Feces as food: The nutritional value of urchin feces and implications for benthic food webs

Abstract Algal subsidies are important to basal consumers of the deep benthos where there is little to no primary productivity. Algal detritus such as pieces of kelp that sink into deep habitats can be an important direct nutritional subsidy, but sea urchin feces may provide an additional, indirect energetic link from shallow-water macroalgae to benthic community members that are too small to handle and consume large detritus directly. Urchins digest macroalgae inefficiently, creating the potential for two key trophic consequences to the benthic food webs they live in. First, urchins act as marine ‘shredders’ creating smaller detrital particles from larger drift; second, the poor digestion may enable microbes to enrich the food value of both the digesta within the urchin guts, and the egesta (feces) after it leaves the gut. We quantified the relative nutritional value of algae and of feces of red and green sea urchins ( Mesocentrotus franciscanus and Strongylocentrotus droebachiensis) fed on monodiets of various algae in laboratory experiments. We then conducted feeding experiments with an epibenthic copepod ( Tigriopus californicus ) to evaluate consequences to a model consumer of different diets including feces. We also quantified assimilation efficiencies of red urchins fed a diet of bull kelp ( Nereocystis luetkeana ). In many cases, key indicators of nutritional value (especially calories and protein content) of algal material increased after being consumed and egested by urchins, and urchin feces “aged” in seawater generally became even more calorie-rich. Benthic copepods raised on diets of urchin feces derived from kelp had faster population growth than those raised on chopped fresh kelp tissue. It is likely that microbiota inside urchin guts are driving these counterintuitive results. The creation of nutritious feces could add to the importance of urchins as a link to benthic communities that rely heavily on detritus for their success.