The significance of seawater viscosity for the tidal transport of common shrimps, Crangon crangon (L.)

Abstract

Experimental data were collected on the activity and direction of locomotion of Crangon crangon exposed to water currents, the temperature and salinity of which were altered periodically. Irrespective of temperature and salinity, the animals always showed a nett catadromous displacement, which suggests a major, passive component in their transport.

The distance an animal is transported is related to its sinking rate which itself is affected by the water viscosity. At low temperatures, animals sink slowly and are displaced horizontally for greater distances than when temperatures are higher. Thus temperature differences between ebb and flood water could result in a nett displacement of animals during a tidal cycle.

Sinking rates are highest at those combinations of seawater temperature and salinity which typify the areas where population densities of C. crangon are highest. When temperature and salinity are both high or both low, sinking rates are low and the frictional resistance of the water is high; conditions which promote tidal transport of sinking shrimps. On the basis of these findings, it is suggested that the seasonal movements of Crangon populations are made by the passive displacements which would occur as a consequence of showing random (regarding tidal phase) periods of swimming alternating with periods of sinking, and in which no orienrational behaviour of the animals is involved.