SENSORY DEVELOPMENT AND CONCURRENT BEHAVIORAL-CHANGES IN ATLANTIC CROAKER LARVAE

Development of the visual and mechanosensory systems of Atlantic croak er larvae was examined and compared to behavioural performance in orde r to determine sensory system functionality. Early larvae were charact erized by a photopic visual system with a high density of cone photore ceptors, but no rods or photoreceptor summation on to higher order cel ls. Acuity of small larvae was comparable to other species. The mechan osensory system of early larvae consisted of a relatively high density of free neuromasts on the head and body. By approximately 10 mm total length (L-T), many sensory attributes of larvae had changed. Visual a cuity had improved to levels slightly better than in other species. Ro ds began forming and summation ratios increased, indicating developmen t of scotopic vision. Cephalic lateral-line canals began to enclose at 15 mm L-T and were complete by 24 mm. The trunk lateral-line began to enclose at 30 mm and was complete by 40 mm. Behavioural performance w as tested using an artificial predatory stimulus under different condi tions to isolate the roles of vision and mechanoreception. Both sensor y systems were necessary to elicit the highest levels of responsivenes s. Responsiveness of larvae <6 mm was equally influenced by visual and neuromast input, while responsiveness of larvae >6 mm was largely inf luenced by visual input. Both vision and mechanoreception influenced o ntogenetic changes in reactive distance of larvae. When there was no v isual input, reactive distances increased slightly through ontogeny, i ndicating the use of mechanoreception. Only when mechanoreception was blocked, the ontogenetic increase in reactive distances was greater th an that of larvae with all sensory systems intact. These results indic ate that Atlantic croaker larvae use both sensory systems throughout t he larval period, but the relative importance of these sensory systems changes with ontogeny. (C) 1997 The Fisheries Society of the British Isles.