Ocean sunfish

• 1. Introduction

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  The ocean sunfish, known to scientists as Mola mola, is an animal whose numbers are hard to believe. A single female can release up to 300 million eggs at one time, more than any other known vertebrate. Those eggs hatch into larvae only 2.5 mm long that weigh less than a gram. A fish that survives can grow to 60 million times its starting weight, arguably the most extreme growth of any animal with a backbone.

  The adult that results looks less like a finished design than an unfinished sketch. So how does a creature built like a swimming head live in every ocean? What does it actually eat, and why does it lie at the surface? Why are people now worried about its future, even as aquariums struggle just to keep it alive?

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• 2. Names and Mistaken Identities

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  In 1554, the French polymath Guillaume Rondelet described the fish in his work de Piscibus and called it Orthagoriscus, meaning sucking pig. He was comparing the shape of its body and mouth to that of the animal. Later naturalists filed it with the pufferfish as Tetraodon mola. The word mola is Latin for millstone, a nod to the fish's gray color, rough texture and round shape. Today it sits in its own genus, Mola, and its own family, Molidae, alongside Mola tecta and Mola alexandrini, the latter once known as Mola ramsayi.

  Mola alexandrini matters because of a long-running mix-up. For years, the ocean sunfish was credited as the heaviest bony fish in the world. That title actually belongs to its close relative, Mola alexandrini.

  The English name sunfish comes from the animal's habit of sunbathing at the surface of the sea. Elsewhere, names fix on its shape. Speakers of Catalan, Dutch, French, German, Spanish, Russian and many other languages say moon fish, because of the round outline. Germans also call it Schwimmender Kopf, or swimming head. In Polish it is samogłów, meaning head alone, since there is no true tail. Swedish klumpfisk, Dutch klompvis and Finnish möhkäkala all mean lump fish. The Chinese translation of its scientific name means overturned car fish.

  The word sunfish on its own can cause confusion. It also names the freshwater sunfish of the family Centrarchidae, which are unrelated to Molidae. Only the terms ocean sunfish and mola point exclusively to the Molidae.

  On the island of Lembata in Indonesia, women in the village of Lamalera wear sarongs patterned with this fish, which they call kebuku. In 2015, two Boston Harbor fishermen, Michael Bergin and Jason Foster, found one and mistook it for a sea turtle, a baby whale, a tuna, Moby Dick and a flounder. They reported it to the Coast Guard in a video that made them briefly famous. In 2017, a rude Facebook copypasta calling the sunfish useless trash went viral.

  The animal's real nature is stranger than any of these names, beginning with a skeleton that is largely not bone.

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• 3. A Body Built Differently

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  The spinal column of Mola mola has fewer vertebrae than that of any other fish, and it is short for the size of the body. The skeleton is mostly cartilage, which is lighter than bone. That lightness lets the sunfish grow to sizes that are impractical for other bony fishes. The axial musculature is lost completely during development, and there is no swim bladder.

  Buoyancy comes instead from a stiff, gelatinous layer under the skin. This layer is about 90 percent water, held in a meshwork of collagen and elastin, and it works somewhat like an exoskeleton. Because of its greasy texture, it may also contain lipids. The larger the fish, the greater the share of its mass this layer makes up. Some sources say the internal organs hold the neurotoxin tetrodotoxin, as in other poisonous relatives, while others dispute it.

  The missing tail is replaced by the clavus, a lumpy pseudotail formed where the dorsal and anal fins converge. It keeps 11-14 fin rays and ends in rounded ossicles, and the fish uses it as a rudder. The dorsal and anal fins are long, so the fish is often as tall as it is long, while the pectoral fins are small and fan-shaped. Near the surface, the protruding dorsal fin is sometimes mistaken for a shark's. The giveaway is motion, because the sunfish sculls its dorsal and anal fins from side to side in a way most fish do not.

