Long-term population dynamics of sea urchins in a central California kelp forest: rare recruitment and rapid decline

109

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.

Section: Mortalitysupporting

confidence: 79%

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

Rowley¹

1990

109

“…It is known that echinoids can feature low levels or even absence of recruitment during several years, p.unctuated by episodic highs (Ebert 1983, Pearse & Hines 1987. Small-scale (of the order of thousands of metres) variation in recruitment is also documented for echinoids (Ebert 1983, Keesing et al 1993.…”

Section: Discussionmentioning

confidence: 99%

Echinodermata:Echinoidea in two contrasting habitats

J¹,

Galera²,

López³

et al. 1995

126

Two populations of Paracentrotus lividus (Lamarck) (Echinodermata: Echinoidea) from the Northwestern Mediterranean were compared to ascertain the plasticity and adaptive value of certain biological parameters. These populations were found in habitats which differed in terms of stab~lity of environmental parameters and food availability. A yearly reproductive cycle was seen in both communities, with a spawning season at the end of spring that lasted up to several months. Recruitment data, inferred from abundance of post-settlement stages, indicated lack of settlement during the study period in the unstable community, while a major recruitment in summer, with less intense episodes occurring until autumn, was recorded in the stable community. The intensity of recruitment varied over an order of magnitude in the 2 consecutive years monitored. The gonadal indices were higher and the feeding indices were lower in the unstable community, indicating greater allocation of resources to reproduction, at the expense of somatic growth, in response to unfavourable conditions The graph of the feedlng and gonadal indices wlth size was parallel In the unstable community, while there was a lag between the sizes at which feedlng activity and gonadal biomass reached their highest values at the stable site, indicating that feeding activity is not so tightly determined by gonad investment needs in the stable, food-rich community.

“…For instance, because sea urchins can structure reef communities (Schiel & Foster 1986, Hughes et al 1987, Pearse & Hines 1987, the fishing of urchin predators can increase the abundance of urchins, which can then overgraze algae (Tegner & Dayton 1981, Tegner & Levin 1983, Siversten 2006. Evidence for this effect comes from comparisons of fished areas with marine reserves, which can help restore food webs to their former state (Sala et al 1998b, Shears & Babcock 2002, Behrens & Lafferty 2004, Lafferty 2004, Guidetti 2006.…”

Section: Introductionmentioning

confidence: 99%

Cascading effects of fishing on Galapagos rocky reef communities: reanalysis using corrected data

Sonnenholzner¹,

Ladah²,

Lafferty³

2009

due to errors in entry of data on sea urchins. We sampled 10 highly fished and 10 (putatively) lightly fished shallow rocky reefs in the southeastern area of the Galapagos Marine Reserve, Ecuador. After the correction, these are the new results: there was a negative association between slate-pencil urchins Eucidaris galapagensis and non-coralline algae. In addition, pencil urchins were less abundant where there were many predators. An indirect positive association between predators and non-coralline algae occurred. Fishing appeared to affect this trophic cascade. The spiny lobster Panulirus penicillatus, the slipper lobster Scyllarides astori, and the Mexican hogfish Bodianus diplotaenia were significantly less abundant at highly fished sites. Urchin density was higher at highly fished sites. Non-coralline algae were nearly absent from highly fished sites, where a continuous carpet of the anemone Aiptasia sp. was recorded, and the algal assemblage was mainly structured by encrusting coralline and articulated calcareous algae.