In Barkley Sound, British Columbia, Canada, newly hatched Nucella emarginata, a direct developing prosobranch gastropod, are almost exclusively located within 3 structurally complex microhabitats: the filamentous algae Cladophora columbiana, clusters of mussels Mytilus spp., and dense assemblages of the barnacle Semibalanus cariosus. When exposed to desiccation or predators for 4 to 5 h, mortality of hatchlings located in these microhabitats (1.7 to 36% mortality) was considerably lower than for individuals on open surfaces (99 to 100%). Also, food (prey species) in C. columbiana, mussel clusters, and S. cariosus microhab~tats was more abundant but of similar energetic value to that on open rock surfaces. The microhabitats used by early juvenile N. emarginata thus provide the double advantage of a considerably reduced risk of mortality and abundant prey. The means by which hatchlings disperse after emerging from their egg capsules were established by laboratory observations and by deployment of collectors in the field. Hatchlings do not actively initiate dispersal by drifting in the water column. Crawling is the usual means of travelling from the capsule to protective microhabitats, with hatchling crawling speed averaging only 3.7 mm min-' on smooth rock surfaces in the laboratory. The abundance and distribution of microhabitats that protect early juveniles could therefore be important determinants of the local abundance and distribution of N. emarginata populations. The use of protective microhabitats may be the key to survival through the vulnerable early juvenile period in many benthic marine organisms.

Delayed metamorphosis can affect post-metamorphic performance in a variety of marine invertebrate species, possibly through effects on larval energy reserves. Nevertheless, the consequences of the 'latent effects' of natural environmental conditions and the proximate causes of the effects have rarely been examined. We documented the combined effect of delayed metamorphosis and sub-lethal exposure of larvae to low-salinity stress on depletion of energy reserves, metamorphic success and juvenile growth rates for the estuarine barnacle Balanus amphitrite, both in the laboratory and in field outplant experiments. Acute (24 h) exposure of 0 d old cyprids to low salinity (10 psu) dramatically reduced juvenile growth rates (measured as basal diameter, dry weight and ash-free dry weight) for the first 5 d after metamorphosis, in both laboratory and field conditions, and the effects were similar to those resulting from delayed metamorphosis. There was an interaction between delayed metamorphosis and salinity effects on juvenile tissue growth rates. To determine how the negative effects of delayed metamorphosis observed in the laboratory might be modulated in the field under different nutrient conditions, we also outplanted juveniles from cyprids that had delayed metamorphosis for either 0 d (control) or 4 d at 2 intertidal sites that experienced different nutrient conditions. Surprisingly, surplus food availability in the juvenile habitat could not entirely compensate for the negative effects imposed by delayed metamorphosis on juvenile growth rates. Additionally, we examined whether or not the detrimental effects of delayed metamorphosis on growth were mediated through effects on juvenile feeding rates and feeding appendage (cirri) morphology. Interestingly, although 2 out of the 3 juvenile feeding cirri lengths were shorter than expected in delayed individuals, mean juvenile filtration rate was significantly higher in delayed individuals than in control individuals. Overall, our results emphasise that acute exposure of competent larvae to low-salinity stress negatively affects juvenile growth rates as severely as delayed metamorphosis, which may ultimately influence juvenile and adult population dynamics in the field. The cause of reduced post-metamorphic performance in young barnacles remains unclear, but does not seem to be caused by reduced capacity for feeding.

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Although depleted energy reserves are not a major direct cause of early benthic phase mortality for many species of marine invertebrates, initial energy reserves might nevertheless play a role in early benthic phase mortality through indirect effects on physiological tolerance thresholds, a mechanism that has not been tested. We therefore examined the extent to which energy reserves affect acute tolerance thresholds of early benthic phase individuals of 2 species (Balanus glandula and Nucella ostrina) to 2 of the most challenging intertidal stressors: desiccation and high emersion temperature. Energy reserve levels were estimated by maintaining individuals without food for different durations of time or using individuals of different body mass. Individuals in each energy level treatment were then exposed to a range of desiccation periods and emersion temperatures. Lower estimated energy levels had no effect on tolerance to emersion temperature, but did significantly reduce the ability to tolerate desiccation. This suggests the survival of individuals through the early benthic phase is likely dependent on initial energy content through an indirect effect on tolerance to desiccation. Also, habitats experiencing intense desiccation likely impose a selective pressure on intertidal invertebrates favoring the evolution of greater energy reserves at the onset of the early benthic phase. These findings suggest depleted energy reserves at the onset of the early benthic phase can influence early benthic phase mortality through indirect effects by making individuals more vulnerable to environmental stressors. In this way, energy reserves may impact recruitment rates and thus influence population dynamics and intertidal community structure.