Zoopedia: Ocean Sunfish

One of the strangest fish to swim in the seas has to belong to the genus Mola: the sunfish. There are three species of Mola with the ocean sunfish, often simply referred to by its scientific name Mola mola, is the most famous. We’re looking at the three species of oceanic sunfish today, and how these evolutionary marvels live.

Taxonomy and Evolution

As mentioned above, the oceanic sunfish belong to the genus Mola which contains most of the members of the family Molidae. These are bony fish which have evolved the strange, bullet-shaped body plan that now only consist of five species – three in Mola, one in Masturus, and one in Ranzania. However, the Mola are by far the largest with the ocean sunfish outweighing them all. The three Mola species are: the ocean sunfish, Mola mola; the Southern sunfish, Mola alexandrini; and the very recently discovered hoodwinker sunfish, Mola tecta. The family Molidae, despite looking prehistoric, are a relatively recent family of fish to evolve, first appearing between 50 to 40 million years. The Molidae family are unique among bony fish, and strangely enough their closest relatives are the pufferfish.

Biology

The oceanic sunfish are such strange looking fish. Extremely flat and disk shaped the scientific name comes from Latin for ‘millstone’, mainly as it resembles one. These giant fish start life as a millimetre long fry – one of 300 million eggs! The ocean sunfish lays more eggs than any other vertebrate. Sex between sunfish, and many other fish, is not an intimate affair; the females expel their eggs and the male expels his sperm fertilising the eggs. How males and females choose their partners is still unknown. Fry hatch out at a size of roughly 2.5 mm in length, meaning that they will grow to be over 60 million times their birth weight – one of the largest, if not the largest, growth rates in any vertebrate. Initially, fry resemble pufferfish with spines covering the body which are lost as the fish grows. Within eight years the sunfish reach their adult size – one in Monterey Bay Aquarium grew from 26 to 399 kilograms in a matter of 15 months. Adults reach a size of 3.3 metres in height and weight over 2,000 kilograms making them the largest bony fish on the planet.

Despite being a bony fish a lot of the ocean sunfish’s body is made out of cartilage – the same thing that our noses are made of. This is just the tip of the iceberg of the sunfish’s weird biology. The strange fins have been for a longtime up for debate – zoologists have struggled to understand for decades whether they are modified caudal fins, or dorsal and tail fins. Luckily, we now know. A 2005 study found that the ancestor’s of Mola lost their caudal fin, with a new fin made by the convergence of the dorsal and anal fins called the clavus was formed. This new tail is used as a rudder as the ocean sunfish rarely needs speed. It also lacks a swim bladder, and pain receptors – often predators will bite chunks out of the sunfish without it registering. Finally, four teeth form a spherical beak which remains permanently open, which allows sunfish to effectively filter-feed.

Hoodwinker sunfish

As expected, sunfish are slow swimmers, but this suits them. They are also found at various depths, being found in the shallows and at depths deeper than 200 metres. This is one reason why it took until 2017 for the hoodwinked sunfish to be officially described – by a very lucky PhD student called Marianne Nyegaard – due to them being found in deeper depths. However, it is relatively common to see Mola mola in the surface, with terrified sailors sometimes seeing their fin break the water and mistake it for a shark fin. While they swim vertically, they do like to bask on the surface horizontally. This is done for several reasons. The main one is so it can warm itself up, but also as a way to remove parasites. Ocean sunfish are heaven for parasites, so rely on cleaning fish to remove these parasites, and bask at the surface to allow seabirds to clean them. Quite hilariously, ocean sunfish can occasionally pick up speed to breach the water as a way to shake off parasites – they can actually reach 3 metres in height doing this! Some unfortunate people have had the misfortune of being hit by a breaching sunfish – all 2000 kg of it.

