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%. Subsequently, juvenile mortality rates have been assumed to be high and to influence 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 among bivalves, gastropods, barnacles, ascidians, bryozoans and echinoderms. High juvenile mortality is widespread among benthic marine invertebrates, with 20 of the 30 studies reporting levels of juvenile mortality >90%. Mortality is particularly high during the first moments of juvenile life, and can exceed 30% during the first day. Pooling survivorship data from all species revealed a general trend, with survivorship 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. Natural variation within this trend would be largely due to variation in intensity of mortality factors. Predation and desiccation are well-documented causes of juvenile mortality, but the current lack of data on factors such as ultraviolet radiation, diseases, and 'internal' causes (energy depletion, developmental and physiological defects) precludes a ranking of factors as selective pressures. Methods used to quantify juvenile mortality vary considerably in the level of resolution they can achieve within the early juvenile period. Studies of early juvenile 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 motile animals with limited dispersal. Early juvenile mortality has been shown to influence population abundance and distribution as well as community structure. Juvenile mortality is also expected to be an important determinant of age at maturity, but only among species maturing within 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 species revealed a bimodal grouping of species: 22% of species maturing within 45 d after beginning juvenile life, and 60% maturing after at least 1 yr. The influence of juvenile mortality on age at maturity will differ substantially among these 2 groups and will therefore not be equal or directly comparable among all species, Given the magnitude of early juvenile mortality and the similarities in mortality patterns across diverse taxa and habitats, a better understanding of early juvenile mortality should help researchers to understand how population parameters are regulated and help elucidate the significance of traits that characterize populations and species.