The selection pressures imposed by invertebrate are of primary significance in the evolution of cladoceran life histories. Small-sized cladoceran species are vulnerable to invertebrates throughout their lives, but minimize the adverse effects of juvenile mortality by producing relatively large offspring, which reach maturity at an early age. Large-sized species are capable of attaining sizes which cannot be handled by invertebrates, and therefore maximize early growth to an invulnerable size. The large planktonic species and some small littoral forms reach an optimal foraging size within their lifetimes and subsequently minimize growth. However, the emphasis on an early production of relatively large young by the small-sized planktonic species prevents them from attaining their optimal foraging size until very late in life; some species apparently never reach it. Nevertheless, these small-sized species grow continously towards their optimal foraging size. They decrease their vulnerability to invertebrates...