Upstream migration and altitudinal distribution patterns of Nereina punctulata (Gastropoda: Neritidae) in Dominica, West Indies

Abstract

The snail Nereina punctulata has been observed performing amphidromous migrations (salt to freshwater migration, post-larval settlement) in the Caribbean, with small- and medium-sized snails achieving maximum fitness at the mid- and high altitudes, but they may be restricted by energy stores. Large snails show no difference in fitness across altitude, but their previous migration history dictates their high-altitude placement in watersheds. The factors determining the rate of migration have not yet been studied. In this study, we sought to understand how migration rate changes with shell size and altitude. We used mark–recapture to track individual snails across seven sites of varying altitude in a single watershed on Dominica and measured the shell length of randomly collected snails at sites. Volunteers were assisted with data collection in both cases. Shell length was positively correlated with distance from river mouth, although smaller snails were more frequently found at high altitude, high flow sites. Snails closer to the river mouth had faster upstream migration rates than those at mid-altitude. While we found large snails at higher altitude sites, there was no significant relationship between migration rate and shell size. Our findings suggest that large snails do not migrate at maximal rates allowed by energy stores. We also observed erosion of the outer shell periostracum and calcium carbonate underneath, which was seen significantly more often on larger shells. We hypothesise that this erosion is a product of exposure of the structural calcium carbonate to low alkalinity in Dominican streams, following an initial chipping of the periostracum.