VARIABILITY, FLEXIBILITY AND PLASTICITY IN LIFE-HISTORIES OF MARINE-INVERTEBRATES

Nearly all aspects of the life histories of individual marine-inverteb rate species are characterized by ranges of;sizes, seasonal variation, and functional flexibility and phenotypic plasticity in response to v arying environmental conditions; that is, they are highly polytypic. F our major areas of polytypy are considered: (1) breeding seasons and c ycles vary greatly in time and duration with latitude and from year to year; (2) egg and larval sizes, while showing both genetic and stocha stic variability in all species, are often also flexible responses to adult and larval nutrition; (3) modes of development are variable with in a number of single species, often reflecting egg-size differences b etween populations, or even as a result of hatching age of siblings fr om a single egg mass; and (4) duration of the pelagic larval phase, bo th before and after the onset of metamorphic competence. Planktotrophi c larvae show plastic responses to phytoplankton abundance in their mo rphologies, and most larvae are flexible in their age at metamorphosis because this complex process requires a more-or-less specialized subs tratum to induce it for most invertebrate species. New data are presen ted that provide additional examples of broad flexibility of sibling l ecithotrophic larvae to hatch and settle at greatly differing ages. La rvae of the patelloidean gastropod Lottia pelta settled from 8 to 28 d ays after fertilization, and those of the fissurelloidean Diodora aspe ra hatched over a three week period from 7 to 30 days post-fertilizati on; larval settlement had a similarly broad range. Even the planktotro phic-lecithotrophic dichotomy breaks down as increasing numbers of spe cies are found to produce larvae that can metamorphose without feeding (i.e. lecithotrophy), or feed and greatly extend their larval duratio ns (planktotrophy) in the absence of suitable settlement substrata. In vertebrate groups with rigidly canalized life histories are noted to b e components of the fouling community, and it is conjectured that an e volutionary history on floating substrata canalized their life histori es toward high inbreeding tolerance, often selfing, and brief pelagic larval durations. We note that most life-history theory has considered extremes and major modes in invertebrate development, and suffers fro m lack of-attention to the abundance of polytypic life-history traits as evolutionary survival mechanisms at the species level. We conclude that response flexibility and plasticity increase both survivorship an d fecundity of individuals, while life-history variability increases t he likelihood of recruitment across populations and persistence over g eological time. More data are needed on the ranges of scalar character s and flexible responses in marine-invertebrate life histories to rigo rously evaluate their contributions to evolutionary success.