Why and how marine-invertebrate larvae metamorphose so fast.

It is argued that larviparous development has evolved at least eight times among extant animals. A 'need for speed hypothesis' is proposed to explain profound convergence on a pattern of small larvae and rapid metamorphosis across six marine invertebrate clades. Shared selection pressures include limits to larval size, the plankton-to-benthos transition, extreme hazards on the benthos, and the profound helplessness of metamorphosing animals. The adaptive mechanisms include: (1) development of juvenile structures in larvae before they are metamorphically competent; (2) external cues trigger metamorphosis; and (3) rapid cell-to-cell conductance of the metamorphic signal to bring about rapid loss of larval structures and release of juvenile structures. Both pattern and mechanisms contrast in every regard with those of the other two major larviparous clades, Insecta and Amphibia.