Control of diel vertical migration: Photoresponses of a larval crustacean

Abstract

The lower threshold for phototaxis of dark‐adapted larvae of the crab Rhithropanopeus harrisii is 10⁻⁷ W·m⁻² at 500‐nm light. This threshold does not change throughout development. In the Newport River estuary (North Carolina), larvae show a pattern of nocturnal vertical migration in which the distribution of later‐stage zoeal larvae centers on the 10⁻⁷ W·m⁻² isolume during the day. The behavioral responses underlying this relationship include a negative geotaxis in darkness which changes to a sinking response in light with a natural underwater distribution. The minimum increase in intensity above the lower threshold which evokes sinking decreases progressively with age. The intensity is well above the phototactic response threshold for stage I zoeae, but within one log unit of threshold for the other three zoeal stages. Thus, during the day larvae are associated with a preferred light level. This association results from a negative geotaxis in darkness that changes to a sinking response upon an increase in light intensity.