  The mouth belongs to the order Tetraodontiformes, which also includes pufferfish, porcupinefish and filefish. Four fused teeth form a beak that prevents the mouth from closing fully, and pharyngeal teeth sit further back in the throat.

  A mature sunfish averages 1.8 m in length and 2.5 m from fin tip to fin tip, and typically weighs between 247 and 1000 kg. Specimens up to 3.3 m in height have been recorded. The largest on record washed ashore in New Zealand in 2006, measuring 3.3 m and weighing 2300 kg.

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• 4. Skin and Parasites

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  The skin of an adult can be up to 7.3 cm thick on the underside, packed with reticulated collagen. It carries denticles and a layer of mucus rather than scales. On the body it can feel as rough as sandpaper, while the skin on the clavus is smoother.

  Adults range from brown to silvery-grey or white, with mottled patterns that vary by region. The back is usually darker and the belly lighter, a form of countershading camouflage. Mola mola can also shift its skin from light to dark, especially when under attack.

  More than 40 species of parasites may live on or inside a single sunfish. One of the most frequent is the flatworm Accacoelium contortum. The fish has several ways of seeking relief. In temperate regions, drifting kelp fields shelter cleaner wrasses and other fish that pick parasites from visiting sunfish. In the tropics, the sunfish asks reef fishes for the same service.

  Basking on its side at the surface lets seabirds feed on parasites from the skin, while smaller fish nibble at the underside. Sunfish have also been reported to breach, clearing the water by about 3 m, in an apparent attempt to shake loose embedded parasites.

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• 5. Life Below the Surface

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  Early researchers believed sunfish mostly drifted with ocean currents, which led some to call them megaplankton. Tracking has shown otherwise, with individuals recorded swimming 26 km in a day at a cruising speed of 3.2 km/h. Adults also spend much of their lives actively hunting at depths greater than 200 m, in both the epipelagic and mesopelagic zones. The popular picture of a fish loafing at the surface is therefore incomplete.

  Temperature sets firm limits. Sunfish are most often found in water warmer than 10 C, and long periods at 12 C or lower can cause disorientation and eventual death. Surface basking may be a way of thermally recharging after cold, deep feeding dives. Sightings in cooler waters outside the usual range, such as those southwest of England, may point to rising marine temperatures. Sunfish are usually seen alone, and only occasionally in pairs.

  Genetic differences between sunfish in the Atlantic and Pacific appear to be wide, yet differences between the Northern and Southern hemispheres are minimal.

  The diet was once thought to be mainly jellyfish and other gelatinous animals such as ctenophores, salps and medusae. Genetic analysis changed that picture. Sunfish turn out to be generalist predators, taking small fish such as flounder, eel larvae, squid, other molluscs, crustaceans and soft-bodied invertebrates. Jellyfish and salps make up only around 15 percent of the diet, and eel grass is sometimes swallowed too. That range suggests feeding at every level, from the surface to the seafloor in some areas.

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• 6. Growing Up

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  A young sunfish at the Monterey Bay Aquarium went from 26 to 399 kg and nearly 1.8 m in height in just 15 months. That is one of the few measured glimpses of growth, because the growth rate in the wild remains undetermined. Sunfish may live up to ten years in captivity, but natural lifespan is still unknown. Estimates put it at 2-23 years for females and 1-16 years for males.

  Mating practices are poorly understood, though spawning areas have been suggested in the North Atlantic, South Atlantic, North Pacific, South Pacific and Indian oceans. Eggs are released into the water and fertilized externally by sperm. The fry look like miniature, spiky pufferfish, with body spines that vanish as they grow. They lack the large pectoral fins and tail fin of the adult form. Young sunfish school for protection, a habit they abandon as they get bigger.

  Bluefin tuna and mahi mahi prey on the young. Adults face few threats because of their size and thick skin, though orcas, sharks and sea lions will eat them.

