Prey-capture in Pomacanthus semicirculatus (Teleostei, Pomacanthidae): Functional implications of intramandibular joints in marine angelfishes

Abstract

We examined prey-capture morphology and kinematics in the angelfish, Pomacanthus semicirculatus (Cuvier 1931), to evaluate the magnitude and role of functional specialisation. The feeding apparatus of P. semicirculatus possess three biomechanical mechanisms of particular interest: (1) a novel intramandibular joint, permitting dentary rotation and protruded jaw closure; (2) an opercular linkage facilitating mandible depression; and (3) a suspensorial linkage with two novel points of flexion, permitting anterior rotation of the suspensorium and augmenting mandible protrusion. Prey-capture kinematics were quantified using motion analysis of high-speed video, yielding performance profiles illustrating timing of onset, duration and magnitude of movement in these three biomechanical systems, and other variables traditionally quantified in studies of teleostean ram-suction feeding activity. Mandible depression and suspensorial rotation both augmented mandible protrusion, and coincided during jaw protrusion, typically increasing head length by 30%. Jaw closure appeared to result from contraction of the adductor mandibulae segment A2, which rotated the dentary by approximately 30° relative to the articular. This resulted in jaw closure with the mandible fully depressed and the jaws at peak-protrusion. Feeding events were concluded by a high-velocity jaw retraction (20-50 ms), and completed in 450-750 ms. Feeding kinematics and morphology of Pomacanthus differed from other biting teleosts, and more closely resemble some long-jawed ram-suction feeders. The structural and functional modifications in the Pomacanthus feeding apparatus are matched to an unusual diet of structurally resilient and firmly attached benthic prey.