The apparent rôle of temperature in breeding initiation and winter population structure in Hyale nilssoni Rathke (Amphipoda): Field observations 1972–1983

Abstract

Data are presented on the population structure of Hyale nilssoni Rathke (an amphipod inhabitant of high littoral seaweeds) over the period of breeding initiation (late February) of 11 yr. By sampling, as near as possible, the same date annually, photoperiodic effects are controlled. The percentage of ovigerous females on this date was significantly correlated with post-winter solstice mean sea temperature. February sex ratios (M : F) did not correlate with either preceding August or September mean sea temperature such as would have been expected was sex determination temperature sensitive. Sex ratio, however, was significantly positively related to mean winter sea temperatures, with warmer winters having male-dominated Hyale populations in late February. Behavioural hypotheses involving amphipod migration and/or size specific elimination of larger males (c.f. females) during harsher winters are offered in explanation. Population size-frequency distributions over 11 yr were remarkably consistent. Male size was more heterogeneous between years than female size. Mean male size (late February) proved to be significantly negatively correlated with mean winter sea temperatures. Mean female size showed no significant relationship. Given that large males are necessary for precopulatory carrying behaviour it is important that, as shown, they are available to initiate spring breeding, even after harsh winters.