The production of subitaneous and diapause eggs: a reproductive strategy for Acartia bifilosa (Copepods: Calanoida) in Southampton Water, UK

Acartia bifilosa from Southampton Water lays two morphologically distinct types of egg which are described for the first time. Eggs with a smooth surface are considered subitaneous, while eggs covered with thick 'spines' are diapause. During the seasonal occurrence of A.bifilosa in Southampton Water, from November/December to June, subitaneous eggs are laid during the first months of this period. The production of diapause eggs is restricted to a 2 month period before A.bifilosa disappears from the water column. There are significant differences between the response of the eggs produced at seasonal field temperatures (5-12C) and those reported for other Acartia species. In particular, the numbers of eggs female^-1 day^-1, both subitaneous and diapause, are lower and typically <4; and the hatching time of subitaneous eggs is longer, at up to 10 days, at optimum temperatures between 15 and 20°C and optimum salinity >20 PSU. Females acclimated to higher laboratory temperature regimes show higher egg production rates at field salinity. A delayed-hatch subitaneous egg is also reported. Results suggest that fecundity in A.bifilosa from Southampton Water might be limited to some degree by temperature and the responses of the eggs produced appear to offer A.bifilosa a reproductive repertoire to ensure its sustained presence in this seasonally influenced environment.