Ontogenetic shifts in resource use by the sea urchin Evechinus chloroticus across an ecotone

Abstract

Sea urchins exhibit dramatic changes in ontogenetic niche between newly settled recruits (many of which occupy cryptic habitats and feed on microalgae or detritus) and mature adults (that may catch drift kelp or actively graze on macroalgae and invertebrates). We examined patterns in the ontogenetic niche of Evechinus chloroticus within the New Zealand fjords, a system with a series of strong environmental and benthic productivity gradients. Using data from 15 long-term monitoring sites, we examined relationships between the diet of juvenile and adult sea urchins and the morphology of their preferred food, the kelp Ecklonia radiata. Stable isotope analysis (δ13C and δ15N) of E. chloroticus stomach contents, muscle tissue and samples along the blades of E. radiata revealed evidence of large variation in nutritional conditions. The contribution of E. radiata to the diet of E. chloroticus declined from the wave-exposed sites near the entrances of fjords to the fully wave-sheltered sites of the inner fjords. Coincident with the decline in prevalence of E. radiata as a food source, the trophic level of E. chloroticus increased at inner fjord sites, indicating prey switching to grazing on invertebrates. Along this gradient, we observed a divergence in the trophic levels of adult E. chloroticus from those of juveniles, indicating that adults maintain a higher trophic level at sites where their preferred food resources are scarce. These results have important implications for understanding how sea urchin populations persist across strong gradients in primary productivity, and their role in subtidal food webs.