Before you might read this, I would like to inform you that instead of a strictly accurate scientific article, I consider it more as my study-note since I am not professional and it contains some of my self-theorized deductions. However, I do try to make sure they are as accurate and logical I can make them to be, but if I do unfortunately make some mistake, you are more than welcome to point it out.

Order Tetraodontiformes

Family Molidae

? Ok, straight to the point. If anyone can actually tell me why the sunfish looks like a fricking flattened piece of bullet, please inform the author immediately. (I mean if you are actually a foreigner researching this, you don’t know how to comment in Bilibili anyways, so maybe not) Because the author simply fails to discover any evolutionary motive that may drive sunfishes to evolve this shape.

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? I mean, sure, you can argue for advantages like avoiding being swallowed by a predator, or more efficient heat absorption when it’s basking. But the author just doesn’t feel like that those indirect effects can directly cause sunfishes to take on such a distinct evolutionary path so different from any other fish on this planet. So what is happening here?

? As with many tetraodontiforms, the early evolutionary transition for their modern shape is not known. The first known molid unearthed came from the Eocene, species name Eomola bimaxillaria. And there’s nothing that can be learnt from this fossil about early molid body-shape evolution since the only remains are some jaw bones and broken crania.[2]

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??Morphological analyses put Eomola sister to the crown Molidae. However, it would be nearly 15 million years before the next molid fossil shall appear in the strata. That leads us into the Miocene, a time of increased plankton productivity that especially benefited pelagic feeders, like sunfishes. The radiation of the three modern genera of Molidae, Mola, Masturus, and Ranzania, presumably occurred around this period.[2]

??I mean, sure, that is only three genera. Some might argue how this can be considered diverse. However, if you look at the modern distributions of the species of Molidae, most of them actually cover the entire globe. Therefore, unlike on land or in reefs where geographical speciation could easily occur, pelagic niches don’t really support that many species. Therefore, I would say that five species, although not a sign of evolutionary success, definitely does not mean that sunfishes are going extinct any time soon. Their distributions are worth more species than they have. So it is plausible to explain the divergence of those genera in the Miocene as proliferation, not to mention the existence of another now-extinct genus, Austromola.[2]

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??The earliest Mola belonging to a determined species is called Mola pileata. It is pretty long-lived, from mid-Miocene to the Pliocene, and pretty widespread as well, from North Carolina to the Netherlands all the way across the Atlantic. This signifies that molid members were already traveling inter-oceanically by the Miocene, and pretty adapted to their lifestyle as well.[2]

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??Masturus doesn’t have any fossils until very recently. Ranzania, on the other hand, though seemingly poor in diversity today, has a long and rich history of fossil records of which its earliest records R. tenneyorum and R. grahami date back to Miocene Maryland. Another notable Ranzania fossil is R. ogaii from Japan, representing the only Indo-Pacific molid fossil to be discovered (not including very recent ones), and extends the distribution of Miocene molids to the rest half of the globe as well. The reason that a genus with one species today can have the richest fossil record within Molidae is due to its tougher carapace and skeletons that are more ossified than other members of its family.[2]

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??As for today’s Molidae, the newest consensus is that there exists five species. Mola mola, Mola alexandrini, and Mola tecta for Genus Mola, Masturus lanceolatus for Genus Masturus, and Ranzania laevis for Genus Ranzania.[2] The taxonomy of Molidae has traditionally been a very convoluted matter, and the system we adopt today is only the newest based on the most recent genetic advancements, and is not consistent for a lot of older sources and articles. Therefore, here’s a flowchart for the especially problematic Mola classification for you to grasp the whole thing.

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??So, with the brief history of Molidae done, why do they grow to be this way?

??Let’s start by examining its locomotion, the most direct way body-shape may influence an organism.

??One very characteristic feature of sunfishes one may notice is the prominent and symmetrical dorsal and anal fins that stick from a molid’s top and bottom. By waving them in synchrony left to right, this through some mechanism produces lift-based propulsion that is the main force enabling sunfishes to swim and maneuver. The small pectoral fins on either side of a sunfish help it keep balance for this seemingly unstable mode of swimming. [2]

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??Sunfishes, as we have already known, are pelagic species. In order to feed, they usually need to cruise long distances in search of plankton food. Thus, the mode of swimming they adopt must be efficient. For example, sunfishes devolved their axial muscles, which in normal fishes are used to undulate the body, but in sunfishes, lateral movements of the body produce drag.[8] Another way to produce efficiency is to make the fins longer, which according to [2] can push water with a slower speed, which also decreases drag. (from here on out, it’s my hypothesis and deduction) Despite this however, having two massive appendages at top and bottom of the body requires its corresponding musculature, and evolution eventually drove the sunfish body to vertically lengthen so as to accommodate such musculature. Finally, since the caudal fin is no longer used for propulsion, it devolved, leaving behind a rudder-like structure called a clavus whose main use is steering.[2][9] This, presumably is the reason why sunfishes appear so compressed and strange-looking.

