Look down in the water! It’s the moon reflection, it’s marine debris, it’s a… sunfish? In case you weren’t nerding out in the marine corners of social media like me last year, you may have missed a debate of whether Ocean sunfish (Family: Molidae) are the most pointless or the coolest fish in the ocean. Now, you may look at that derpy face and make a quick judgement, but for scientists, it is the animals that seemingly make no sense that are the most fascinating to study! Think about it- this large, dense, ocean-disk of a fish has survived for over 40 million years¹; evolutionarily, they must be doing something right!

Sunfish are the largest bony fish [Source]

Sunfish (or Molas) are relatives of pufferfish, and as young larvae, you can tell the family resemblance³, but as they mature they lose their spines, and their dorsal (top) fins fuse with their caudal (tail) fins.^1,2 While they may be tiny as larvae, they grow to be one of the heaviest bony fish in the world- weighing between 500 and 2,200lb (250-1,000kg)³! Wild growth rates are not known, but a captive sunfish at the Monterey Bay Aquarium in Monterey, CA, had to be airlifted out and released into the bay because it gained 800lb (364kg) in 14 months and outgrew the million gallon tank⁴! With an ability to grow that heavy, you may think that some epic protein shakes are involved; however, a sunfish’s diet consists mostly of crustaceans and fish (Infraclass: Teleosts), but they will also munch on sea jellies, Portuguese man-o-wars, sponges, and squid⁵. Their lack of a swim bladder allows them to travel up and down the water column, and their upper and lower teeth fused together like a parrot’s beak, is great for chomping on prey. (Coincidentally, that permanently gaping mouth paired with continuously surprised eyes may be the culprit for their signature goofy face.)

 

Sunfish’s generalist diet combined with both benthic (bottom) and pelagic (open ocean) foraging strategies may be surprising, because they are often seen floating sideways at the surface of the water, mistaken for dead until they clumsily move their fins. Considering this giant head with fins can zip up and down the water column for food, what are sunfish doing basking on the sea surface? There are two possible reasons: One is that sunfish are known to have a heavy parasitic load. Their odd shape prevents them from being able to clean and rid themselves of up to 40 kinds of parasites, so they float at the surface to encourage a symbiotic relationship with sea birds; sea birds get a quick and easy buffet of parasites, and sunfish get a healthy scrub⁶! The second possibility is that due to their deep dives (up to 150m) for benthic prey, they return to the ocean surface where it is warmer to regulate their body temperature⁷. So, evolution may have dealt ocean sunfish a weird combination of adaptations to deal with, but they certainly have taken it in successful stride, even if they look ridiculous in the process.

There are three species of Mola, the common sunfish (Mola mola), the southern ocean sunfish (Mola ramsayi), and the aptly named hoodwinker sunfish (Mola tecta), which was discovered in 2017- hiding in plain sight of scientists until researchers discovered mystery DNA among their samples from 150 sunfish⁸! Molas are fascinating because they are incredibly cosmopolitan, found through tropical and temperate ocean basins in both the Atlantic, Pacific, and Indian Oceans³. However, despite their wide range, they are considered vulnerable by IUCN. The fact that sunfish are pelagic and leisurely hang out at the surface of the water mean they often end up as bycatch through commercial fishing. Just in California, 29 percent of the bycatch from the swordfish fishery is ocean sunfish⁷, with higher numbers estimated in other countries.

Aside from human predation via bycatch, ingestion of ocean debris (plastic bags look a lot like sea jellies), and being considered a delicacy in Asian countries, sunfish also can be targeted by fellow marine predators such as sea lions. Interactions that have been observed indicate that sea lions either chew on Molas and pull them around just for fun or the animals get frustrated with the tough skin and dense meat of the Molas, that they give up after tearing off the Molas’ fins (much to the fatal detriment of our ocean-disk friends). That may be depressing, but sunfish have one of the highest fecundity rates (abundance of offspring) of any vertebrate (animal with a backbone), with a female carrying 300 million eggs each about 1.3mm in diameter at one time (so in theory, there is a lot of Mola to go around)³.

Mola mola larvae [Source]

Although sunfish do give the impression that they are just slow, heavy flesh disks with a face that imitates permanent astonishment that they exist in the marine ecosystem, they actually are behaviorally and morphologically complex and quite literally, shouldn’t be taken at face value. If you don’t take my word for it, take it from the loyal sunfish scientists who clearly love their study species and fight to break the stereotype that Molas aren’t just a joke evolution played on marine researchers. So, you had better not make fun of sunfish characteristics too much or you may have 300 million baby Molas and a group of passionate sunfish scientists coming for revenge.

 

Karli Chudeau is in the Animal Behavior Graduate Group and a part of the UC Davis Center for Animal Welfare. She is interested in conservation management and assessing animal welfare in wildlife rehabilitation settings. Her current research examines how we can use behavioral management interventions, such as environmental enrichment, to improve reintroduction success with pinnipeds. She is also an avid ocean nerd.

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References

[1] Santini, F. & Tyler, J.C. (2002). Phylogeny of the ocean sunfishes (Molidae, Tetraodontiformes), a highly derived group of teleost fishes. Italian Journal of Zoology, 69(1), 37-43, DOI: 10.1080/11250000209356436.

[2] Bass., A.L., Dewar, H., Thys, T., Streelman, J.T., & Karl., S.A. (2005). Evolutionary divergence among lineages of the ocean fish family, Molidae (Tetraodontiformes). Marine Biology, 148(2), 405-414, DOI: 10.1007/s00227-005-0089-z.

[3] https://oceansunfish.org/lifehistory.php

[4] Powell, D.C. (2003). A fascination for fish: Adventures of an underwater pioneer. University of California Press/Monterey Bay Aquarium series in marine conservation. Berkeley & Los Angeles, California

[5] Sousa, L.L., Xavier, R., Costa, V., Humphries, N.E., Trueman, C., Rosa, R., Sims, D.W., & Queiroz, N. (2016). DNA barcoding identifies a cosmopolitan diet in the ocean sunfish. Scientific Reports, 6, 28762, DOI: 10.1038/srep28762.

[6] https://video.nationalgeographic.com/video/weirdest-mola-mola

[7] Cartamil, D.P. & Lowe, C.G. (2004). Diel movement patterns of ocean sunfish Mola mola off southern California. Marine Ecology Progress Series, 266, 245-253, DOI:10.3354/meps266245.

[8] Nyegaard, M., Sawai, E., Gemmell, N., Gillum, J., Loneragan, N.R., Yamanoue, Y., & Stewart, A.L. (2017). Hiding in broad daylight: Molecular and morphological data reveal a new ocean sunfish species (Tetraodontiformes: Molidae) that has eluded recognition. Zoological Journal of Linnean Society, 182(3), 631-658, DOI: 10.1093/zoolinnean/zlx040.

[Source for main image]