Everything you need to know about ocean sunfish

Everything you need to know about ocean sunfish

Sunfish are among the most misunderstood animals on the planet. Strange-looking, they’ve been called everything from “the most useless animal” to “a baby wheel”, but the truth about sunfish is that they are far more amazing, and capable, than first impressions may lead you to believe.

They may look awkward in our world, but let’s see what the world of a sunfish is really like.

Mola mola. Photo courtesy of LA STarS.

What is a sunfish?

Sunfish are indeed strange. They have no true tail, and instead look like a fish that has been cut in half. Their elongated dorsal and anal fins flop about in sync, appearing as though a stingray has been turned on its side. When they do breach the surface, their fins might trick you into thinking that they are sharks, or their wide eyes might lead you to believe they are scared or confused. What is really happening here?

Sunfish comprise a group of five unique species that roam the deep waters of the world. Though they are fast-growing and cumbersome, don’t be fooled into thinking that sunfish can’t fend for themselves! Sunfish are voracious predators and one of the ocean’s greatest eaters.

Sunfish are also endless record breakers – as we’ll learn. They are the largest bony fish – the biggest one ever weighed being over 2.7 tons, growing to over 3m long and 4m from fin to fin!

The perfect name

In German, ocean sunfish are called “schwimmender kopf” – literally “swimming head”. In Polish they are called “samogłów” – “lonely head”. Some Hawaiian names for them include “kaumakanui” – “eyes stuck on it” – and “kunehi apahu” – “it’s been cut off”. In Afrikaans, Albanian, Icelandic, and Russian, they are “maanvis”,”peshku hënë”, “tunglfiskur”, “луна-рыба” – “moonfish”. In Hebrew they are “דאג השמש” – “sun watcher”. In Taiwan, they are known as “the fish that looks like a toppled car”.

Their modern scientific name Mola was coined by Swedish naturalist Carl Linnaeus in the 1700s. Linnaeus noted that when they basked in the sun, they looked like large, grey millstones – “mola” in Latin.

The 5 species of sunfish

Ocean sunfish (Mola mola)

The Mola mola is the most common of all sunfish species – and the one that most of this article is based on. They have round, knobbly tails, and long dorsal and anal fins. Their bodies are almost circular.

Their skin is gritty like sandpaper, covered with a thick layer of mucus. Their skin can vary from grey to blue or white and they can be covered in white spots, which are sometimes only visible when the fish is stressed.

Credit: Hectonichus (CC BY-SA 3.0)

Sharptail mola (Masturus lanceolatus)

Like the ocean sunfish, the sharptail also grows to huge sizes. Their distinguishing feature is their tail, which grows to a distinct point, rather than being rounded like all other species.

Their skin is smooth, with only a thin mucus layer and similar colouration to the Mola mola. Unlike other sunfish, they do not regularly bask in the sun.

Credit: Erik van der Goot (CC BY-SA 4.0)

Southern sunfish (Mola alexandrini)

Southern sunfish look almost identical to ocean sunfish.

Despite being known as the “southern” or “southern ocean” sunfish, they are found globally.

Slender sunfish (Ranzania laevis)

Slender sunfish are the smallest sunfish, never growing more than a metre in length. They have silver skin with striking blue stripes, by far the most colourful sunfish.

Their mouths are vertically orientated.

Slender sunfishare the rarest sunfish – but we do see them in South Africa on ocassion.

Credit: Explorasub (CC BY-SA 4.0)

Hoodwinker sunfish (Mola tecta)

Hoodwinker sunfish made headlines in July 2017, when they became the first new species of sunfish to be discovered in over 130 years. The common name “hoodwinker” alludes to the fact that they were hiding in plain sight – people simply always assumed they were Mola mola.