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The following two paragraphs are completely my self-theorizing:

??Some problems with my theory, though. First, why do sunfishes adopt dorsal and anal fins as means of propulsion? Other animals with similar niches as sunfishes usually adopt the symmetrical pectoral fins/appendages as means of propulsion. It is simply more logical that way: dorsal and anal fins have different developmental origins and different muscular arrangements altogether, and it would take additional specialization to make those two fins symmetrical.[2][7] The only reason I can think of is inertia from sunfish ancestors who undulate the back of their torso to move forward. And when they shift to a pelagic lifestyle where bursts of movement aren’t required anymore, the previous caudal movements make swaying the nearby dorsal and anal fins a natural transition? This can potentially be peeked from some tetraodontiform relatives of Molidae who also incorporate dorsal and anal fins as a main means of thrust.[11] Of course, sometimes evolution doesn’t make sense, so there might not be a specific reason at all for sunfishes taking this path.

??Second, why do sunfishes have to appear compressed? If they can protract their height, why can’t they protract their width as well? Instead appearing so strange-looking? This one is fairly easy since protracting the width means significantly enlarging its mass, and all kinds of problems concerning surface area arise. Besides, to move such a large body, sunfishes would require larger fins, and thus larger muscles and larger bodies, a vicious cycle. So remaining compressed is I guess beneficial for the sunfish?

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??Besides their superficial appearance, sunfishes have a lot other interesting adaptations. Let’s start from their birth. You’ve probably heard of this myth before, how sunfishes can produce tens of millions of eggs and are the most fecund vertebrate to counter their high death rate. Well, according to [2], the counting method of the original paper that says this is problematic, but it is still safe to say that sunfishes are one of the most fecund vertebrates, though further research would have to be done to affirm this.

??After being fertilized externally, the eggs eventually gave birth to tiny molid larvae very different to their adult counterparts. In fact, premature sunfishes have a funny resemblance to their tetraodontiform relatives boxfishes or pufferfishes, and a stage in molid larval development is even called “ostracioniform.”[2]

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??Change, however, soon starts to emerge. The larvae shed their spikes and degenerated what should have developed into their caudal fin, instead combining the posterior fin rays of their dorsal and anal fins to combine into a structure called clavus, which we mentioned before, replacing the caudal.[2] And do they grow! From such a tiny critter they grow tens of millions of times into a behemoth, reaching 2-3 meters in Masturus and Mola and up to 1 meter in Ranzania (which is still pretty impressive).[14][2] Especially notable is the species Mola alexandrini, weighing a wholly 2300 kg, it is the heaviest bony fish in the world. (Many sources also claim it to be the largest bony fish, but since there’s not really a standard metric measuring “l(fā)argeness,” and sunfishes are not really the longest bony fish either, the author decides to leave it out) Reaching such a large size ensures heat being lost slower during one of their deep dives, which as we’ll see later, are very important to successful preying by sunfishes.[2]

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??If you have read my last article on tetraodontiform evolution, you may remember me saying how along with boxfishes, sunfishes also possess a carapace formed by fused scale plates. They provide protection, and sometimes they are quite hard, especially in Ranzania.[2]

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??Beneath the carapace, however, is a unique structure known as the hypodermis. A thick layer of spongy, greasy, and gelatinous tissue intent on making the sunfish less desirable to eat. It has other functions as well, such as providing insulation and reducing the density of the sunfish, and the latter is pretty important. We might all be familiar with the swim bladder, a bony fish organ that fills air to control the density and thus buoyancy of the entire fish. However, swim bladders cannot work outside a depth range, which sunfishes frequently go over and would have to degenerate their swim bladders during development, so how can they control their diving?[2][8] Well, it turns out that the hypodermis is actually mostly water-filled, and by taking up a large portion of the molid body, it makes the density of the entire sunfish roughly equal to the water’s density. And that makes the sunfish float and dive to hunt at ease.[2] Now that we mention how sunfishes eat…

