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

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 CitedBass, 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. Journal of Experimental Marine Biology and Ecology 370, 134–143. 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 sunfishes. 2. Ontogeny of the median fins and axial skeleton of Ranzania laevis (Teleostei, Tetraodontiformes, Molidae). Journal of Morphology 266, 11–21. doi:10.1002/jmor.10242

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. doi:10.1002/jez.b.21375

Nakamura, I., and Sato, K. (2014). Ontogenetic shift in foraging habit of ocean sunfish Mola mola from dietary and behavioral studies. Marine biology 161, 1263–1273.

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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 of Fish Biology 87, 1118–1126. doi:10.1111/jfb.12769

Pough, F. H., Janis, C. M., Heiser, J. B., and others (1999). ‘Vertebrate life’. (Prentice Hall Upper Saddle River, NJ.) Available at: https://www.researchgate.net/profile/F_Harvey_Pough/publication/253242233_Vertebrate_Life_9e/links/0046351f7bc9e5d763000000.pdf [Verified 14 April 2016]

Sims, D. W., Queiroz, N., Doyle, T. K., Houghton, J. D. R., and Hays, G. C. (2009). Satellite tracking of the World’s largest bony fish, the ocean sunfish (Mola mola L.) in the North East Atlantic. Journal of Experimental Marine Biology and Ecology 370, 127–133. doi:10.1016/j.jembe.2008.12.011

Watanabe, Y., and Sato, K. (2008). Functional Dorsoventral Symmetry in Relation to Lift-Based Swimming in the Ocean Sunfish Mola mola. PLOS ONE 3, e3446. doi:10.1371/journal.pone.0003446

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