Habitat and Distribution

Ocean sunfish live in open waters, and, as mentioned above, they swim from 200 metres up to the surface. Following food and heat they spend their time leisurely moving from these depths. Typically, they are found in temperate and tropical waters, but are most commonly found in tropical waters. Sunfish are found worldwide, with the ocean sunfish being the most widely distributed species. Mola mola are found in all warm oceans and seas, being found as far north as Ireland and as far south as Australia. The Southern sunfish is found in similar areas, but are more common in southern waters. Finally, the hoodwinked sunfish has only been found around New Zealand, Australia, South Africa, and southern Chile. Hoodwinker sunfish, initially believed to be Mola mola, have been found washed up on Californian and Dutch shores, but there have been two cases within the last century.

Diet and Predators

Sunfish have a varied diet, with them initially thought to solely exist on a diet of jellyfish and plankton. However, more recent research, especially since sunfish have now been successfully kept in captivity, have found them to be generalist eaters, and would eat small fish, squid, crustaceans, larvae, and sea grass. If its edible and can fit in its beak, the sunfish will eat it. Young sunfish have far more predators than the adults, as adults have much thicker flesh and their size will put many predators off. Large sharks, orcas, and sea lions all prey on adult sunfish, with sea lions often attacking sunfish for fun. Due to their lack of pain receptors it is not unheard of to see sunfish with large bites taken out of them. Humans have historically fished for sunfish, with it being a delicacy in Japan, Korea, and Taiwan – the EU has, however, banned its consumption due to a fear that the intestines are toxic. Finally, sunfish are a host for lots of parasites. One is very interesting – the larvae for tapeworm that live in the guts of sharks. The larvae develop in the sunfish and wait for a shark to bite into the sunfish; when this happens the tapeworm can enter the shark.

Threats and Conservation

Currently, only Mola mola has been ranked by the IUCN, and it does not look good. They are currently rated as ‘Vulnerable’, with their numbers decreasing for various reasons. Despite being popular with divers – especially as sunfish are really curious about seeing divers – they are at risk due to human actions. While sunfish are becoming more and more popular as a dish, the big reason for their numbers dropping is due to bycatch. This is when an animal is accidentally caught in fishing, with gillnets being especially dangerous for sunfish (as well as many other animals). Plastic, especially plastic bags, also resemble their main food source – jellyfish – so are increasingly dying due to plastic ingestion. On top of this, we do not fully understand the biology of sunfish, so we do not yet understand the impact climate change, water poisoning, and the disappearance of other organisms will have on the ocean sunfish. For the other two species we know even less.

Bibliography:

• ‘Ocean Sunfish (Mola)’, National Geographic, [Accessed 20/09/2022]
• Thomas Murphy and Declan Quigley, ‘Ocean sunfish (Mola mola) stranded at Old Head Quay, Louisburg, Co. Mayo’, The Irish Naturalists’ Journal, 31:1, (2010), 70
• ‘Ocean Sunfish’, IUCN, (07/06/2011), [Accessed 20/09/2022]
• Marianne Nyegaard, Etsuro Sawai, Neil Gemmell, Joanne Gillum, Neil R Loneragan, Yusuke Yamanoue, Andrew L Stewart, ‘Hiding in broad daylight: molecular and morphological data reveal a new ocean sunfish species (Tetraodontiformes: Molidae) that has eluded recognition’, Zoological Journal of the Linnean Society, 182:3, (2017), 631-658
• Anna L. Bass, Heidi Dewar, Tierney Thys, J. Todd. Streelman, Stephen A. Karl, ‘Evolutionary divergence among lineages of the ocean sunfish family, Molidae (Tetraodontiformes)’, Marine Biology, (2005)
• Yuuki Watanabe and Katsufumi Sato, ‘Functional Dorsoventral Symmetry in Relation to Lift-Based Swimming in the Ocean Sunfish Mola mola‘, PLoS ONE, 3:10, (2008)
• Animalogic, ‘Mola Mola: Behemoths of the Deep’, YouTube.com, (25/05/2018), [Accessed 20/09/2022]
• Joseph Nelson, Terry Grande, and Mark Wilson, Fishes of the World, Fifth Edition, (Hoboken: 2016)
• David Burnie and Don Wilson, (eds.), Animal: The Definitive Visual Guide, (New York: 2017)