Newly settled sea urchins in a kelp bed and urchin barren ground, a comparison of growth and mortality

Abstract

Growth of newly settled purple sea urchins Strongylocentrotus purpuratus was studied using laboratory rearing of individuals at natural densities, size-frequency analysis of a recently settled cohort collected in field samples, and mark-recapture techniques with individuals injected with tetracycline. In the laboratory, sea urchins were reared on crustose coralline algae as characteristic of a sea-urchin barren ground (barrens), and on foliose red algal turf as characteristic of a kelp bed. Growth in a barrens was compared with growth in a kelp-bed habitat using all 3 methods. Field work was done at 2 subtidal sites near Santa Barbara, California, USA, from 1985 through 1987. S. purpuratus in the laboratory grew at similar rates for the first 50 d after settlement regardless of substrata (0.29 to 0.45 mm mo-l), and in the field grew slightly, but significantly, faster in the barrens (0.42 mm mo-') than in the kelp-bed (0.31 mm mo-l). At roughly 50 d of age and 0 8 to 1.2 mm test diameter, sea urchins switched from feeding by surface scraping to grazing on fleshy algae. Thereafter, growth was signlflcantly faster in the presence of macroalgae (0.56 to 1.67 mm mo-l), than in habitats or treatments without fleshy algae (0.12 to 0.27 mm mo-'). Based on these results, S purpuratus diameter at 1 yr old was estimated to be 3.6 mm in barrens and 17 mm in kelp-bed habitats. Newly settled sea urchins suffered higher mortality in the kelp bed than in the barrens, but individuals at slightly greater ages showed no detectable difference in mortality between the 2 habitats. The attainment of a threshold size for switching diets and increasing growth rate (if algae is available) may be a critical event in a sea urchin's life-history.