Natural mortality of marine invertebrate larvae

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Abstract
Planktonic marine invertebrate embryos and larvae lead transitory lives of great risk and grave uncertainty. Estimation of the magnitude of risk and mortality, however presents a formidable challenge. Three methods are currently in use to estimate levels of mortality among populations of marine invertebrate larvae: 1) theoretical models where rates of instantaneous mortality are correlated with other life history parameters, 2) estimation of mortality based on laboratory observations of predator-prey interactions and 3) analysis of relationships among gamete production, larval populations, and densities of post-larvae in the field. Theoretical exercises indicate that rates of larval mortality are correlated with duration of the planktonic period, and that differences in mortality rates may be offset by differences in egg size and fecundity. Results from laboratory investigations suggest that mortality rates change with larval size or age. Attempts to monitor cohorts of larvae in the plankton offer the most direct evidence for natural mortality, although they often lack confidence that larvae were sampled from a continuous population. Because factors that contribute to the mortality of marine invertebrate embryos and larvae operate concurrently and mortality is a density-dependent and age-specific demographic process, detailed investigations of several synergistic ecological parameters are required to provide meaningful estimates of instantaneous mortality rates in larval populations.