Egg to juvenile period, generation time, and the evolution of larval type in marine invertebrates

Many hypotheses have been put forward to explain the evolution of different larval types in marine invertebrates. These hypotheses have generally ignored the impact which the larval (developmental) type can have on the adult stage of the life cycle. A conceptual model presented here shows how reduction of the pre-metamorphic, or 'egg-to-juvenile' period either increases size at reproduction [by increasing the benthic (post-metamorphic) development period in the case of species with fixed duration life cycles], or decreases overall generation times (in species with fixed size or age at reproduction, and variable life cycle duration). In either case the net result is an increase in fitness. A simple analytical version of this model is derived and is illustrated with data from 2 species of nudibranch mollusc. The model reveals an 'optimal' egg-to-juvenile period [where the intrinsic rate of increase (r) is maximised] for all life-cycle durations modelled. Reduction of the egg-to-juvenile period leads to an increase in r in all cases except when egg-to-juvenile period is very short. At egg-tojuvenile periods typical of species with feeding larvae, increases in r may be considerable (a reduction in egg-to-juvenile period from 100 d to 90 d yields a 4 to 13 % increase in r). Available evidence from several taxa indicates that developmental type and egg-to-juvenile period are correlated, such that species with non-feeding larvae tend to have shorter egg-to-juvenile periods. Consequently, the selective pressures to reduce egg-to-juvenile period which are illustrated by this model highlight a previously overlooked pressure for the evolution of non-feeding larvae in marine invertebrates.