EFFECTS OF CROWDING AND DIFFERENT FOOD LEVELS ON GROWTH AND REPRODUCTIVE INVESTMENT OF DAPHNIA

The effects of daphniid crowding on juvenile growth rate, length at fi rst reproduction, clutch size and egg size of four species of Daphnia were compared with the effects of food level. Juvenile Daphnia were gr own to primipary in a flow-through system in water conditioned by diff erent densities of the same, or another, species. At high ambient food levels, water from Daphnia that had been crowded at densities greater than or equal to 150 l(-1) depressed growth rate and lowered body siz e and clutch size of D. hyalina and D. galeata; effects on the same tr aits of D. magna and D. pulicaria were variable (stimulation, depressi on, or no effect). D. hyalina and D. geleata responded to gradients of increasing daphniid density (0-300 l(-1)) by altering egg mass, somat ic mass and clutch size to maintain a relatively constant reproductive investment; egg mass increased with crowding and then decreased in a pattern consistent with Glazier's (1992) hypothetical model of changes in offspring size in relation to food quantity and maternal demand. E ffects of crowding by conspecifics were indistinguishable from those o f other species. This study, which uncouples the effect of crowding pe r se from ambient resource depletion, shows that chemical substances r eleased by high densities of Daphnia can cause changes in life-history traits comparable to those that occur in response to low food levels.