The Ocean Sunfish or Mola (Mola mola) is the world's heaviest bony fish. The distinctive body shape is laterally compressed and appears bluntly terminated to the rear, as if the tail had been lopped off. Molas have a reduced skeleton, with fewer vertebrae than any other fish. Metamorphosis from larva to adult is remarkable in that, unlike most fish, Molas pass through two distinct larval phases—a typical Tetraodon pufferfish-like larval and another highly transformative stage resulting in the complete absorption of the tail (Fraser-Brunner 1951). Molas have been claimed to be the most fecund vertebrates known, with a single female reportedly producing several hundred million eggs at once (Schmidt 1921, cited in Pope et al. 2010).
Molas have a very broad global distribution, occurring in both temperate and tropical waters of the Atlantic, Indian, and Pacific Oceans. Due to their primarily pelagic (open water) distribution, studying their ecology and behavior is challenging. They are most easily observed basking at the surface, a behavior for which several functions have been proposed, including warming themselves after deep dives into cold water and presenting themselves to seabirds and other fishes that remove parasites (such as Pennella copepods) from their bodies (Abe et al. 2012; Pope et al. 2010 and references therein). Although Molas were long believed to be sluggish swimmers, drifting passively in ocean currents, based on investigations in recent years (e.g., Cartamil and Lowe 2004; Sims et al. 2009; Dewar et al. 2010) it is now clear that Molas do not necessarily travel with prevailing currents and instead appear to be relatively active predators that are capable of migrating at least moderate distances, perhaps in response to shifts in regional productivity and temperature. Gelatinous zooplankton (such as jellyfishes, salps, and pyrosomes) comprise a large fraction of the diet of these fish, although Syväranta et al. (2012) have used stable isotype analyses to argue that the extent of dependence of Molas on gelatinous zooplankton may be overstated, as was suggested by Pope et al. (2010) (but see Logan and Dodge 2013 and Harrod et al. 2013). Stable isotope studies of Molas in the Mediterranean Sea by Cardona et al. (2012) supported the assertion that gelatinous zooplankton are at least a major component of the diet. Hays et al. (2009) compared foraging depths for Molas and Leatherback Turtles (Dermochelys coriacea)--the heaviest bony fish and the heaviest sea turtle, both of which are believed to feed heavily on gelatinous zooplankton. They found that while Molas can feed from the surface to depths greater than 500 m, Leatherbacks are limited to relatively shallow waters (<200 m), presumably because of the constraint that they must return to the surface to breathe air.
Although Molas are caught and sold in only a few parts of the world, such as Japan and Taiwan, in recent years they have been taken incidentally in substantial numbers in many fisheries, including the swordfish drift gillnet fishery off California and Oregon (U.S.A.), the illegal Spanish driftnet swordfish fishery off the Gibraltar Straits in the Mediterranean, and the the tuna and swordfish longline fishery off the coast of South Africa (Dewar et al. 2010 and references therein).
The taxonomic treatment of fishes in the family Molidae has fluctuated considerably through time. Current molecular, morphological, and distributional data appear to support the recognition of at least two species in the genus Mola: M. mola and the far less familiar M. ramsayi (which is likely limited to the southern hemisphere), as well as possibly at least one more. Because of the unresolved taxonomic issues in the genus Mola, it is possible that some studies supposedly done on M. mola may actually apply to a different closely related species. Two additional species are included in the family Molidae, Masturus lanceolatus and Ranzania laevis. The Molidae are closely related to the Tetraodontidae (pufferfishes) and Diodontidae (porcupine fishes). (Bass et al. 2005; Yoshita et al. 2009; Pope et al. 2010 and references therein)
Pope et al. (2010) provide a wide-ranging and thorough review of the limited available data on the taxonomy, morphology, ecology, and conservation of Molas.
(Bass et al. 2005 and references therein; Pope et al. 2010 and references therein)