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Ocean Sunfish


16 May 2018, by Matthew Oldfield

Sunfish are animals of extremes. They are the world’s heaviest bony fish, produce more eggs than any other vertebrate and exhibit the greatest increase in size from hatching as a fry to mature adulthood - up to 60 million times their body weight at birth. Whilst there are several places around the world where sunfish can be seen on a regular basis, Bali is the only dive destination in the world where these remarkable fish can be encountered with any reliability. The island’s famous ‘Mola mola’ arrive every summer around the islands of Nusa Penida and Lembongan and have spawned an entire dive tourism industry that draws visitors around the world hoping for an encounter with these incredible animals.

EVOLUTION


Sunfish belong to the Tetraodontiformes family, which include pufferfish, porcupinefish and filefish. Like their cousins, the front four teeth of sunfish are fused together to create a beak which prevents sunfish from shutting their mouths. These fish have also taken body modification to an extreme, shortening their spine and losing much of the bony tissues from their skeleton - the reason why sunfish are able to grow to such remarkable sizes.

The first ancestors of modern sunfish date from the Eocene, over 40 millions years ago, and 9 extinct species have been identified so far. In today’s oceans five different species of sunfish remain - the ocean sunfish, southern ocean sunfish, slender sunfish, sharptail mola and the recently described hoodwinker sunfish.

The smallest of these unusual animals is the slender sunfish which can grow to around 1m in length, whilst the largest is the ocean sunfish that can reach a truly monstrous 3.3m in length, 4.2m across the fins and 2,300kg in weight. In fact, the Mola mola is  the heaviest known bony fish in the world.

DISTRIBUTION

Sunfish can be found throughout the world’s temperate and tropical oceans, their range limited only by the temperature of the water in which they find their food. The oceanic sunfish, slender sunfish and sharptail mola are found around the world, whilst the southern oceanic sunfish and hoodwinker sunfish have only been identified in the southern hemisphere. In Asia, divers have a good chance of encountering the southern oceanic sunfish in locations such as Bali, Alor and the Maldives.

All sunfish are pelagic - open ocean fishes - spending their lives drifting with the currents or actively swimming in search of food. The different species of sunfish prefer to spend their lives at different depths: the sharptail mola for instance prefers the sunlit waters of the top 200m during the day, making regular descents into deeper water to feed, whilst the oceanic sunfish is capable of diving down into the twilight world of the mesopelagic zone beyond 200m. For many years it was thought that these fish would just drift with the currents and their unusual body shape prevented them from swimming well. However, tracking studies have shown they can cover great distances whilst cruising at a slow-but-steady speed of over 3 kilometres an hour, as well as accelerating fast enough to leap clear of the water - an incredible feat for such a heavy animal!

ECOLOGY & BEHAVIOUR


Very little is known about the mating behaviour of sunfish, although they are known to produce vast numbers of eggs - over 300 million were found in the ovaries of an average sized female oceanic sunfish, a record for any vertebrate animal. After being fertilised, the eggs drift in the plankton before hatching and releasing tiny larvae just 2.5mm in length and weighing less than a gram. These larvae then undergo colossal growth, first developing into fry that resemble miniature, spiky pufferfish, then slowly taking on the adult form of a sunfish. From larvae to fully-grown adult, the sunfish may have grown up to 60 million times their birth weigh - one of the most extreme developments of any vertebrate animal.

It was long thought that sunfish fed exclusively on jellyfish buts scientists have since learnt that their tastes are a little wider and adult sunfish will feed on jellyfish, salps and other jelly-like animals, as well as small fish, larvae, squid, crustaceans, worms and even seagrass. Much of the behaviour of sunfish is dictated by their constant search for food. In the colder depths below the sunlit layers of the oceans, jellyfish and other species can be found in abundance, and so sunfish must dive down repeatedly to feed. Like all fish, sunfish are cold-blooded and so they must return to the warmer waters closer to the surface to warm up and ‘recharge’ after feeding, even lying flat at the surface to expose the maximum area of their bodies to the warming rays of the sun.

Sunfish are well known for their interactions with other species. These fish are susceptible to skin parasites and will often use the services of cleaner wrasse and other fish at floating rafts of seaweed or on coral reefs. Seabirds have even been observed landing on ‘sun bathing’ sunfish and picking off parasites. At locations such as Bali, sunfish are normally seen at cleaning stations where reef fish and cleaner wrasse remove and feed on skin debris and parasites from the surface of the sunfish.

THREATS

Little is known about populations of sunfish around the globe and whilst none of different species are considered threatened or endangered, there is growing concern that sunfish numbers may be declining as a result of fisheries that employ gillnets - hanging walls of nets that drift in the ocean currents and sweep up everything that swims by. In particular, the swordfish industry in the Pacific and Mediterranean has been shown to catch huge numbers of sunfish - the vast majority of which are simply discarded as by catch. Sunfish are also threatened by the huge increase of plastic waste in the oceans, which closely resemble jellyfish - sunfishes main diet. Scientists are now starting to get a better understanding of the movements of sunfish by employing satellite tags, aerial surveys and collecting information about sightings and bycatch figures in the hope that these incredible fish can be better protected.

MOLA WHAT?

Bali's sunfish are commonly known by a scientific name - Mola mola - however this identification is incorrect and the sunfish found around Penida and Lembongan are actually Mola ramsayi - the bumphead sunfish, named after a shallow bump on the head. To further complicate matters, it was discovered in 2017 that this species had actually been described back in 1839 as Mola alexandrini, and then promptly forgotten - until now. With precedence given to the first formal description of a species, Bali's sunfish are now correctly identified as Mola alexandrini - although local divers will probably always call them Mola mola!

WANT TO BE A BETTER SUNFISH DIVER?

In conjunction with marine scientists, divers and marine managers, a code of conduct for diving with Mola mola was established in the early 2000’s. Follow these simple steps and help reduce your impact as a diver:

  • Always approach Mola Mola very slowly within its field of view;
  • If the fish are just entering the cleaning station, do not approach until the cleaning has begun and the fish have been stationary for at least 1 minute;
  • Maintain a minimum distance of 3m (or 2 body lengths) from the closest sunfish when animal is at a cleaning station;
  • Maintain a minimum distance of 10m (or 5 body lengths) when an animal is unsettled (not in cleaning) and considering approach to the reef;
  • Do not touch or feed the sunfish;
  • Do not swim behind the Mola Mola as this can startle the animal;
  • Do not swim under the fish as your bubbles will disturb cleaning behaviour;
  • Wherever possible, do not block the Mola Mola’s escape route off the reef or pathway onto a cleaning station;
  • If a Mola Mola approaches you, remain still and do not touch it. If you touch it, you will remove the layer of mucus that protects it against infection;
  • Do not use flash photography as this often disturbs the fish;
  • Do not use personal underwater motorized propulsion vehicles or make unnecessary loud noises;
  • Be courteous to other divers and restrict your interaction time to 5 minutes when other groups are present;
  • Only dive with companies which have endorsed and adhere to the Code of Conduct. Follow the directions of your dive guide.

Images courtesy of Scubazoo.

Matthew Oldfield
Co-founder of ZuBlu