??Sunfishes eat jellyfishes, or more accurately, gelatinous zooplankton, since their diet is not limited to the phylum Cnidaria. There has been trend recently hypercorrecting that sunfishes are not slow jelly-eaters as many claim them to be, which is true. Sunfish diets are diverse. But there has never been dispute that the majority of sunfish prey is jellyfishes. Sunfish migration has matched global jellyfish patterns, and diving near the coast is specifically intended for feeding on the jellies that bud from continental shelves.[2]

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??Sunfishes are equipped with large and mobile eyes and acute olfactory systems. They are used to the best of their advantage when diving, when a sphere encountering nearby prey is maximized to the largest. One problem is that sunfishes are ectothermic, presumably another measure to conserve energy by sacrificing swimming speed.[8] But by having one of the largest sizes among fishes, their heat can be conserved longer in the cold depths, thus lengthening?their efficient feeding time. Not only so, when they resurface, they can bask in the Sun and effectively reheat their bodies by lying horizontally at the surface, plus exposing a sizeable surface area to sea birds who can help sunfishes clean their parasites.[2][17] Two birds with one stone.

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??It is frankly questionable whether sunfishes going to all this hassle actually can have their expenditures covered by feeding on a bunch of energy-poor jellyfishes. But other pelagic animals with very similar niches, like the leatherback turtles we mentioned earlier, are proven to be able to sustain on jellyfishes. With sunfishes’ professional energy-conserving measures, this should work out. Besides, energy output during jellyfish blooms can be sizeable anyways.[2] One significant advantage sunfishes have over marine reptiles like leatherbacks is that leatherbacks can’t dive too long due to need to breathe, while sunfish, being a fish, is only restrained in diving time by body heat loss.[18] Thus, leatherbacks tend to need to swim wider horizontally in order to obtain sufficient food. This phenomenon also shows the universal impediment faced by marine tetrapods, the inability to access the deep sea, and thus less capability to survive through turbulent mass extinctions.

? Btw, Ranzania don’t eat jellyfishes. They seem to occupy a different niche of fast-movingness entirely and feed on crustaceans and fish larvae. Therefore, the above analyses are pretty much only suited for Mola and Masturus.[2]

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??Throughout the centuries, the unique shape of sunfishes has drawn countless people, like me, into its mysteries, and the contents above have been the hard-work of many scientists and hobbyists and even youtubers. As a pelagic species, studying sunfishes is understandably very difficult due to their wide distribution and low density in this distribution.[9] Many data including species occurrence and size records come from fishery recordings where sunfishes are often caught as bycatch. But, naturally, the ambiguity of species demarcation has even bugged professional ichthyologists for centuries, how can one expect species reports that aren’t even the main focus of fisheries be reliable? Besides, fisheries usually use the formula (total weight)/(weight of each individual) to estimate the number of sunfishes, which as we’ve seen before, sunfish weights can vary a few million times in the most extreme cases. This has thus made the assessment for the vulnerability of this unique lineage in modern days difficult.[2]

??Nevertheless, from how I see it, sunfishes don’t really face that much of a threat from humans. They are so useless, after all, and many fisheries just release them back into the ocean every time they caught one. And aside from specific places, they aren’t consumed as food either, the use of which has even been banned by the EU. Sunfishes have even been seen migrating to the poles as adaptive response to the exacerbating global warming nowadays.[2]

??As for their long-term fate? Well, no one can be certain, and especially for those who see sunfishes as an evolutionary failure, they are headed for demise. But their way of living has worked this long after all. Sure, sea lions, sharks, and orcas eat them occasionally, but their astronomical fecundity and global distribution seem to be able to withstand anything. Even if they do eventually go extinct due to their excessive specialization, at least they made our world a more diverse place by adding yet another colorful stroke on what evolution is capable of.

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Works Cited

[1] Farber, Madeline. “Massive Sunfish Found on California Beach is one of the World’s Rarest, Scientists Say.” Fox News, 1 Mar 2019, www.foxnews.com/science/massive-sunfish-found-on-california-beach-is-one-of-the-worlds-rarest-scientists-say. Accessed 6 Aug 2023.

[2] Thys, Tierney, et al. The Ocean Sunfishes-Evolution, Biology, and Conservation. Boca Raton, CRC Press, 2021. Accessed 6 Aug 2023.

[3] avancna. “Eomola bimaxillaria.” Deviant Art, 10 Aug 2008, www.deviantart.com/avancna/art/Eomola-bimaxillaria-94341927. Accessed 6 Aug 2023.

[4] Tyler, James and Bannikov, Alexandre. “New Genus of Primitive Ocean Sunfish with Separate Premaxillae from the Eocene of Southwest Russia (Molidae, Tetraodontiformes).” Copeia, vol. 1992, no. 4, 18 Dec 1992, pp. 1014-1023. Accessed 6 Aug 2023.

