LIFE-STRATEGY SHIFT BY INTRASPECIFIC INTERACTION IN DAPHNIA-MAGNA - CHANGE IN REPRODUCTION FROM QUANTITY TO QUALITY

In Daphnia magna, high animal density causes a mutual intraspecific in fluence, called life-strategy shift by intraspecific interaction (LiSS II). We investigated mothers (F-0), living singly or in groups of four per 50 ml, and their F-1 offspring. Clutch sizes of F-0 daphnids livi ng in groups were smaller than those of singly living ones. However, t he F-1 neonates were bigger in size, had higher dry weights and contai ned more lipids. They were also able to survive longer starvation peri ods. If these F-1 daphnids were again kept in groups of four, the effe cts of LiSSII on some life-history parameters were stronger. As compar ed to group-living F-0 daphnids, juvenile development time was prolong ed from 7.5 to 10.0 days. In contrast, body size was reduced and the f urther reduction in offspring number was slight. The F-2 neonates exhi bited the same change towards bigger size and higher dry weight as obs erved in the F-1. In contrast F-1 daphnids, that were the offspring of daphnids living in groups but were kept singly were bigger and produc ed more offspring than single daphnids that were the offspring of daph nids kept singly. The neonates of the former were bigger in size and h eavier. It is suggested that for the F-0 generation the ecological sig nificance of the LiSSII effects is an increase in both the parent and offspring survival probability when food is limited. LiSSII acts befor e food conditions deteriorate seriously. Daphnids affected by LiSSII i nvest less of their biomass into offspring. Offspring release is well- timed and quality is better, at the expense of quantity. Under favoura ble environmental conditions, F-2 daphnids that are the offspring of m others affected by LiSSII appeared well adapted to fast recolonisation when living at low density.