A couple of weeks ago a large Ocean Sunfish or Mola mola washed out on the beach at Nature’s Valley. The Nature’s Valley Conservation Trust posted some photographs on social media. 

Mola mola, the largest bony fish in the world, can weigh over 2 200 kg – the same as an adult white rhino. The fish is flattened laterally – from side to side. (The one above is lying on its side.) PHOTO: Nature’s Valley Trust

There are actually three species of sunfish that are found in the temperate waters off the South African coast. Some have even been spotted in Walker Bay.

It is probably one of the more bizarre-looking fish in the ocean. For a fish it certainly is – the largest bony fish in the world. There are larger fish in the sea, but they are all elasmobranchs (cartilaginous fish), like sharks, rays and whale sharks. Instead of a bony skeleton, they have cartilage (the same stuff your ears and the tip of your nose are made of). 

Sunfish do not have scales to protect them like most bony fish do. They have a sandpapery skin, more like a shark. This lack of scaly protection makes them more vulnerable to parasites – but more of that later.

As you can see from the photograph, the fish is flattened laterally – from side to side. (The one in the photo is lying on its side.) The body is almost disc-shaped, with two large fins projecting, one from its dorsal side (back) and one from the bottom (anal fin). There is also a stubby pseudo-tail. 

And it’s big – really big. An adult can weigh over 2 200 kg – the same as an adult white rhino. The length from the tip of the dorsal fin to the tip of the anal fin can be over 2 metres. 

It’s the shape of the fish that gave it its Latin name Mola mola.  ‘Mola’ is Latin for a millstone – a large, round, flat, rough-surfaced, disc-shaped stone.

Maybe repeating the ‘mola’ part for both the genus and the species name gave it a pleasing ring. ‘Mola mola’ is not easily forgettable. It’s probably one of the few animals where the scientific name is regularly used to describe it. Not many people use the common name, ‘Ocean Sunfish’, or the German name that translates to ‘swimming head’.

The ‘Incredible Hulk’ starts out life as a tiny piece of popcorn. Mola babies are protected by a star-shaped, transparent covering, and the spikes gradually disappear as the young mola grows. PHOTO: Kerryn Parkinson

Molas produce many strange-looking offspring

As you might expect from their size, female molas produce more eggs than any other vertebrate on earth – about 300 million at a time. The baby fish that hatch out of them are tiny and equally strange-looking. Little mola fry are protected by a star-shaped, transparent covering that looks like it should be hanging from a Christmas tree. Those spikes gradually disappear as the young mola grows.

So how does the equivalent of an underwater flying saucer swim around? Molas move around on their sides flapping their dorsal and anal fins like a pair of wings, and steering with the lumpy pseudo-tail (called a clavus). It’s not efficient, but it works.

Why do they seem to say “ooh”?

The reason why molas all seem to be saying “ooh”, is because their upper and lower teeth are fused into a parrot-like beak that’s stuck permanently open – perfect for slurping up jellyfish and other delicacies. PHOTO: National Geographic

When you look at mola photographs, you will notice they all seem to be saying “ooh”, which is pretty much what we would say if we are lucky enough to see them. 

The reason is that they have fused, beak-like teeth. Their upper and lower teeth are fused into a parrot-like beak that’s stuck permanently open. It’s perfect for slurping up jellyfish and other delicacies, but it’s not pretty.

Why are they called ‘Sunfish’?

Originally, it was thought that molas spend their days near the surface of the sea, basking in the sun. But recent research has shown that they dive to depths of up to 200 metres or more to snack on jellyfish and siphonophores (invertebrate organisms that include jellyfish, corals, and sea anemones). In addition to jellies and zooplankton, they are also known to eat squid, sponges, eelgrass, crustaceans, small fishes and deep-water eel larvae.

So why are they so often seen at the surface of the sea, sunbathing? The reason seems to be that diving so deep for their food chills them, so in between dives they soak up the rays to warm up. This allows the molas to dive repeatedly for food and so extend their foraging time. Their large size is another benefit. The larger the animal, the smaller its relative surface area and the slower their heat loss in deep cold water.

They’re covered in parasites

An adult Mola mola can weigh over 2 200 kg – the same as an adult white rhino. The length from the tip of the dorsal fin to the tip of the anal fin can be over 2 metres. PHOTO: deepoceanfacts.com

It’s not easy for a sunfish that has no scales to avoid parasites. More than 50 species of parasites have been found on molas. They therefore have to pay visits to ‘cleaning stations’ where small fish will pluck the parasites from their skins. However, some parasites are so deeply embedded in the mola skin that they can only be surgically removed. This service is kindly provided by seagulls and other seabirds that can dig them out with their sharp beaks – another reason for molas to hang out in surface water.

Conservation status

The IUCN ranks Mola mola as a vulnerable species. Bycatch (fish caught unintentionally while harvesting other species) is a serious problem for molas. They swim about in the open ocean, they’re massive, and they like to float at the surface… You can see where this is going. Molas are frequently captured as bycatch in many fisheries that use long lines, drift gillnets, and midwater trawls. For example, scientists estimate that longline fisheries in South Africa alone account for as many as 340 000 Mola molas as bycatch each year. As is the case with most ocean fish, molas are under serious threat.

Based on these high bycatch rates and population declines of up to 80 % in some areas, the IUCN suspects that the global Mola mola population has declined by at least 30 % over just three mola generations. 

Will our grandchildren ever have the awesome pleasure of seeing this incredible hulk?

About the Author

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Whale Coast Conservation passionately lives by its slogan “Caring for your environment”.

Its small staff and volunteers are dedicated to

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WCC increases general public awareness of sustainability through environmental education, citizen-science research projects, community projects and campaigns.

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WCC facilitates educator development programmes to improve the capacity of educators to offer informed environmental content in their lessons across all learning streams.

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