An ecological transition during juvenile life in a marine snail

Ecological shifts occurring after metamorphosis in benthic marine invertebrates have received much less attention than the more conspicuous transition occurring at metamorphosis 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 examines the ontogeny of vulnerability to desiccation, of susceptibility 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 mo based on growth rates in the laboratory. The coordination of these ecological changes in N. emarginata over the 3-8 mm SL size range constitutes an ecological transition that partitions postmetamorphic life into 2 periods, early juvenile and late juvenile/adult, each with distinct 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