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.
‘It was demonstrated that despite the missing swim-bladder, ocean sunfish are neutrally buoyant (mean body density 1,027 ± 4 kg/m3, N = 20) in seawater (density ca. 1,026 kg/m3) and that a thick layer of low-density, subcutaneous, gelatinous tissue plays a major role providing this buoyancy (Watanabe and Sato 2008). The degenerate, cartilaginous skeleton of M. mola (…) also likely contributes to buoyancy (…). Importantly, the gelatinous tissue is incompressible, enabling rapid depth changes without the changes in buoyancy that would be experienced by fish possessing swim-bladders (…). This combination of a lift-based swimming mode [This results from the one-stroke cycle movement of the dorsal and anal finsthat effectively act as a pair of vertical hydrofoils. The presence of active swimming appendages that are not bilaterally symmetrical isanother unique and interesting characteristic of the ocean sunfish] and neutral buoyancy from incompressible, gelatinous tissue appears to allow M. mola to move over considerable distances, despite its unusual morphology.’ (Pope 2010: 7).
[The gelatinous layer in ocean sunfishes has a thickness ranging from 3.9 cm (2 kg individuals) to 21.0 cm (959 kg individuals) and a mean density of 1,015 kg/m3. It may account for 26 % to 44% of the body mass and supports from 69% to100% of the fish weight in water. Watanabe and Sato 2008: 3]