ABSTRACT
The unusual appearance of ocean sunfish and their relatives (Molidae), including exaggerated dorsal and anal fins and a missing caudal fin, raises a question of how they swim. In this chapter, the locomotory systems and biomechanics of ocean sunfish are reviewed based on recent morphological, kinematic, and behavioral studies. Ocean sunfish flap their dorsal and anal fins laterally to produce lift-based propulsion forces, a unique swimming style in which two fins with different origins function as paired vertical wings. The two fins are symmetrical within individuals, despite ontogenetic changes in shape. Unlike many other teleosts with axial musculature that drive caudal fins, sunfish have two sets of muscles, separated by the horizontal septum. These muscles run dorsoventrally to drive the dorsal and anal fins. Aerobic red muscles are located medial to anaerobic white muscles, an arrangement reminiscent of endothermic (i.e., high body temperature) fishes, although sunfish are apparently ectothermic. The thick, white sub-skin layer (hypodermis) has exoskeletal and possible buoyancy functions as well (adult Molidae lack a swim bladder). It is suggested that their conspicuous swimming style and associated morphology enable energy-efficient swimming at a cost of reduced acceleration and manoeuvring abilities, allowing sunfish to search the open ocean for patchily distributed prey (e.g., gelatinous zooplankton).
