Life Cycle Evolution in Asteroids: What is a Larva?

The diversity of larval forms and developmental patterns in asteroid echinoderms has become increasingly apparent over the past 10-15 years. However, the classification of developmental patterns has been ambiguous because the patterns have not been defined as unique sets of ecological and developmental character states. In addition, character states have not been defined consistently. Thus attempts to understand the evolutionary changes in development (e.g., heterochrony and heterotopy in morphogenesis) that underlie larval diversity have been hampered. We propose a multifactor classification of asteroid developmental patterns that uses an explicit set of characters that provide information on habitat (e.g., pelagic or benthic) and mode of nutrition (e.g., feeding or nonfeeding) of the developing young, as well as the type of morphological development (indirect = larval; direct = nonlarval). We conclude that direct development is exceptionally rare. All asteroids whose development has been studied, except Pteraster tesselatus, have the indirect type of development. We also propose definitions of some important terms that have been used inconsistently in the literature (e.g., larva, metamorphosis, indirect development, and direct development). Our definitions take into account the continuous nature of development and the evolutionary diversification of ontogenetic sequences. These definitions are intended to provide a clear conceptual basis for analyzing asteroid life cycle evolution. We argue that the ancestral asteroid life cycle involved pelagic larval development with both bipinnarian and brachiolarian stages. We then present a series of hypotheses for six types of evolutionary transitions in development that can account for the diversity of larval forms and developmental patterns in starfish.