Sensory development and concurrent behavioural changes in Atlantic croaker larvae

Development of the visual and mechanosensory systems of Atlantic croaker larvae was examined and compared to behavioural performance in order to determine sensory system functionality. Early larvae were characterized by a photopic visual system with a high density of cone photoreceptors, but no rods or photoreceptor summation on to higher order cells. Acuity of small larvae was comparable to other species. The mechanosensory 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 acuity had improved to levels slightly better than in other species. Rods began forming and summation ratios increased, indicating development 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 was tested using an artificial predatory stimulus under different conditions to isolate the roles of vision and mechanoreception. Both sensory systems were necessary to elicit the highest levels of responsiveness. Responsiveness of larvae <6mm was equally influenced by visual and neuromast input, while responsiveness of larvae > 6 mm was largely influenced by visual input. Both vision and mechanoreception influenced ontogenetic changes in reactive distance of larvae. When there was no visual input, reactive distances increased slightly through ontogeny, indicating the use of mechanoreception. Only when mechanoreception was blocked, the ontogenetic increase in reactive distances was greater than that of larvae with all sensory systems intact. These results indicate that Atlantic croaker larvae use both sensory systems throughout the larval period, but the relative importance of these sensory systems changes with ontogeny.