A creature of legend

Sunfish have inspired myths and legends in all the cultures that have encountered them, as this very informative article on OceanSunfish.org describes. The oldest written description of ocean sunfish was by ancient Roman philosopher Pliny the Younger – initially called porcus (pigfish), they were said to be the largest fish and would grunt at fishermen when caught. Pliny the Younger also noted that sunfish made loud noises when caught – eluding to the “croak” that they use to communicate when in distress, made by grinding their teeth together.

Polynesians, such as indigenous people of Hawaii, believe that killing slender sunfish is bad luck. They believe that slender sunfish are “makua”, the king of the mackerel, and that by killing a sunfish, mackerel will not find their way to the fishing grounds that the Polynesians depend on.

A slender sunfish. Photo courtesy of Hakai.

In 1600s Japan, the shōgun would accept a sunfish in lieu of taxes. In modern Japan, sunfish are beloved and swimming with the Mola is a popular pastime. The “sunfish Pokemon”, Alomomola, is even heart-shaped as a sign of the love that Japan has for them.

A 300-year-old text describing the anatomy of the ocean sunfish. Image courtesy of Deep Blue Home.

In British legend, St Piran, a famous healer, was kidnapped by pagans. He was tied to a millstone and thrown off the cliffs into the ocean. As he was pulled underwater, the millstone suddenly began to rush to the surface and he was able to grab onto it and be transported to a nearby beach. Was his millstone saviour really a sunfish? We’ll never know.

Ancient Californian indigenous people were the first people known to have encountered Mola mola, the ocean sunfish, almost 5000 years ago.

Where do sunfish live?

Sunfish inhabit the epipelagic zone, the upper 200m of the ocean where sunlight illuminates the world. However, sunfish can easily dive to depths of over 600m – into the mesopelagic zone, also known as the twilight zone. In this zone, waters become much darker, pressure increases and temperatures drop to almost freezing.

All sunfish inhabit temperate and tropical seas. The ocean sunfish, southern sunfish, slender sunfish and sharptail mola have all been identified circumglobally, in all five oceans. The newly discovered hoodwinker sunfish has only been identified in the southern Pacific and Indian oceans, from Chile to South Africa, but it likely has a similar distribution to other species. All five species have been spotted right here in Cape Town.

Lifecycle of a sunfish

Sunfish lay more eggs than any other kind of animal – in fact, a single Mola mola can lay over 300 million eggs at a time! This is necessary, as their reproductive strategy is quite risky – males and females spawn huge amounts of eggs and sperm into the water column, and basically hope for the best. They have no specific courtship rituals or mating seasons – they simply take advantage of being near to other sunfish (multiple research attempts have failed to link mating to any specific pattern).

When sunfish larvae hatch they are only 2mm long, and they stay in small schools as they develop into fry, for protection from predators. They grow rapidly, and once they develop their mature body shape they leave the school and brave the ocean on their own.

Larva of a Mola mola. Image courtesy of Sylvian TK.

Not only are ocean sunfish (Mola mola) the largest bony fish, but they also grow the most of any vertebrate – they grow to 60 million times their size from when they hatch! A typical growth rate for an ocean sunfish is 500g a day, but one kept the the Monterey Bay Aquarium grew 373kg in just 15 months!

Photo courtesy of sailrou59.

The lifespan of ocean sunfish is currently unknown, particularly the length of time they spend in the juvenile stage. Current best estimates for Mola mola and Masturus lanceolatus are approximately 20 to 25 years to attain their full size.

Postlarval stage of a Ranzania laevis. Photo courtesy of Frank Baensch.

Strong swimmers, keen sunbathers

Sunfish are the ultimate sunbathers – they can spend a lot of time on their sides on the water’s surface soaking up the sun’s rays. They do this to regulate their body temperature – after a deep dive into icy waters, they spend time on the surface to warm up. By using the sun and warm surface waters to regulate and raise their body temperature, sunfish are able to spend more time foraging in the cold depths. This dependence on the sun for thermoregulation means that sunfish are only able to hunt in deep waters during the day. At night they simply rest and scavenge in warmer surface waters.

