THE DISTRIBUTION, GROWTH AND REPRODUCTION OF PONTOCRATES ARENARIUS AND P-ALTAMARINUS (CRUSTACEA, AMPHIPODA) AT MILLPORT, SCOTLAND

Results of a comparative study on the distribution, growth and reprodu ctive bionomics of two species of sand-dwelling oedicerotid amphipod, Pontocrates arenarius and P. Altamarinus are presented. Pontocrates ar enarius is a littoral species with highest population densities betwee n mean tide level (MTL) and low water springs (LWS) while P. altamarin us lives in the immediate sublittoral between LWS and -2 m Chart Datum . Both species are iteroparous, have female biased sex ratios, year-ro und breeding and life cycles largely based on two generations per year . The percentage of ovigerous females in the total population indicate s breeding activity, and temporal changes in this percentage (arcsin t ransformed) have been tested for correlations against seasonal variati on in five environmental factors (mean maximum weekly air temperature, mean minimum weekly air temperature, mean weekly sea temperature, day length, and mean weekly windspeed). Based on these analyses, differen ces in the timing of peak reproductive activity between these two spec ies are interpreted as indicating the operation of different mortality agents, i.e. abiotic factors in the littoral P. arenarius and biotic effects (e.g. predation by fish) in P. altamarinus from the sublittora l.