Size and shape of Daphnia longispina in rock-pools

Abstract

The body size and shape of D. longispina in small and large rock-pools was measured. The mean body length of Daphnia in large rock-pools with vertebrate planktivores was smaller than that of Daphnia in small rock-pools without vertebrates, but the variability in body lengths within pools over the season was as great as that found between pools and predator regimes. We did find that D. longispina in large rock-pools produced one egg at a smaller body length and had fewer eggs per individual than did Daphnia in small rock-pools. D. longispina populations also showed different body shapes both as the season progressed and in rock-pools having different predator composition. As expected in Daphnia cyclomorphosis, the core body length decreased during the summer, accompanied by significantly more pointed heads and longer tail spines than in early summer. D. longispina in large rock-pools (with vertebrate predators) have significantly larger exuberances than those in small rock-pools (lacking vertebrate planktivores). The results are discussed in the context of size-selective predation.