(full paper is archived in the Miller Library)
Title: The Tegula funebralis shore-level size gradient: settlement and effects of susceptibility to predation, and settlement
Student Author(s): Polk, R. Eben
Faculty Advisor(s): Watanabe, James
Location: Final Papers Biology 175H
Date: June 1998
Abstract: Past studies have found that the black turban snail, Tegula funebralis, exhibits a shore-level size gradient, with mean shell size either increasing or decreasing in a down-shore direction, at different sites. Predation has been implicated as a driving factor. A shore-level size gradient was found at Monterey Bay, CA. Snails > 15 mm were more abundant than other size classes in the Low intertidal zone, while snails 9 - 15 mm were the most abundant class in the Mid zone, and snails £ 9 mm were the most abundant class in the High zone. Mean size increased in a down-shore direction. The study site had virtually no Pisaster ochraceus, a known predator.
Laboratory experiments revealed that small snails are significantly less susceptible to predation by Pisaster, compared with both medium and large snails. Experimental responses to direct contact with tube feet show some size-related differences in short-term behavior. However, these would tend to make small snails more susceptible than larger T. funebralis; non-behavioral mechanisms must keep Pisaster from preying on small snails.
A qualitative search for juvenile T. funebralis < 2 mm was undertaken at two sites in Monterey Bay. Juveniles were abundant in the Mid zone at both sites, and rare or not present in the High and Low zone, indicating that settlement occurs in the Mid zone, with migration of T. funebralis becoming possible at greater sizes.
Results suggest that other factors besides predation, such as settlement, are responsible for some components of a size gradient at a protected rocky shore in south Monterey Bay, including the general absence of small T. funebralis from the Low intertidal zone.