Latitude and coastline shape correlate with age-structure of Chlorostoma (Tegula) funebralis populations

Between 2005 and 2008, size-frequency distributions from 22 populations of the intertidal snail Chlorostoma (Tegula) funebralis were studied to determine the effects of latitudinal region and coastal topography. Upwelling, which may affect larval recruitment, varies along a latitudinal gradient, while coastal topography may mitigate the effects of latitude. Sites were categorized by latitude: Northern (Oregon, 15 sites), and Southern (California, 5 sites; Baja California, 2 sites); and by coastal topography: protected (bays and coves, 7 sites in north, 3 in south), or exposed to the open ocean (7 sites in north, 5 sites in south). In Oregon, population structures at protected and exposed sites were significantly different (p = 0.001). At protected sites, a large percentage of individuals were pre-reproductive juveniles. At exposed sites, the population was weighted towards large individuals capable of high reproductive output with few juveniles present in the population. This pattern may reflect inter-annual variation in recruitment in Oregon, with protected areas experiencing more constant recruitment. In California and Baja California all populations were predominately composed of juveniles, regardless of coastal topography (p = 0.336). Differences in age-structure of the population create significant differences in relative reproductive potential. The potential reproductive output of a population was calculated by quantifying the eggs produced by snails covering the size range of reproductive individuals. Populations in the north at exposed sites produce eggs at a rate up to 2 orders of magnitude higher than populations in the south (p = 0.004). These variations in size-structure of C. funebralis populations with coastal topography and latitude may have important impacts for the population distribution.