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 ...

To determine ~f the onset of intertidal 11fe is a critical time for survival, we examined the fate of newly settled Balanus glandula, an intertidal barnacle, on natural rocky substrata in Barkley Sound, Br~tish Columbia, Canada. In total, 552 indiv~dual settlers were mapped and followed daily during their first 3 to 5 d after settlement in May 1992 and were then periodically re-examined over a 45 d period. A sharp decline in survivorship occurred during the first day after settlement. For settlers arriving on 16 and 17 May, mortality during the first day after settlement (38.0%) was almost as high as all mortality during the following 44 d (40.1 %). Mortality during the first day after settlement was 1.5 to 6.0 times higher than during the second day for settlers arriving on 16, 17 and 18 May. There was no relationship, however, between percent first day mortality and densities of recruits or of grazers (limpets, litto~ines). The elevated levels of first day mortality were also not consistent with desiccation stress, wave exposure, or size-specific changes in vulnerab~l~ty. The first moments after settlement may const~tute a bottleneck for the surv~val of barnacles settllng In the intert~dal zone. For organisms such as B glandula, selective pressures for traits such as t~m e of settlement relative to the tidal cycle, selection of settlement micro-site, and energy reserves at settlement could be most Intense during the first day in the benthic habitat.

No abstract

Ecological shifts occurring after metamorphosis in benthic marine invertebrates have received much less attention than the more conspicuous transition occurring at metan~orphosis and settlement. It remains unclear whether postmetamorphic shifts occur simultaneously or at different times, and whether the shifts occur over brief, discrete periods or are extended or even continuous through juvenile life. The present study of the muricid gastropod Nucella emarginata exarmnes the ontogeny of \r.ulnerabillty to desiccation, of susceptibll~ty to hatchling predators, of shell coloration, and of distribution among microhabitats as a function of snail size. All the above parameters changed substantially over approximately the same size range. Individuals acquired the ability to survive direct exposure to desiccation for the duration of a low tide over the 3.1-6.5 mm shell length (SL) size range, and also became virtually invulnerable to hatchling predators when they reached 6.5 mm SL. The shift in mortality factors was paralleled by a change in shell colour over the 3-7 mm SL size range, and in distribution over the 3-8 mm SL size range. All shifts were therefore completed by the time individuals reached 8 mm, or by the age of -4 n~o based on growth rates in the laboratory. The coordination of these ecological changes in N. emarglnata over the 3-8 mm SL size range constitutes an ecological transition that partitions postinetamorphic life into 2 penods, early juvenile and late juvenile/adult, each with d~stinct selective environments and corresponding adaptive traits. A similar ontogenetic transition has also been documented in juvenile lobsters, and studies of juveniles of other species reveal that comparable ecological changes are common among benthic marine invertebrates. Interspecific variation is nevertheless expected in the exact nature and timing of the transition, particularly as a result of differences in initial juvenile size, growth rate, adult size, ability to learn, and motility.

We examined the sensitivity of newly settled Mytilus trossulus to heat and desiccation, as well as the ontogeny of sensitivity through the early benthic phase. Laboratory experiments were conducted to determine the sensitivity of mussels to the full range of temperatures and desiccation levels experienced in the field. Mussels of 1 to 2 mm shell length (SL) experienced a threshold of heat tolerance at 34°C and a threshold of desiccation tolerance at vapour pressure deficit levels of 1.01 kPa. Extended periods of temperatures reaching or exceeding lethal levels for newly settled M. trossulus occurred relatively rarely in Barkley Sound, British Columbia, Canada, whereas lethal levels of desiccation occurred often during the recruitment season and were usually sustained for several hours. Desiccation, therefore, appears to be a substantially greater threat to recently settled M. trossulus than heat. A final laboratory experiment characterized the changes in sensitivity to desiccation that occur as mussels increase in size. Mussels became highly tolerant to desiccation when they reached a size of 2 to 3 mm SL. This size closely corresponds to the size at which juvenile M. trossulus relocate from protective filamentous algal habitat to adult habitat, suggesting ontogenetic shifts in habitat use by juvenile M. trossulus are a response to changing sensitivity to desiccation. If so, the future survival of newly settled mussels, and thus possibly the local persistence of mussel populations, may depend upon the persistence of protective algal microhabitats under changing climate conditions.