ABSTRACT: It has often been observed that the abundance of sea urchins is lower inside kelp forests and along their nearshore edges than outside their offshore boundaries. We tested the hypothesis that this distributional pattern is a reflection of settlement patterns by monitoring settlement on artificial surfaces at 3 kelp forests near San Diego, California, USA. We estimated settlement at paired sites under the canopy and outside the offshore edge of the kelp forests and at paired sites located 20 m outside the canopy near the inshore and offshore boundaries of each kelp forest. There was no evidence of an effect of dense stands of giant kelp on the settlement of purple sea urchins. The results for red sea urchins were ambiguous; in 3 of 7 comparisons, average settlement was higher or observations of higher settlement were more frequent offshore from the kelp forest than elsewhere. Although this could result from different effects on the 2 species, we think it probably was due to accidents of sampling. We conclude that the distributional patterns of purple and red sea urchins relative to kelp forests are unlikely to reflect larval availability or settlement, but are more probably a function of post-settlement events. We qualify our conclusions because of the tremendous temporal variability in the physical and biological factors that can potentially affect the local distribution of sea urchin larvae. The 2 years of this study may be short relative to natural cycles.

KEY WORDS: Echinoidea . Kelp . Larval filter hypothesis . Sea urchin . Settlement

Published in MEPS Vol. 133 (1996) on March 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996