Abstract
Results of a comparative study on the distribution, growth and reproductive bionomics of two species of sand-dwelling oedicerotid amphipod, Pontocrates arenarius and P. Altamarinus are presented. Pontocrates arenarius is a littoral species with highest population densities between mean tide level (MTL) and low water springs (LWS) while P. altamarinus lives in the immediate sublittoral between LWS and -2 m Chart Datum. Both species are iteroparous, have female biased sex ratios, year-round breeding and life cycles largely based on two generations per year. The percentage of ovigerous females in the total population indicates breeding activity, and temporal changes in this percentage (arcsin transformed) have been tested for correlations against seasonal variation 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, differences in the timing of peak reproductive activity between these two species 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 sublittoral.
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