A basking sunfish. Photo courtesy of Richard Dolan.

Trackers placed on sunfish have revealed that despite their reputation for being lazy, they only spend about half their lives in the top 10m of the ocean. Surprisingly little of this time is spent sunbathing – only 50% of their time is spent in shallow waters. This 50% accounts mostly for night time when they never dive into deep waters due to being unable to warm up again. The low nutrient content of their prey means that they must eat huge quantities – there isn’t time for rest if you are a sunfish.

Mola tecta. Photo courtesy of Ilse Reijs and Jan-Noud Hutten.

During a typical day, sunfish will dive approximately 40 times to depths of 90 to 170m, although the deepest dive ever observed for a sunfish was 644m. While some dives are far longer, the average deep dive for a sunfish lasts less than 10 minutes before they return to shallow waters. These short dives are not only necessitated by the need for the fish to warm up, but they are also a searching strategy for the largest abundances of jellyfish. This behaviour is also exhibited by leatherback turtles, blue sharks and swordfish.

Sunfish also migrate, following seasonal changes in jellyfish populations, although these migrations are usually only a relatively short distance when compared to other ocean wandering species.

Unlike most other fish, sunfish do not have a swimbladder – the organ used by fish to adjust their buoyancy. Instead, sunfish have a layer of jelly under their skin that makes them neutrally buoyant, meaning they neither sink nor float in the water (much like a scuba diver).

Interestingly, sunfish are able to warm their bodies so effectively that scientists suspect there are underlying adaptations we have yet to discover. This is further supported by the fact that large sunfish lose heat slower than small ones.

What do sunfish eat?

All sunfish are active predators, swimming many kilometres a day to find their prey. Until recently, it was believed that sunfish preyed only on jellyfish, but recent research has shown that their prey is much more diverse. Notably, sunfish have been found to spend a great deal of time in waters up to 200m deep hunting for siphonophores – animals that travel in colonies, like blue bottles.

Although tiny, floating hyrdozoa like this by-the-wind sailor (Velella velella) are a tasty snack for a sunfish. Image ©Jodi Frediani, source GrindTV.

Sunfish are able to eat these animals using their unusual teeth, which are fused together to form two bony plates, which look like a parrot’s beak.

Photo by Ruth Hendry/Te Papa.

Deeper in their throats, they have large sets of pharyngeal jaws – with their long claw-like teeth, these certainly look a lot more intimidating than the beak! These teeth are used to grind and shred food on its way to a sunfish’s stomach.

Photo by Ruth Hendry/Te Papa.

Despite their immense size, sunfish pose no risk to humans. The only humans who have been killed or injured by sunfish have been accidental, when the fish has jumped onto or damaged a boat after being hooked by a fisherman.

Sunfish give us a glimpse into the ocean’s health. We know very little about the ocean’s jellyfish population, but we do know that sunfish eat them. By surveying the population of sunfish in an area, it is possible to work out how many jellyfish are present – useful for studying the “rise of slime” being caused by ocean acidification and overfishing.

Why do they have such big eyes?

There is a lot more to the large, cartoonish eyes of sunfish than you may think at first. It’s easy to forget that sunfish are predators, and large eyes allow them to better spot prey over great distances and in relative darkness.

Photo by Devon Bowen/Two Oceans Aquarium.

Their visual acuity (the ability of the eye to focus) is far greater than many other ocean predators, such as catsharks, dolphins and beluga whales. This incredible focus helps them spot jellyfish that may be almost invisible in the water column. To put this in perspective, their vision is about 20/100, which means that what a human could see clearly 100m away, they need to be 20m away from to focus on – not bad considering they have a brain weighing less than 6g to process that information. Yes, that’s grams not kilograms.

Unlike most fish, sunfish can blink! They have strong muscles around their eyes that they can use to clean their eye, or to pull it back into its socket when feeling threatened.

