Distribution and Tidal Rhythmicity of a Littoral Amphipod

Abstract

Combined percentage size frequency histograms for Hyale grandicornis associated with Celidium prisioides on St. Croix Island in Algoa Bay, South Africa show that small size classes dominated throughout the year. On average 82% were less than 4 mm total length and there appears to be continuous recruitment of juveniles to the population. Very few large specimens were found to be associated with G. pristoides and of the 1436 amphipods measured, less than 0.5% were ovigerous females. G. pristoides thus appears to be a nursery for juveiles. H. grandicornis is extremely motile and the numbers associated with Gelidium decreased during high tide and increased again at low tide. Using Spearman's rank correlation for H. grandicornis (numbers per gram dry mass of Gelidium against tidal height) this rhythmicity was found to be highly significant (p <0.01; 12 d.f.). Observations on the behaviour of H. grandicornis revealed that the amphipods remain in the littoral zone. As the tide rises and water splashes into the Gelidium belt the amphipods become active and move about on the shore only to take shelter again as the tide recedes.