A comparison of laboratory feeding rates with in situ capture of drift algae by the red urchin Strongylocentrotus franciscanus
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The red sea urchin (Strongylocentrotus franciscanus) is a common subtidal herbivore throughout the northeast Pacific. In the San Juan Archipelago (SJA), Washington, red urchins are subject to little predation pressure and are generally exposed and sedentary. Recent research has shown that detached drift algae are common and abundant at all subtidal depths surveyed (>150 m) in the SJA. Here, we investigated whether red urchin feeding rates observed in the laboratory were consistent with field observations of drift capture. Feeding rates were quantified for captive red urchins; from most to least rapidly consumed (grams per hour), 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 taxonomic composition 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 least at this site, availability of particular algae is more important in determining overall drift capture rates than is urchin preference. However, captured Agarum constituted a smaller proportion of total algal mass when urchins were given six days to collect drift, indicating that they 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 lowquality subsidy, at least for urchins.