Tissue provides neutral buoyancy: ocean sunfish
The thick layer of low-density, subcutaneous tissue of the ocean sunfish enables rapid depth changes by having a incompressible, gelatinous composition.
‘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 fins that effectively act as a pair of vertical hydrofoils. The presence of active swimming appendages that are not bilaterally symmetrical is another 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]
Learn more about this functional adaptation.