  In 2016, researchers from China National Genebank and A*STAR Singapore, including Nobel laureate Sydney Brenner, sequenced the sunfish genome. They found several genes that might explain its rapid growth and large body. The genome is compact, at 730 Mb, as in its relative the fugu. The data suggest sunfish and pufferfishes diverged about 68 million years ago. Extinct relatives of Mola mola lived in the Oligocene and Miocene epochs, while the earliest known fossils of Mola mola itself come from Holocene archaeological middens.

  Much still remains unknown, including migration patterns, population structure, diet ecology and survival after release. Scientists are working on aerial surveys, pop-off satellite tags, genetic analysis of tissue samples and amateur sighting data.

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• 7. Appetite and Bycatch

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  In Japan, Korea and Taiwan, the flesh of the ocean sunfish is considered a delicacy, and Taiwan and Japan hold the largest markets. Every part is used in cooking, from the fins to the internal organs, and some parts appear in traditional medicine. The European Union forbids fishery products from the family Molidae under Regulation (EC) No 853/2004, citing toxins harmful to human health.

  The larger threat is accidental. In California, sunfish make up nearly 30 percent of the catch in the swordfish fishery that uses drift gillnets. In the Mediterranean swordfish industry, the figure climbs to 71-90 percent. The IUCN lists the species as vulnerable with a decreasing population, and drift gillnets are the main threat. Bycatch and destruction of sunfish are unregulated worldwide.

  Some fishermen treat sunfish as worthless thieves of bait and cut off their fins. Without the dorsal and anal fins the fish cannot propel itself, so this finning leads to its eventual death. Floating litter adds another danger, since plastic bags resemble the jellyfish that sunfish sometimes eat. A bag can choke the fish or fill its stomach until it starves.

  Sunfish themselves pose little danger to people. In 2005, a 1 m sunfish leaped onto a family's boat off the coast of Pembrokeshire, Wales, and landed on a 4-year-old boy, causing slight injuries. Collisions with boats are common in some regions and can damage hulls or the propellers of larger ships, as well as hurt the fish. Divers have little to fear, and at popular dive sites some sunfish have reportedly grown familiar with them.

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• 8. Life in Captivity

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  Kukey, a sunfish at Kamogawa Sea World in Japan, arrived in 1982 at 72 cm. He set a captivity record of 2,993 days, or eight years, and measured 187 cm when he died. Few aquaria hold sunfish at all, because their care is so demanding. At the Kaiyukan Aquarium in Osaka, the sunfish is reportedly as popular as the larger whale sharks, and the Lisbon Oceanarium in Portugal shows them in its main tank.

  The Monterey Bay Aquarium is said to have received the first ocean sunfish held in a United States aquarium in August 1986. Earlier cases exist, however. Marineland of the Pacific on the Palos Verdes Peninsula held at least one by 1961, and in 1964 it held a 650 lb specimen, then said to be the largest ever captured. A 1000 lb sunfish had been brought alive to Marineland Studios Aquarium near St. Augustine, Florida, in 1941.

  Because nobody had kept sunfish on a large scale, Monterey staff had to invent their own methods for capture, feeding and parasite control. By 1998 they could hold one for more than a year before releasing it. Mola mola became a permanent part of the aquarium's Open Sea exhibit. The largest Monterey specimen was euthanized on the 14th of February 2008 after a long period of poor health.

  Tank design is a constant worry, since a sunfish cannot easily turn its body and may scrape itself on the walls. In a small tank, a hanging vinyl curtain has been used to turn a boxy shape into a rounded one. A better answer is enough room to swim wide circles, in a tank deep enough for a fish that may stand 3.2 m tall. Feeding next to faster, more aggressive fish is hard, but sunfish can learn to go to a floating target and take food from a pole or a human hand.

  One sunfish at an aquarium in Kaikoyan stopped eating and seemed stressed after the aquarium closed for renovations. Owners suspected digestive trouble or parasites. Staff eventually concluded the fish missed its visitors, so they placed cardboard cutouts of people around the tank. The sunfish recovered and began waving its fins again.

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