Basal consumers are an essential link between primary production and upper trophic levels in all food webs. In marine environments, benthic primary consumers, such as sea urchins, are a key macroalgal consumer and may act as ecosystem engineers. Sea urchin growth and feeding rates can influence the presence and abundance of macroalgal species that provide support and cover for a variety of species. Marine macroalgae are diverse and not uniform in their availability to urchin consumers in the field. The relative food quality of sympatric algal diets to urchins and potential indirect ecological consequences of urchins consuming different algal diets is unknown. The purpose of the present study was to evaluate feeding rates, growth, gonad development, and biochemical composition (lipids, fatty acids, proteins) of adult purple urchins (Strogylocentrotus purpuratus) when maintained on diets representing taxa from each of the key macroalgal phyla: Ulva sp. (Chlorophyta), Porphyra sp. (Rhodophyta), or Nereocystis luetkeana (Ochrophyta). For 10 weeks, adult urchins were maintained on controlled, monoculture diets during which time we collected algal samples for biochemical analysis. We also quantified the biochemical composition of algal diets and resulting urchin fecal material to evaluate the potential food value of urchin-derived fecal subsidies to other consumers.
Urchin wet weight and tests grew by an average of 56% and 50% respectively over the experimental period, but there were no differences growth based on diet. However, there were significant differences in feeding rates, with urchins consuming Nereocystis (mean ± SD ww mg/hr, 134 ± 0.4) at a higher rate than either Ulva (41 ± 7) or Porphyra (44 ± 11. Urchins fed Nereocystis generated significantly less fecal material (mean ± SD dw mg /hr, 2.9 ± 1.6) than those fed either Ulva (8.4 ± 0.1) or Porphyra (8.0 ± 0.4). At the end of the feeding trial, experimental urchin gonad indices (mean ± SD, 0.12 ± 0.03) were greater than conspecifics from the same low-intertidal location (0.03 ± 0.03). The combination of macroalgal and urchin biochemical composition will allow us to determine the role of compensatory feeding on algae of differing food quality and the role that these differences may play in the production of nutritional subsidies. Direct quantification of urchin trophic modification of consumed primary producers, via analyses of both the urchin tissues and their fecal material, will be used to better inform predictive models to understand trophic interactions and benthic community dynamics.