CHARACTERIZING TEMPERATE ROCKY SHORES FROM THE PERSPECTIVE OF AN EARLY JUVENILE SNAIL - THE MAIN THREATS TO SURVIVAL OF NEWLY-HATCHED NUCELLA-EMARGINATA

Mortality factors most likely to constitute substantial selective pres sures for early juvenile gastropods on temperate rocky shores were ide ntified by examining the vulnerability of hatchlings of an intertidal snail, Nucella emarginata, to heat stress, desiccation, and predation in 1992 and 1993. The highest temperature of substrata measured at tid al heights colonized by N. emarginata in Barkley Sound, British Columb ia, Canada, was 28.5 degrees C. This temperature was not lethal to hat chlings in laboratory tests. In laboratory and field desiccation exper iments, all hatchlings died within 6 h of emersion. Early juveniles co uld not survive direct exposure to even moderate drying conditions for the duration of a low tide. Hence, intertidal microhabitats which dry up even for short periods during low tides would prove lethal. Of 45 intertidal animal species to which hatchlings were exposed in the labo ratory, small decapod crustaceans were the only organisms to cause sub stantial hatchling mortality. Of these, Pagurus hirsutiusculus and Hem igrapsus nudus were by far the most abundant in the field, and are pro bably the only important predators of early juvenile N. emarginata at most sites. Total predator densities in the field were as high as 438 individuals m(-2), suggesting that predation pressure may be intense. Desiccation and predation by decapod crustaceans appear to be the most significant threats to early juvenile N. emarginata. These factors co mmonly occur on most temperate rocky shores and undoubtedly constitute major selective agents influencing population parameters and shaping life-history strategies and early juvenile traits of intertidal invert ebrates.