Behavioral Responses of Crustacean Larvae to Rates of Salinity Change

The ontogeny of behavioral responses of larvae of the crabs Rhithropanopeus harrisii and Neopanope sayi to rates of change in salinity were analyzed with a video system. A salinity increase evoked an ascent in both species. For R. harrisii the threshold rate of increase was 1.1 x 10^-3 ppt s^-1 for the first and last zoeal stages and changed little with acclimation salinity. N. sayi larvae were more sensitive, as thresholds were 2.8 x 10^-4 ppt s^-1 for Stage I zoeae and 7.0 x 10^-4 ppt s^-1 for Stage IV. This difference in sensitivity may relate to the magnitude of salinity gradients in the estuarine/coastal areas inhabited by the larvae. At threshold rates of salinity increase the absolute amount of change before a response was lower for Stage I zoeae (0.09-0.11 ppt) than Stage IV zoeae (0.21-0.29 ppt) for both species. Decreases in salinity did not induce the expected descent response in either species at rates up to 4.7 x 10^-2 ppt s^-1. The different responses in a salinity gradient may have resulted because the rate threshold and absolute amount of change before a response to a salinity increase were below those for a salinity decrease. Considering larval sinking rates and normal environmental salinity gradients, larvae of both species can respond to rates and amounts of salinity increase in their environment. The ascent response may be important for keeping larvae up in the water column and reducing the likelihood that they will encounter the bottom.