Juvenile mortality in benthic marine invertebrates

T h ~r t y years ago Thorson (1966 Neth J Sea Res 3 267-293) calculated that juvenile mortallty in m a n n e 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 luvenile mortality IS common or to determine if lnterspecific trends in l u v e n ~l e mortality exist To address t h ~s Issue we revlewed 30 studies of age-specif~c mortality among bivalves, gastropods, barnacles ascid~ans, bryozoans and echinoderms High juvenile mortality is widespread among benthic marlne Invertebrates with 20 of the 30 s t u d ~e s reporting levels of luven ~l e mortal~ty > 9 0 % Mortal~ty is part~cularly h ~g h during the first moments of luvenile life and can exceed 30% during the f ~r s t day Pool~ng surv~vorship data from all species revealed a general trend with survivorsh~p decreasing exponentially during the first days or weeks of juvenile life until by the age of 4 mo, virtually all cohorts were reduced to < 2 0 % of their ~nitial numbers ~nortality remained low thereafter We suggest that extreme vulnerability at the onset of luvenile life is a shared t r a ~t that is largely responsible for the surv~vorship 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 lnternal causes (energy depletion developmental and physiological defects) precludes a ranking of factors as select~ve pressures Methods used to quantify juvenile mortality vary cons~derably in the level of resolution they can achieve w t h i n the early luvenile perlod Studies of early juvenile mortality should ideally monitor the fate of individuals from the onset of luvenile l ~f e using sampling intervals 5 1 d Mapping and imaging techniques can provide accurate results for sess~le organisms, whereas mark and recapture can be effective for motile animals w ~t h l ~m ~t e d dispersal Early juvenile mortahty has been shown to influence population abundance and distnbution as well as community structure Juvenile mortality is also expected to be an important determinant of age at matunty, but only among species m a t u r ~n g within 4 mo of postlarval l ~f e since mortality remalns low after the age of 4 mo A compilation of data on age at first reproduction In 92 species revealed a b~modal grouping of species 22% of species matunng within 45 d after b e g i n n ~n g luvenile life and 60% maturlng after at least 1 yr The influence of luven~le mortality on age at inaturlty will d ~f f e r 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 ]uvenile mortality should help researchers to understand how population parameters are regulated and help elucidate the s~gnificance of traits that characterize populations and specles