1. Swimming Ability and Feeding Strategy of the Ocean Sunfish,
Mola mola, Allow for Occupation of a Unique Niche
Bruce DeMarais
Vertebrate Biology
Keven Henley
4/14/16
The ocean sunfish (Mola mola) falls into the large class of ray-finned fishes
known as Actinopterygii. Before we dive into a discussion of body function and form that
is specifically related to the ocean sunfish, we will cover its more primitive origins.
Actinopterygii are one of the major groups of bony fishes, which are broadly termed
Osteichthys. Early actinopterygians didn’t have many of the anatomical features that we
now see in extant individuals. Their fin membranes and basal fin elements were much
less pliant due to more bone being present in these structures. The earliest
actinopterygians were identified by their one dorsal fin in contrast to sarcopterygians that
possessed two. Towards the end of the Paleozoic Era, modifications in fin structure
appeared that are more indicative of modern teleosts. Fins became more flexible, which
allowed for better locomotion/swimming control. This also allowed for a reduction in
dermal armor because fish were better suited to evade predators. Jaws modified for
suction and other functions also became present (Pough et al. 1999). These modifications
laid the foundation for increased diversification of teleosts, which are now the most
diverse group of fishes.
Here, we are discussing the order Tetraodontiformes. It is a unique monophyletic
group that includes 10 families and over 300 species (Johnson and Britz 2005). This
group is unique in that it evolved in coral reefs and it is considered to be the most derived

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group of teleosts. Tetra refers to the fact that individuals belonging to the order
Tetraodontiformes have four teeth in the outer jaws. Many of the bones present in the
head and body of other groups have either been fused together or lost in this group. This
is hypothesized to be due to evolution in a diverse coral habitat. In this habitat, fast travel
for long periods of time was not necessary. Individuals would feed in microhabitats
provided by coral species. The pelvic fins also become obsolete. The head and jaw were
highly modified; premaxilla and maxilla fused to allow the individual to eat hard
organisms such as urchins. However, some families have become prevalent offshore,
away from reefs where they feed on gelatinous zooplankton. This is the case with the
ocean sunfish (Helfman et al. 2009). Research done on divergence of Tetraodontiformes
suggests that familial lineages had already diverged and were present around 35 million
years ago (Bass et al. 2005).
One of the most unique morphological traits that Tetraodontiformes possess is the
adductor mandibulae complex. It is a trait that has allowed them to feed in coral reef
habitats. Though the ocean sunfish does not actually feed in reefs, we must mention this
in order to understand where its unique jaw structure came from. This specialized jaw
muscle complex is one of the morphological traits that set Tetraodontiformes apart from
other orders. It has been highly modified among families as a result of niche partitioning
and diet specialization. The trait is derived from a single myogenic anlage and it is
interesting to note that this same anlage has evolved into differing jaw muscle complexes
in other fish taxa; it is highly conserved (Konstantinidis and Harris 2011).
As stated before, Tetraodiformes is the most derived order of teleosts and within
that order is the family Molidae, which is the most advanced teleost family known. It

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consists of three lineages and five species. Lineages within the Molidae family are
estimated to have diverged around eighteen million years ago but extensive evidence for
that claim is lacking. The most widely distributed lineage is the Mola mola. For that
reason, it has been difficult for researchers to distinguish between evolutionary origins of
M. mola and other Molidae (Bass et al. 2005). That is one of the countless facets that
comprise its outright uniqueness. The ocean sunfish appears circular from the side, with
giant dorsal and anal fins that guide and facilitate the fish’s movement. The caudal fin
has, for the most part, been lost and is therefore referred to as pseudocaudal. These
individuals can grow to be three meters long and four meters tall. Some weigh up to
2,300 kg. They are the largest known bony fish. They primarily feed on jellyfish but are
known to graze for benthic crustaceans as well. Most individuals are tracked in pelagic
zones but they are sometimes found in coastal waters (Helfman et al. 2009).
Not only do ocean sunfishes possess the fused skeletal elements present in all
Tetraodontiformes, much of their skull and fin structure is comprised of cartilage. Being
the most derived of all fish species, it is interesting to see a return of some of the most
primitive characteristics; the cartilaginous skeleton is present in primitive groups such as
Chondrichthyes. This provides harmonious evidence for current evolution theory. It
reinforces the idea that evolution has no predetermined goal or destination that results in
a species of higher fitness (Helfman et al. 2009). It is a process that wanders aimlessly,
wrought with trial and error.
An effective way of understanding the life history of the ocean sunfish is to look
at it in the context of its niche. Ocean sunfishes graze far off shore and are found in both
the pacific and the Atlantic. They feed on gelatinous zooplankton, which is a food source