[5] Gregorova, Ruzena, et al. “A Giant Early Miocene Sunfish from the North Alpine Foreland Basin (Austria) and Its Implication for Molid Phylogeny.” Journal of Vertebrate Paleontology, vol. 29, no. 2, Jun 2009, pp. 359-371. Accessed 6 Aug 2023.

[6] Bran, Jaime. “Austromola, a Giant Sunfish from the Early Miocene of Europe.” Twitter, 20 Feb 2021, twitter.com/BranArtworks/status/1362944412767109120. Accessed 6 Aug 2023.

[7] Davenport, John, et al. “The Locomotor System of the Ocean Sunfish Mola mola (L.): Role of Gelatinous Exoskeleton, Horizontal Septum, Muscles and Tendons.” Journal of Anatomy, vol. 233, no. 3, Sep 2018, pp. 347-357. Accessed 6 Aug 2023.

[8] 用盡力氣順其自然. “翻車魚的奇妙生物特性.” Bilibili, 26 Jul 2023, www.bilibili.com/video/BV1Xx4y197Zt. Accessed 6 Aug 2023.

[9] Arostegui, Martin, et al. “Spatiotemporal Segregation of Ocean Sunfish Species (Molidae) in the Eastern North Pacific.” Marine Ecology Progress Series, vol. 654, Nov 2020, pp. 109-125. Accessed 6 Aug 2023.

[10] Berrington-Blew, Philippa. “Scientific Illustration.” Pinterest, www.pinterest.com/pin/79587118389315220/. Accessed 6 Aug 2023.

[11] 木節(jié)天蟲. “【鲀形目的演化史·其一】刺鲀，箱鲀，河鲀，鱗鲀.” Bilibili, 4 Jun 2023, www.bilibili.com/read/cv24130440. Accessed 6 Aug 2023.

[12] Science Photo Library. “Leatherback Turtle.” Pixels, 7 Oct 2018, pixels.com/featured/4-leatherback-turtle-scubazooscience-photo-library.html. Accessed 6 Aug 2023.

[13] InterestCaptured. “The Effects of Light Intensity and Photoperiod on Marine Fish Larvae.” Hub Pages, 17 Sep 2017, discover.hubpages.com/education/The-Effects-of-Light-Intensity-and-Photoperiod-on-Marine-Fish-Larvae. Accessed 6 Aug 2023.

[14] Blue Corner Marine Research writers. “Ocean Sunfish (Mola sp.).” Blue Corner Marine Research, bluecornerconservation.org/mola-mola-ocean-sunfish. Accessed 6 Aug 2023.

[15] Learn, Joshua. “Meet the Sunfish’s Cuter, Faster, More Agile Cousin.” Hakai Magazine, 16 Jun 2017, hakaimagazine.com/news/meet-sunfishs-cuter-faster-more-agile-cousin/. Accessed 6 Aug 2023.

[16] Katayama, Eri and Matsuura, Keiichi. “Fine Structures of Scales of Ocean Sunfishes (Actinopterygii, Tetraodontiformes, Molidae): Another Morphological Character Supporting Phylogenetic Relationships of the Molid Genera.” Bulletin National Museum Nature and Science Series, vol. 42, no. 2, 20 May 2016, pp. 95-98. Accessed 6 Aug 2023.

[17] Bass, Anna, et al. “Evolutionary Divergence among Lineages of the Ocean Sunfish Family, Molidae (Tetraodontiformes).” Marine Biology, vol. 148, 1 Sep 2005, pp. 405-414. Accessed 6 Aug 2023.

[18] Hays, Graeme, et al. “Vertical Niche Overlap by Two Ocean Giants with Similar Diets: Ocean Sunfish and Leatherback Turtles.” Journal of Experimental Marine Biology and Ecology, vol. 370, no. 1-2, 1 Mar 2009, pp. 134-143. Accessed 6 Aug 2023.

[19] Two Oceans Aquarium writers. “Why did this Sunfish Jump out of the Water?” Two Oceans Aquarium, 19 May 2022, www.aquarium.co.za/news/why-did-this-sunfish-jump-out-of-the-water. Accessed 6 Aug 2023.

Cover: Mancini, Mark. "Dive Inside the Weird World of the Ocean Sunfish." How Stuff Works, 20 Jun 2019,?animals.howstuffworks.com/fish/sunfish.htm. Accessed 12 Aug 2023.