(full paper is archived in the Miller Library)
Title: Ocenebra circumtexta in the intertidal: prey selection and stratification of a predatory snail
Student Author(s): Dusek, Eva
Faculty Advisor(s): Watanabe, Jim
Location: Final Papers Biology 175H
Date: June 1999
Abstract: Foraging behavior of Ocenebra circumtexta, an abundant predatory gastropod in the intertidal zone at Hopkins Marine Station, was studied. Ocenebra in the field were observed consuming barnacles, small limpets, tube worms, and mussels, with two barnacles, Tetraclita squamosa and Chthamalus spp. composing 85% of their diet. As a population, Ocenebra appear to be consuming prey proportional to its abundance in the field. This is consistent with a theory of random prey selection. However, snails marked and monitored over a five week period through 2 to 4 feeding cycles showed consistency in prey choice that differed from the population average. Prey items chosen by each individual were also consistent with a particular vertical zone in the intertidal. Ocenebra occur vertically from the subtidal to approximately +5 feet. This range encompasses 3 major zones, divided according to domination by algae, Tetraclita, or Chthamalus. Marked snails remained within the vertical zone corresponding to their choice of prey at a much higher rate than would be expected if the individuals moved about randomly. Two possible conclusions are that Ocenebra stratify according to prey type in the vertical zone, or that Ocenebra stratify for non-foraging reasons. Evidence for the former is that in tanks with 4 types of prey items, all 8 individuals monitored throughout 3 to 5 feeding cycles chose prey consistent with a specific zone in the intertidal. This supports a correlation between zonation and prey choice in Ocenebra. This could imply that learning, conditioning, or genetic variation is present in Ocenebra circumtexta, and could lead to a competitive advantage over other predatory snails in the intertidal.