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A comparison of laboratory feeding rates with in situ capture of drift algae by the red urchin Strongylocentrotus franciscanus
Abstract
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.