Visual acuity in a species of coral reef fish: Rhinecanthus aculeatus.

Coral reef fish present the human observer with an array of bold and contrasting patterns; however, the ability of such fish to perceive these patterns is largely unexamined. To understand this, the visual acuity of these animals - the degree to which they can resolve fine detail - must be ascertained. Behavioural studies are few in number and anatomical analysis has largely focused on estimates of ganglion cell density to predict the visual acuity in coral reef fish. Here, we report visual acuity measures for the triggerfish Rhinecanthus aculeatus. Acuity was first assessed using a series of behavioural paradigms and the figures were then contrasted with those obtained anatomically, based on photoreceptor and ganglion cell counts. Behavioural testing indicated an upper behavioural acuity of 1.75 cycles·degree(-1), which is approximately the same level of acuity as that of the goldfish (Carassiusauratus). Anatomical estimates were then calculated from wholemount analysis of the photoreceptor layer and Nissl staining of cells within the ganglion cell layer. Both of these anatomical measures gave estimates that were substantially larger (7.75 and 3.4 cycles·degree(-1) for the photoreceptor cells and ganglion cells, respectively) than the level of acuity indicated by the behavioural tests. This indicates that in this teleost species spatial resolution is poor compared to humans (30-70 cycles·degree(-1)) and it is also not well indicated by anatomical estimates.