Evolution of Larvae and Developmental Modes
This chapter examines the origin and diversification of metazoan larvae and developmental modes. It discusses the major features of larval evolution, and considers their developmental bases, ecological causes, and evolutionary consequences. Feeding larvae have diversified substantially within many phyla. All of the large phyla, and many of the small ones, contain both planktotrophic and lecithotrophic larvae. As with feeding larvae, repeated evolutionary derivations can provide evidence of adaptive responses. The evolutionary transition to lecithotrophy almost always involves an increase in egg size and usually involves a general simplification of larval structure. The embryos and larvae of marine invertebrates need to do two things: produce the next life-history phase and survive until this can be accomplished. The evolutionary history of metazoan larvae covers immense temporal, ecological, and phylogenetic scopes. The very existence of larvae suggests that some degree of evolutionary uncoupling is possible between early development and adult phenotype.