DEMOGRAPHIC COSTS OF ANTIPREDATOR DEFENSES IN DAPHNIA-PULEX

Juvenile Daphnia pulex develop neck spines in response to a chemical agent released to predatory Chaoborus larvae. While these defensive structures reduce the vulnerability of Daphnia to the predator, they also entail a demographic cost. The authors investigated the nature and degree of this cost through an analysis of cohort life tables involving both the typical morph (TM), which lacks neck spines, and spined morph (SM) at 22°C. No consistent pattern of differences between TM and SM occurred with respect to survivorship, clutch sizes, mean egg volume, or number of juvenile instars. Development rates of both juvenile and adult instars, however, were significantly slower in SM. Presence of neck spines increased the age at maturity for D. pulex by 8.4-14.6%, and the duration of adult instars exposed to Chaoborus-factor, whose eggs will develop into SM, was 2.8% longer than for those not exposed. This caused delayed reproduction in SM and resulted in a population growth rate 8-9% lower than in TM. -from Authors