What eats them?

Remember that layer of jelly under their skin? It makes sunfish taste absolutely terrible. Fortunately for sunfish, and the thousands of parasites they host, their gooey flesh and awkward size and shape are all reasons that humans throughout history couldn’t be bothered to eat sunfish except in a few rare occurences.

Sunfish are predated on by orcas, great white sharks and seals. Sea lions seem like the most vicious predator – often they will simply eat the fins off of the sunfish and leave it to die slowly (warning: link is upsetting). When young, sunfish are preyed on by bluefin tuna (Thunnus thynnus) and dorado (Coryphaena hippurus).

Photo courtesy of Richard Herrmann.

A fish with friends

Because they are large and slow moving, sunfish are susceptible to a wide variety of skin parasites – sometimes over 40 species on a single fish – and they are literally habitats that move. Many of these parasites are hosted by the jellyfish that the sunfish consume.

Fortunately, being charismatic has its perks and sunfish have developed some unique interspecies relationships to help them cope with this.

Cleaning station

Mola mola have a symbiotic relationship with a large variety of species, from seagulls to wrasse. Sunfish signal these species whenever they need a cleaning, flapping flippers on the surface to attract gulls, or hanging around near kelp forests to attract cleaner fish.

Where did weird fish like this come from?

Sunfish have some very weird relatives. Most marine fish have the ability to “cough”, to get food waste out of their mouths. A family of fish called Tetraodontiformes has members with a few common characteristics: bony or leathery armoured skin, the inability to bend backwards, fused or absent caudal fins, covered gill plates, etc. From this group, several species evolved that were able to use this coughing action to blow jets of water to dislodge or uncover prey – so far, sunfish match all the traits of Tetraodontiformes.

But what are these Tetraodontiformes that sunfish are so closely related to? If you looked closely at the baby sunfish, you might have a clue – they are puffer fish!

The postlarval stage of a Mola mola. The resemblance with a pufferfish is uncanny. Photo courtesy of Fishes of Australia.

Sunfish are, surprisingly, very recent additions to the tree of life – with the first “sunfish” being seen in the fossil record about 50 million years ago. Two ancient species are known: Eomolaive bimaxillaria and Austromola angerhoferi. To keep it simple, the main change from Eomola to Austromola and finally to modern sunfish is the adaptation of the beaked jaw from a flatter, toothier jaw. This is likely linked to an increased dependence on jellies as a food source – a survival adaptation needed to skirt competition with the increasing marine mammal population, much like the leatherback turtle did.

The sunfish tree of life. Image courtesy of San Francisco State University.

Ocean wanderers at risk

The International Union for Conservation of Nature (IUCN) recognises Mola mola as Vulnerable to extinction. To put this in perspective, other animals such as the polar bear, giant panda and cheetah are also Vulnerable – but have you heard of the plight of the sunfish before? This truly highlights the need for ocean ambassadors to bring to light the plight of pelagic species.

The greatest risk facing sunfish is being caught as bycatch – in fact in some industries, more ocean sunfish are caught than the actual target species! In South Africa alone over 340 000 are unintentionally caught a year! Longline, gill nets and mid-water trawls are all hazards to sunfish, commonly making up more 20-50% of all bycatch in industries such as swordfish, tuna and Cape horse mackerel.

Photo courtesy of Fishbio.

Littered plastic, particularly plastic shopping bags, are another major risk facing ocean sunfish. In the water column, these bags drift just like Mola mola’s favourite snack – a tasty jelly. A sunfish could choke to death immediately, or starve slowly from intestinal blockages. This is a threat facing many pelagic species, and another great reason to Rethink the Bag.

Smaller species such as the sharptail mola and the slender sunfish are not currently recognised as being at risk, and the southern and hoodwinker sunfish are listed as being data deficient, although likely face the same perils as Mola mola, due to sharing a range and lifestyle.

Photo courtesy of Scuba Diver Life.

