Demographic costs of antipredator defenses in Daphnia pulex

Juvenile Daphnia pulex develop neck spines in response to a chemical agent released by predatory Chaoborus larvae. While these defensive structures reduce the vul­nerability of Daphnia to the insect predator, they also entail a demographic cost. We 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 in two different food regimes: natural food conditions (53-,um filtered pond water) and ideal food conditions (1 x 105 cells/mL Chlamydomonas sp.). No consistent pattern of differences between TM and SM occurred with respect to survivorship, clutch sizes, body sizes, mean egg volume, or number of juvenile instars. Development rates of both juvenile and adult instars, however, were significantly slower in SM. The 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 (r) that was 8-9% lower than in TM. This relatively large demographic cost of spine formation in D. pulex produces a strong selection pressure to forgo the formation of these spines when Chaoborus predators are absent.