Feeding Mechanisms in Sharks

Although many sharks have a rather general vertebrate body plan,they display a number of specializations for feeding that beliethe notion that they are "primitive." These specializationsinclude a battery of highly developed exteroceptive systemssuch as vision, olfaction, acoustico-lateralis sense and electroreception;and a cranial morphology that has been molded into a numberof functionally adaptive forms. These forms result in grasping,sucking, crushing, gouging, cutting and filtering systems offeeding. With relatively few exceptions elasmobranch feedingmechanisms share such features as subterminal or inferior mouths,a dynamic tooth replacement system, hyostylic jaw suspensionand a kinetic, protractile upper jaw. The importance of eachof these components is discussed. The evolution of the highdiversity of mechanical feeding systems in such a small groupof vertebrates has probably been facilitated by the morphologicalsimplicity of the basic feeding mechanism. This radiation wasaccomplished by modifications in jaw length, the length andsupporting angle of the hyomandibula, the size of the gape,dentition and changes in the relative size of the cranial musculature.The evolutionary pattern of shark feeding mechanisms is complex,there being several examples of both parallelism and convergence.A long-jawed, grasping form (similar to, but not identical withChlamydoselachus) is here considered primitive. From a subsequentbenthic sucking and grasping ancestor, similar in many respectsto some living batoids,radiated crushing, ray-like forms; cutting,squaloid forms; and gouging, lamniform and carcharhiniform types.From the latter developed sucking and grasping, or crushingforms such as modern orectolobiforms, triakids and heterodontiformsharks. From several levels (primary crushing, secondary crushingand gouging) there emerged filter-feeding forms representedtoday by mobulids, rhiniodontids and cetorhin.