The Ocean Sunfish Releases 300 Million Eggs and Still Loses Ground

A single female ocean sunfish can release up to 300 million eggs at one time, the highest fecundity ever measured in a vertebrate, and almost none of them will ever become a fish.

A Body Built From Statistical Improbability

The number belongs to Mola mola, the heaviest bony fish species alive. The record specimen washed ashore in New Zealand in 2006, measuring 3.3 metres in length and weighing 2,300 kilograms. Typical adults are more modest, sitting between 247 and 1,000 kilograms with a mean length of 1.8 metres, but even the median animal is unlike any other fish a diver is likely to meet in open water. When its dorsal and anal fins are fully extended, a mature sunfish is roughly as tall from fin tip to fin tip as it is long, up to 3.3 metres in vertical span.

That symmetry is unusual, and it is a clue that the sunfish is not built like other fish. The lineage split from the pufferfishes about 68 million years ago, according to comparative analysis of the Mola mola genome, which was sequenced in 2016 at approximately 730 megabases. Over that interval, evolution has trimmed the animal's spinal column so severely that it contains fewer vertebrae, and is shorter relative to body size, than in any other fish. The genus name is Latin for millstone, a reference to the fish's grey colour, rough hide and disc-like shape. Linnaeus described the species in 1758; Johann Heinrich Linck the Younger established the genus Mola as valid in 1790.

No Tail, and a Sculling Gait

What is missing from that outline is the piece of anatomy every other fish takes for granted. The sunfish has no true caudal fin. As the fish matures, its tail folds inward to form a rounded, rudder-like structure called a clavus, which retains 11 to 14 fin rays and terminates in rounded ossicles. Propulsion is generated instead from the top and the bottom, by synchronized sweeps of the long dorsal and anal fins moving in the same direction. The result is a distinctive sculling gait that reads, from a distance, as awkward, and that closer study now recognises as efficient enough to carry the animal across genuine distances.

Individual sunfish have been tracked swimming 26 kilometres in a single day at a cruising speed of about 3.2 kilometres per hour. They spend a large portion of their lives actively hunting at depths greater than 200 metres, occupying both the epipelagic and mesopelagic zones. That descent is not casual. Mola mola is most often found in water warmer than 10 degrees Celsius, and prolonged exposure to water at 12 degrees or below can lead to disorientation and death. The famous behaviour of surface basking, floating on one side with the largest possible profile turned toward the sun, is now interpreted as a thermal-recharging strategy that lets the fish rewarm after cold, deep hunting dives. It is not laziness; it is a return trip.

The diet along those dives has also been revised. Genetic gut analysis has shown that jellyfish and salps account for only about 15 percent of what Mola mola eats. The rest is small fish such as flounder, fish larvae, squid, molluscs, crustaceans and other soft-bodied invertebrates, most of it taken hundreds of metres down. The older summary from the Mola genus entry, that "their food of choice is jellyfish, though they will eat small fish and large amounts of gelatinous zooplankton and algae," describes a diet that gut analysis has since displaced. The teeth are fused into a beak-like plate, and the fish cannot fully close its relatively small mouth.

The Arithmetic of Getting That Big

The animal that hatches from one of those 300 million eggs is 2.5 millimetres long and weighs less than a single gram. A surviving fry can grow to roughly 60 million times its birth weight before adulthood, an increase greater than that of any other known vertebrate. The math is easier to see in captivity: at the Monterey Bay Aquarium, one juvenile grew from 26 kilograms to 399 kilograms in 15 months, reaching a height of nearly 1.8 metres in the same window. At Kamogawa Sea World, a captive sunfish known as Kukey was held from 1982 for 2,993 days, roughly eight years, and measured 187 centimetres at death.

Protecting that mass takes armor. The skin on the ventral surface can be up to 7.3 centimetres thick, underlain by a gelatinous layer that is roughly 90 percent water, laced with a meshwork of collagen and elastin. More than 40 species of parasite have been recorded living on or inside the ocean sunfish, one of the most common being the flatworm Accacoelium contortum, and the hide is what keeps most of them from mattering. Adult sunfish are preyed on at sea by sea lions, orcas and large sharks; younger sunfish are taken by bluefin tuna and mahi mahi. Encounters with people are rare and, for the most part, harmless. In 2005, a one-metre sunfish leapt from the water and landed on a four-year-old boy on his family's boat off Pembrokeshire, Wales, causing slight injuries.

The Paradox at the Driftnet

None of that engineering matters at the surface. In the California swordfish fishery, ocean sunfish make up nearly 30 percent of the total drift-gillnet catch. In Mediterranean swordfish operations, sunfish bycatch has been recorded at between 71 and 90 percent of the total catch. In the busiest sunfish waters of the Mediterranean, nine out of every ten animals hauled up in a swordfish net is a fish that no one set out to catch.

The Wikipedia entry for the genus Mola summarises the reproductive strategy plainly: "They breed by producing around 300 million eggs directly into the ocean, which are externally fertilized by sperm into the water column." That is the whole plan. Broadcast into the open water column and hope the numbers hold. They no longer do. The IUCN Red List classifies Mola mola as Vulnerable, with a decreasing population trend, and identifies drift gillnets as the principal threat.

What the Fish Actually Is

The ocean sunfish is not a curiosity. It is the endpoint of 68 million years of evolutionary drift from a common ancestor with the pufferfishes, an animal that traded its tail for a clavus with 11 to 14 fin rays, its skeleton for the shortest vertebral column of any fish, its speed for 26 kilometres of steady daily range at 3.2 kilometres per hour, its skin for a 7.3-centimetre wall over a gelatinous layer that is 90 percent water, and its reproductive caution for 300 million eggs at a time. Three hundred million eggs, released into the open ocean, and still the animal that hatches from them is losing ground.

Sources

• Ocean sunfish (Wikipedia)
• Mola (genus) (Wikipedia)