Sunfish have a critical conservation question to ask us: “How is it possible that an animal that nobody eats, and which has no commercial value, is being fished to extinction?”

Remember that sustainable seafood doesn’t just mean eating species that aren’t endangered, it means choosing ones that are caught without damaging the populations of others. It’s easy to make the green choice and help protect species like the ocean sunfish – simply choose MSC Certified frozen food and use the WWF SASSI app to ensure that you make the green choice when eating out. Stay sunny!

Learn more about the Two Oceans Aquarium’s sunfish conservation and research work.

Photo courtesy of Wikimedia Commons.

Sunfish in captivity

The ocean sunfish is an unusual fish that draws a lot of public attention, but is rarely kept in aquariums. The Two Oceans Aquarium is one of a select few that keep them on occasion, others being the world-class Nordsøen Oceanarium in Denmark, Monterey Bay Aquarium in the USA, Lisbon Oceanarium in Portugal, Kaiyukan Aquarium in Japan and L’Oceanogràfic in Spain.

The reason that sunfish are so rarely kept in captivity is because to their large size and cumbersome swimming patterns – an exhibit hosting one needs to be big enough for the sunfish to move freely. Their rapid growth also poses challenges when it comes time to release them. Capturing an ocean sunfish without injuring it is also a challenge requiring special equipment, and in most cases sunfish that are displayed have been rescued, e.g. bycatch survivors from fishing vessels or, as is the case for sunfish at the Two Oceans Aquarium, ones that become trapped and/or injured in ports, drydocks and harbours.

As an ocean wandering species, ocean sunfish are surprisingly well suited to captivity. They do not have strong migratory instincts; in nature, they simply follow the food. They also do not display social behaviours or mating cues – other factors that may affect an animal’s suitability to spend time at an aquarium.

Always on the lookout for nutrient-rich food sources, this Mola mola has no hesitation about accepting squid from an Aquarium diver. Photo by Cleeve Robertson.

Ocean sunfish are naturally curious and friendly, likely due to their reliance on other species that rid them of parasites. In an aquarium they have little trouble interacting with other exhibit inhabitants or divers – in fact, the ocean sunfish brought to the Two Oceans Aquarium at the end of 2017 was readily eating from the hands of scuba divers within a day.

Parasites are removed by hand. Photo by Devon Bowen/Two Oceans Aquarium.

Sunfish only dive deep into the mesopelagic zone during the day as their prey, such as siphonophores, take shelter in this relative darkness. If food is available near the surface, such as floating hydrozans, the sunfish will not dive unnecessarily. Some sunfish spend large parts of their lives in shallower coastal waters where dives to great depths are not possible – they are highly adaptable and quickly learn how to exploit alternative foods, despite that tiny brain!

Although most of their feeding takes place at great depths, where food is abundant, ocean sunfish are adaptable enough to feed near the surface if the opportunity arises. Photo courtesy of Jodi Freduani/GrindTV.

In nature, the sunfish’s diet is composed primarily of hundreds of kilograms of jellyfish a day – a diet that cannot be replicated in an aquarium. However, despite jellyfish being their main source of food, sunfish will preferentially eat squid, fish and brittle stars in nature as these are more nutritious. Squid is thus the preferred food for the ocean sunfish in captivity – and has the added benefit of not infesting the sunfish with parasites like jellyfish do. As sunfish receive their water from the food they eat, special gel meals are also formulated to keep them healthy and hydrated.

Compared to their prey, sunfish are huge, necessitating their dive to great depths where jellies are more abundant. The availability of more nutrient-rich food sources negates this need. Photo courtesy of Jodi Freduani/GrindTV.

These factors allow well looked after sunfish to do well in captivity, provided care is taken to ensure that their enclosures are large enough and that they are released before they outgrow them Their adaptable natures and diverse feeding and hunting techniques enable them to do well in this space, and to thrive when they are once again released into the wild.

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