魨於日本的捕獲體長介於 50-300 cm,紐西蘭記錄的體長介於 37-236 cm (Paulin et al., 1982;Sawai et al., 2011 et al., 1982;Araujo et al., 2010;Das et al., 2012),然 而目前國際間關於翻車魨科魚類漁獲統計資料幾 (Nakatsubo et al., 2007;Sawai et al., 2011 長多小於 100 cm 或大於 200 cm (屬於性成熟個體) (Yoshita et al., 2009) (Table 4) (Lee, 1986;Sims and Southall, 2002;Dulcic et al., 2007;Fulling et al., 2007;Sims et al., 2009a, b;Thys et al., 2017)。 臨界溫度在 8-10 °C Thys et al., 2015),臺灣東部海域的海面水溫在冬季時的海水 ...
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Sunfishes (Molidae) comprise the largest bony fishes inhabiting the epipelagic to mesopelagic realms in tropical and temperate ocean regions. Sunfishes are principally bycatch species captured by longline, harpoon, drift net, and set net fisheries in eastern Taiwan. To fill in some of the necessary gaps in the knowledge of sunfishes for local fisheries management, we investigated variability in species composition, abundance, size composition,and the possible correlations of these phenomena with sea surface temperature (SST) in eastern Taiwan. Fishery data regarding sunfishes was collected from Shingang fish market, Taitung, from January 2016 to October 2017, and from the set-net complex, Hualien, from January 2003 to December 2011. The species comprised Masturus lanceolatus (80%) followed by Mola spp. (20%). The species abundance, average standard length, and size composition exhibited seasonal variability, with the abundance reaching a peak in winter when the SST was low. Small, immature individuals first appeared in winter, and then the number of fish caught decreased sharply in summer while the proportions of the largest individuals were highest in summer. The catch number of Masturus lanceolatus was highest in winter and late spring/early summer, and it was caught mainly at water temperatures above 20 °C. Variability in the catch data, a proxy of population dynamics, appeared to be closely associated with water temperature, the life history traits of the fish, and their migratory behavior.
... Examples are found among some large reptiles (e.g., alligators, Lance, 2003) and fish (e.g. ocean sunfish, Pope et al., 2010), which grow large and slowly, have long lifespans, but produce hundreds to thousands of small offspring each reproductive bout (this is also common in large trees). Only when there is low adult extrinsic mortality, and the effect of increased parental investment on offspring reproductive value is large, should we find slower life histories also being characterized by lower fertility rates. ...
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Human evolutionary demography is an emerging field blending natural science with social science. This edited volume provides a much-needed, interdisciplinary introduction to the field and highlights cutting-edge research for interested readers and researchers in demography, the evolutionary behavioural sciences, biology, and related disciplines. By bridging the boundaries between social and biological sciences, the volume stresses the importance of a unified understanding of both in order to grasp past and current demographic patterns. Demographic traits, and traits related to demographic outcomes, including fertility and mortality rates, marriage, parental care, menopause, and cooperative behavior are subject to evolutionary processes. Bringing an understanding of evolution into demography therefore incorporates valuable insights into this field; just as knowledge of demography is key to understanding evolutionary processes. By asking questions about old patterns from a new perspective, the volume—composed of contributions from established and early-career academics—demonstrates that a combination of social science research and evolutionary theory offers holistic understandings and approaches that benefit both fields. Human Evolutionary Demography introduces an emerging field in an accessible style. It is suitable for graduate courses in demography, as well as upper-level undergraduates. Its range of research is sure to be of interest to academics working on demographic topics (anthropologists, sociologists, demographers), natural scientists working on evolutionary processes, and disciplines which cross-cut natural and social science, such as evolutionary psychology, human behavioral ecology, cultural evolution, and evolutionary medicine. As an accessible introduction, it should interest readers whether or not they are currently familiar with human evolutionary demography.
... The same is true for the species with the largest number of records, H. platessoides (Pleuronectidae), C. hippurus(Coryphaenidae), and M. mola (Molidae); while the first two are species exploited by the fishing industry, sunfish (M. mola) has a wide distribution and is mostly associated with scientific and recreational interests (Pope et al., 2010). ...
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