JUVENILE MORTALITY IN BENTHIC MARINE-INVERTEBRATES

Thirty years ago, Thorson (1966; Neth J Sea Res 3;267-293) calculated that juvenile mortality in marine bivalves could exceed 98.6%. Subsequ ently, juvenile mortality rates have been assumed to be high and to in fluence the evolution of life history traits. However, there have been no attempts to establish whether high juvenile mortality is common or to determine if interspecific trends in juvenile mortality exist. To address this issue, we reviewed 30 studies of age-specific mortality a mong bivalves, gastropods, barnacles, ascidians, bryozoans and echinod erms. High juvenile mortality is widespread among benthic marine inver tebrates, with 20 of the 30 studies reporting levels of juvenile morta lity >90%. Mortality is particularly high during the first moments of juvenile life, and can exceed 30% during the first day. Pooling surviv orship data from all species revealed a general trend, with survivorsh ip decreasing exponentially during the first days or weeks of juvenile life until, by the age of 4 mo, virtually all cohorts were reduced to <20% of their initial numbers; mortality remained low thereafter. We suggest that extreme vulnerability at the onset of juvenile life is a shared trait that is largely responsible for the survivorship trend. N atural variation within this trend would be largely due to variation i n intensity of mortality factors. Predation and desiccation are well-d ocumented causes of juvenile mortality, but the current lack of data o n factors such as ultraviolet radiation, diseases, and 'internal' caus es (energy depletion, developmental and physiological defects) preclud es a ranking of factors as selective pressures. Methods used to quanti fy juvenile mortality vary considerably in the level of resolution the y can achieve within the early juvenile period. Studies of early juven ile mortality should ideally monitor the fate of individuals from the onset of juvenile life, using sampling intervals less than or equal to 1 d. Mapping and imaging techniques can provide accurate results for sessile organisms, whereas mark and recapture can be effective for mot ile animals with limited dispersal. Early juvenile mortality has been shown to influence population abundance and distribution as well as co mmunity structure. Juvenile mortality is also expected to be an import ant determinant of age at maturity, but only among species maturing wi thin 4 mo of postlarval life since mortality remains low after the age of 4 mo. A compilation of data on age at first reproduction in 92 spe cies revealed a bimodal grouping of species: 22% of species maturing w ithin 45 d after beginning juvenile life, and 60% maturing after at le ast 1 yr. The influence of juvenile mortality on age at maturity will differ substantially among these 2 groups and will therefore not be eq ual or directly comparable among all species, Given the magnitude of e arly juvenile mortality and the similarities in mortality patterns acr oss diverse taxa and habitats, a better understanding of early juvenil e mortality should help researchers to understand how population param eters are regulated and help elucidate the significance of traits that characterize populations and species.