Undergraduate Honors Theses

Thesis Defended

Fall 2012

Document Type



Ecology and Evolutionary Biology

First Advisor

Dr. Andrew Martin


The red sea urchin (Strongylocentrotus franciscanus) is a common subtidal herbivore throughout the northeast Pacific. In the San Juan Archipelago, Washington, red urchins are subject to little predation pressure and generally exposed and sedentary. Recent research has shown that subtidal red urchins feed primarily on detached drift algae, abundant at all subtidal depths surveyed in the San Juans. Here, I investigated whether field observations of urchin drift capture were consistent with red urchin feeding rates in the laboratory and feeding preferences shown in other studies. Feeding rates were quantified for captive red urchins; from most to least rapidly consumed (g/hr), these were: Nereocystis luetkeana, Mazzaella splendens, Saccharina sp., Agarum fimbriatum, and Ulva sp. In the field using SCUBA, we repeatedly collected all algae captured by urchins at one-day and six-day intervals within a 25 m2 permanent transect at a depth of 18 m. We identified, blotted, and massed the “stolen” algae to compare proportions and mass captured over different time frames, assuming that drift held after a longer time period would more closely reflect urchin preference. Results indicate that at this site, availability of an alga is more important in determining its proportion in captured drift than is urchin preference. However, comparing proportions of each specific alga between time frames revealed some tentative trends of selectivity. The most statistically and ecologically significant of these were kelps of the genus Agarum, which constituted a much smaller proportion of total mass when urchins were given six days to collect drift, indicating that urchins are likely discarding this alga. This result is consistent with current and previous lab preference studies and suggests that the large quantity of Agarum drift into deep water is a low-quality subsidy, at least for urchins.