AN ECOLOGICAL TRANSITION DURING JUVENILE LIFE IN A MARINE SNAIL

Ecological shifts occurring after metamorphosis in benthic marine inve rtebrates have received much less attention than the more conspicuous transition occurring at metamorphosis and settlement. It remains uncle ar whether postmetamorphic shifts occur simultaneously or at different times, and whether the shifts occur over brief, discrete periods or a re extended or even continuous through juvenile life. The present stud y of the muricid gastropod Nucella emarginata examines the ontogeny of vulnerability to desiccation, of susceptibility to hatchling predator s, of shell coloration, and of distribution among microhabitats as a f unction of snail size. All the above parameters changed substantially over approximately the same size range. Individuals acquired the abili ty 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 becam e virtually invulnerable to hatchling predators when they reached 6.5 mm SL. The shift in mortality factors was paralleled by a change in sh ell 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 similar to 4 mo based on g rowth rates in the laboratory. The coordination of these ecological ch anges in N. emarginata over the 3-8 mm SL size range constitutes an ec ological transition that partitions postmetamorphic life into 2 period s, early juvenile and late juvenile/adult, each with distinct selectiv e environments and corresponding adaptive traits. A similar ontogeneti c transition has also been documented in juvenile lobsters, and studie s of juveniles of other species reveal that comparable ecological chan ges are common among benthic marine invertebrates. Interspecific varia tion is nevertheless expected in the exact nature and timing of the tr ansition, particularly as a result of differences in initial juvenile size, growth rate, adult size, ability to learn, and motility.