SEX-RATIO VARIATION IN THE INTERTIDAL ISOPOD, JAERA ALBIFRONS

Thirty 'rock-populations' of the intertidal isopod, Jaera albifrons (I sopoda: Asellota), from South Wales area were found to display a signi ficant and persistent sex ratio bias, with females outnumbering males by up to 12 to 1. This thelygeny was not mirrored in broods that were isolated in aquaria after insemination, nor in broods that developed u nder various temperature and light regimes. Such a discrepancy suggest s that the mechanism generating sex ratio biases in natural population s is not innate, nor is influenced by cytoplasmic or environmental fac tors. The possible causes of thelygeny in natural populations are disc ussed.