Feeding patterns and brain evolution in ostariophysean fishes.

The sense of taste plays a crucial role in a fish's ability to locate and select appropriate food. Functionally, the taste system is divisible into two subsystems, with external taste, utilized to locate food in the environment, being mediated by the facial nerve while intraoral taste, crucial for triggering swallowing, is mediated by the vagus nerve. Each of these nerves connects to its own portion of the medullary viscerosensory column. In most teleosts, the viscerosensory column forms a continuous, relatively undifferentiated column of neuropil in the dorsomedial medulla. The taste bud-bearing surfaces of the fish are mapped onto this column with external taste buds being represented anteriorly and pharyngeal taste buds caudally. Taste information reaching the vagal taste area, the "vagal lobe", is relayed directly to motoneurons that control the oropharyngeal musculature. In goldfish, unlike most teleosts, the vagal lobe is laminated, highly differentiated structure containing both sensory and motor layers. This derived neural structure is related to the specialized palatal food sorting apparatus utilized by the fish to separate food from substrate material. Despite the complex morphology of the vagal lobe in goldfish, the underlying circuitry is essentially identical to that of other fishes, i.e. after an obligatory synapse in the sensory layers, the gustatory input is relayed to the oropharyngeal motoneurons comprising the motor layer. Thus evolution of the derived, laminated brain structure did not entail generation of new connectivity but merely involved rearrangement of previously existing neuronal populations.