Initial juvenile size and environmental severity: influence of predation and wave exposure on hatching size in Nucella ostrina

Abstract

According to life history theory, offspring survival increases with offspring size, mainly because large offspring are more resistant to environmental stress. Populations and species living in habitats with severe environmental conditions have therefore been predicted to produce larger off- spring than populations in more benign environments. We studied 10 populations of the rocky inter- tidal gastropod Nucella ostrina in Barkley Sound, Canada, to determine the extent of variation in average hatching size at a small spatial scale (the sites being ≤10 km apart), and to determine the influence on hatching size of 2 components of environmental stress: predation and wave exposure. In a 3 yr study of these 10 populations, average shell length of newly hatched snails differed by 15 to 25% among the populations in a given year, and estimated total organic carbon per hatchling varied by 97 to 210%. These are considerable differences given the proximity of these populations. The rel- ative differences among populations were broadly consistent over the 3 years of the study, some pop- ulations consistently producing larger average hatching sizes than other populations. Based on the abundance and size structure of the predator populations at 4 of these field sites, predation pressure was predicted to favour different average N. ostrina hatching sizes at these sites: large at 2 sites and small at 2 sites. However, the average hatching size of N. ostrina populations at those sites was not consistent with that prediction, suggesting predation pressure may not have been an important determinant of hatching size. This is a surprising finding, as predation is often considered to be a major cause of early juvenile mortality and thus an important selective pressure influencing the evo- lution of early juvenile traits. However, average population hatching size did correlate with the rela- tive degree of wave exposure of the site, indicating that either wave action itself or factors covarying with wave action have a substantial influence on initial juvenile size in this species.