The thick layer of low-density, subcutaneous tissue of the ocean sunfish enables rapid depth changes by having a incompressible, gelatinous composition.

The ocean fish displays a laterally compressed body that is equipped with rather large dorsal and anal fins, and terminates with a broad, stiff lobe (the clavus) instead of the usual caudal fin. Internally, the fish lacks an active swim bladder and has large deposits of low-density, subcutaneous, gelatinous tissue. While these unusual characteristics and awkward shape could at first suggest Mola mola to be a planktonic fish with poor swimming capabilities, the ocean sunfish is able to move over considerable distances, both horizontally and vertically. This ability is directly related to the morphologic adaptations described above. The fish swims by stroking its dorsal and anal fins laterally and in a synchronous manner, thus generating a lift-based thrust that enables it to cruise at speeds of 04.-0.7 ms-1. This mode of swimming which resorts to fins that are not bilaterally symmetrical has not been found in other organisms so far. At the same time that it moves forward, the ocean sunfish is also able to undergo substantial vertical movements in the water column. This behavior is facilitated by the fact that the fish is neutrally and stably buoyant independently of the depth, a crucial property that is related to the absence of a swim bladder (the presence of which would otherwise change volume with hydrostatic pressure) and to the water-rich gelatinous tissue deposits.

M. mola in typical swimming position

M. mola in typical swimming position

A partial skeleton of an ocean sunfish, showing the structure of the fins

An enormous ocean sunfish (mola mola), caught by W.N. McMillan of E. Africa, at Santa Catalina Isl., Cal. April 1st, 1910. Its weight was estimated at 3,500 pounds.

Last Updated September 14, 2016