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Date of Award


Document Type


Degree Name

Master of Science (M.S.)


Marine Sciences

First Advisor

Steven Obrebski

First Committee Member

Edmund H. Smith

Second Committee Member

James A. Blake


Two common limpets, Collis~ (Acmaea) digitalis and Collisella (Acmaea) scabra have overlapping distributions in the upper int.ertidal .spray zone. Haven (1971) found that Q. digitalis is more abundant on vertical surfaces while Q. scabra dominates horizontal surfaces. He assumed this preference to be due to differential abilities to withstand desiccation. In this study I report field work demonstrating a high correlation between the-ratio of abundance of Q. digitalis/ Q. scabra and angle of slope of substrate. Length (mm) of Q. scabra declines linearly with angle, but Q. digitalis shows no such trends. Results of laboratory measurements of angle of substrate and small and large members of the species do not alone explain the size distribution and abundance relationships found be·tween the two species in the field .• Although resistance to desiccation may play a part in determining distribution and abundance, especially in the upper intertidal, it is clear that other factors may be important, such as food resources. Fecal pellets were used as a technique to study food resource partioning. Limpet size and abundance is related to the availability of microalgae on a particular slope of substrate. Differences in the amount of movement between the two species may be. related to time and distance traveled in foraging for food. The ratio of abundances of these limpets and its relation to substrate angle needs to be studied from the point of view of competitive interactions with respect to food resource partioning, since these species seem to have tolerances to desiccation greatly exceeding environmental stress conditions.