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of relatively low nutrition. Leatherback sea turtles also feed on zooplankton. They are the
largest turtle species in existence and possess some of the same overall body
characteristics. It possesses analogous structures such as a modified jaw that functions in
a similar way that the adductor mandibulae complex does in the ocean sunfish.
Geolocation tracking enabled scientists to conclude that leatherbacks and ocean sunfishes
move broadly throughout the same area in the oceans. Although both species have
gravitated to a niche that is slightly overlapping, there are certain constraints on them
physiologically that will never allow them to overlap to a large degree; turtles will never
be able to dive as deep as ocean sunfish due to their inability to extract oxygen from the
water with their gills (Hays et al. 2009). By taking a look at where the body form of the
ocean sunfish was derived from and then observing its current behavior, we begin to see
that it was merely a coral reef fish that adopted more K selected traits over evolutionary
time and found itself in a niche comparable to the leatherback sea turtle.
Although zooplankton is thought to be the primary diet of ocean sunfishes around
the world, there is still a lot to be discovered about their diet. A paper by Itsumi
Nakamura and Katsufumi Sato postulated an “ontogenic shift” in the foraging habits of
the ocean sunfish. At a young age, the fish feeds on benthic crustaceans and mature
individuals feed on larger zooplankton (Nakamura and Sato 2014). This could be
indicative of its evolutionary origin; feeding on benthic crustaceans is a strategy similar
to the one used by many other Tetraodontiformes in the coral reefs. Inherent locality of
juveniles and their inability to swim long distances also suggest that ocean sunfish were
derived from a coral reef dwelling order (Dewar et al. 2010). Therefore, it is evident that
ocean sunfish have adopted a strategy related to size, swimming ability and feeding that

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is quite different from other Tetraodontiformes. They were thus able to exploit the large
abundance of jellyfish colonies in pelagic zones.
Remotely operated vehicle footage has shown that deep water grazing of ocean
sunfishes is more prevalent than expected. They have been filmed swimming at depths of
up to 600 m (Phillips et al. 2015). Satellite tracking of adult individuals was observed in
the North East Atlantic. They were observed migrating north in the spring and south in
the autumn. This is to be expected for any migratory species. Distances up to 2,000 km
were logged, showing that these fish have an incredible swimming ability in terms of
longevity (Sims et al. 2009). We will now transition into some of the major body design
modifications that are apparent in the ocean sunfish that allows it to exhibit these long
distance swimming abilities. Using motion sensor tags, researchers were able to show
what kind of thrust ocean sunfish use to propel themselves through the ocean waters. By
using their modified dorsal and anal fins at the same time, they were able to generate a
lift-based thrust that is similar to the type of movement exhibited by penguins that use
flippers to swim. This finding is quite influential because it illustrates just how the body
of the sunfish was modified to swim in this manner. The dorsal and anal fins are
symmetrical in shape, externally. They also contain the same amount of muscle mass,
internally. However the muscle shape differed a lot because the structures are non-
homologous. The apparent paired fins act as vertical hydrofoils. The vertically wide and
horizontally short pseudocaudal fin acts as rudder of sorts. Ocean sunfish were also able
to descend to great depths in the pelagic zone without the use of a swim bladder; they
instead have subcutaneous gelatinous tissue that is light and incompressible. (Watanabe
and Sato 2008). The loss of the swim bladder in Molidae is probably due to energy

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conservation during growth and development but research hasn’t been done on this
specifically. The swimming strategy that they have adopted is efficient in that it allows
them to travel long distances both horizontally and vertically. We can now conclude by
saying that the ocean sunfish truly represents the most advanced bony fish in that it is
able to migrate across vast reaches of ocean and feed on organisms that large K selected
species feed on.
Literature Cited
Bass, A. L., Dewar, H., Thys, T., Streelman, J. T., and Karl, S. A. (2005). Evolutionary
divergence among lineages of the ocean sunfish family, Molidae (Tetraodontiformes).
Marine Biology 148, 405–414. doi:10.1007/s00227-005-0089-z
Dewar, H., Thys, T., Teo, S. L. H., Farwell, C., O’Sullivan, J., Tobayama, T., Soichi, M.,
Nakatsubo, T., Kondo, Y., Okada, Y., Lindsay, D. J., Hays, G. C., Walli, A., Weng, K.,
Streelman, J. T., and Karl, S. A. (2010). Satellite tracking the world’s largest jelly
predator, the ocean sunfish, Mola mola, in the Western Pacific. Journal of Experimental
Marine Biology and Ecology 393, 32–42. doi:10.1016/j.jembe.2010.06.023
Hays, G. C., Farquhar, M. R., Luschi, P., Teo, S. L. H., and Thys, T. M. (2009). Vertical niche
overlap by two ocean giants with similar diets: Ocean sunfish and leatherback turtles.
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doi:10.1016/j.jembe.2008.12.009
Helfman, G., Collette, B. B., Facey, D. E., and Bowen, B. W. (2009). ‘The Diversity of
Fishes: Biology, Evolution, and Ecology’. (John Wiley & Sons.)
Johnson, G. D., and Britz, R. (2005). Leis’ conundrum: Homology of the clavus of the ocean
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(Teleostei, Tetraodontiformes, Molidae). Journal of Morphology 266, 11–21.
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Konstantinidis, P., and Harris, M. p. (2011). Same but different: ontogeny and evolution of the
Musculus adductor mandibulae in the Tetraodontiformes. Journal of Experimental
Zoology Part B: Molecular and Developmental Evolution 316B, 10–20.
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Nakamura, I., and Sato, K. (2014). Ontogenetic shift in foraging habit of ocean sunfish Mola
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Phillips, N. D., Harrod, C., Gates, A. R., Thys, T. M., and Houghton, J. D. R. (2015). Seeking
the sun in deep, dark places: mesopelagic sightings of ocean sunfishes (Molidae). Journal
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