The World's Largest Bony Fish

Reaching a weight of 1500 kg, the pelagic Ocean Sunfish (Mola mola) is a spectacular animal. Its distinctive laterally compressed shape and “chopped off” appearance make it one of the most distinctive of fish (1). Its truncated shape lends it its common name, as well as its tendency to bask on its side, close to the surface. One of only five members of the Molidae family, it has some unique features including the largest number of eggs produced by a vertebrate (300,000,000 eggs at one time) and larval metamorphosis that passes through two distinct stages (1). M. mola is distributed throughout the worlds oceans, with reports from the North and South Atlantic, the Pacific, the Mediterranean and the Gulf of Mexico (2). It feeds mainly on jellyfish and comb jellies, making it one of the few animals to do so. The increase in jellyfish numbers worldwide as commented on in this blog (here and here) means that there has been a marked increase in research into M. mola and the other sunfish species world wide. Satellite tracking of individuals has shown that they move up and down in the water column during the day and night, most probably in synchrony with their prey (2). They are also shown to migrate north in spring/summer and south in winter, again in response to prey movements but possibly also due to its preferred temperature range of 10-19°C (2). M. mola is preyed upon by relatively few other animals with reports of sharks, orcas and sea lions reported as feeding on them (3). They are, however, prone to high levels of parasitism, possibly due to their large surface area.

Ocean Sunfish, Mola mola

Unfortunately the picture used to illustrate this post is of a dead M. mola, caught off the south coast of Ireland and photographed in a fishmongers*. And while a live example would have been much more preferable, this picture does point to the fact that sunfish are being caught in huge numbers as a result of commercial fishing of swordfish and tuna species. These bycatch numbers are alarming: in South African swordfish and tuna fishing, 170 sunfish are caught for every 1000 hooks deployed (2). In Californian drift gillnet fishing for swordfish M. mola comprised 29% of total catch – much more than the target species (4). The enigmatic nature of this sunfish means that the effect of fishing practices on populations remains unknown.

*Note: The sale of all Molidae species is banned in the EU as they are classed as poisonous under Chapter II part G of Regulation (EC) no 854/2004 of the European Parliament and of the Council. I can only assume that the fish was for display purposes only.

References:

1. Bass et al., 2005. Marine Biology 148 pp. 405–414
2. Sims et al., 2009. Journal of Experimental Marine Biology and Ecology 307 pp. 127-133
3. Pope et al., 2010. Reviews in Fish Biology and Fisheries 20 pp. 471-487
4. Cartamil and Lowe, 2004. Marine Ecology Progress Series 266 pp. 245-253