Number of Eggs and Body-Size in the Freshwater Copepod Diaptomus gracilis Sars in the English Lake District

Abstract

Clutch-size and body-size of adult females of spring-caught Diaptomus gracilis Sars were found to have a statistically significant correlation with depth of water but not with total ions (mequiv./l.) in the water of 42 lakes and tarns in the English Lake District. 2. Seasonal variations in clutch-size and body- size in Windermere and in Wise Een Tarn are described. In the tarn adult females lay more eggs in late April to early June than in other seasons and are larger throughout the year than in the lake. 3. The percentage of egg-bearing females ranges from 0-93 %, and rarely falls below 20% except from October to January when egg- laying may cease altogether in some, but not all, years. 4. Diaptomus gracilis Sars was more frequently present and more frequently abundant in a productive lake (Esthwaite Water) than an unproductive one (Buttermere) but only formed 34 % of the total planktonic Crustacea in the former compared with 52 % in the latter. 5. Earlier reports by Thomas (1961) that mean number of eggs and percentage of adult females of this species vary inversely with the total ions in the water, and that water with higher amounts of dissolved salts is less suitable for D. gracilis, are thought to be based on insufficient observations. 6. It is suggested that a possible link between large body-size, large clutch-size and shallow- ness of water may be greater accessibility of benthic and littoral algae in shallow water. Pe3oIMe