The ocean sunfish, Mola mola, is found in every ocean in the world. Known for its unusual body shape resembling a large “head” with no tail and long fins, and its large size (up to 5,000 lbs), the biology of this species is poorly understood. Many of the basic biological questions about ocean sunfish remain a mystery. These include: how the fish are distributed in the world's oceans, how fast they swim, how they behave towards each other and other kinds of animals, or how and when they mate. Recent studies have shown that the number of ocean sunfish may be declining due to the large numbers caught as bycatch in commercial fisheries in the Pacific and Mediterranean, and its increasing popularity as a food fish in Asia . Though it is commonly found in the Atlantic Ocean, there is no data on how many sunfish there are in the area during different times of year, or if there are occurrences of ocean sunfish bycatch there. Because there is little information on its basic biology, scientists have no idea how the global population of ocean sunfish is faring.
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Four species of Molidae are currently recognized: Ranzania laevis (slender mola) , Masterus lanceolutus (sharptailed mola) Mola mola (common mola), and Mola ramsayi. Until recently, M. ramsayi was infrequently mentioned, but a 2005 study on genetic divergence showed strong evidence suggesting M. ramsayi is a separate species from the more common M. mola. M. mola is known for its unusual body shape resembling a large “head” with no tail and long fins, and its large size (reaching 14 feet and weighing up to 5,000 lbs). Ocean sunfish lack caudal bones, ribs, pelvic fins, a swim bladder, and have fewer vertebrae than any other fish. Lacking a tail, sunfish instead have a broad, stiff lobe at the end of the body which acts as a ruder, called the clavus). Also unusual is the mola skeleton which lacks bony tissue and resembles cartilage. The tough skin of the mola measures up to three inches in thickness and forms a rubber-like armor around the body. The skin is covered with a layer of slimy mucus and many parasites, abrasions and scars. M. mola are known to host some 40 genera of parasites.
Little is known about Mola mola's basic ecology, distribution, or population dynamics and the global status of the species is completely unknown. Sunfish were so named for the common behavior of lying on their side near the surface, appearing to “sunbathe”, often with the dorsal fin protruding from the water. As adults, M. mola are mostly found alone and occasionally in pairs. They feed primarily on gelatinous zooplankton such as jellyfish and ctenophores in the water column, but other items found in M. mola guts include squid, sponges, starfish, eel grass, crustaceans and small fishes. Known predators of M. mola larvae include bluefin tuna and mahi mahi. Adult M. mola in the Pacific are preyed on by sea lions and orcas.
Members of the family Molidae are recognized as the most fecund vertebrate, with single females producing as many as 300,000,000 eggs at one time. Five spawning areas have been suggested for members of the Molidae family in the central gyres of the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Oceans. However, there are few M. mola larvae on record anywhere in the world.
Ocean sunfish are globally distributed, and M. mola is a common resident of the northeast United States in offshore waters during the spring and summer months with an estimated summer population of 18,000. It is virtually absent in these areas in the winter and fall. M. mola spend the majority of their time in near surface waters (75%) making periodic dives between 40 – 150 meters (and as deep as 500 m) which are thought to be related to prey acquisition. It has been suggested that mola spend time floating in surface waters as a mechanism of “thermal recharging” after deep dives in cold water). Molas breach, leaping out of the water by as much as two body lengths, which may be a mechanism of parasite removal. There are conflicting reports about the swimming ability of ocean sunfish. Most existing information suggests M. mola move primarily by passive transport via ocean currents but a recent study using acoustic telemetry reported a highly directional swimming ability not related to current direction. Because ocean sunfish are such large fish that are easily sighted from both the air and sea and they prefer to swim in nutrient-rich waters, information on the distribution of Mola mola may serve as a useful “indicator” of nutrient rich areas of the ocean where other endangered marine animals may be found such as tuna, sea turtles, and whales.