ABSTRACT: I examined the interaction between the unusual, canopy dwelling echinoid Holopneustespurpurescens and its main host plant, the kelp Eckloniaradiata. During a 4 yr study at Cape Banks, New South Wales, Australia, H. purpurescens reached densities as high as 1 ind. per thallus of E.radiata and >17 m^-2, with densities declining strongly in the latter years of the study. These sea urchins also occurred, although at lower densities, on the dictyotalean alga Homoeostrichus sinclairii. H. purpurescens consumed laminae of E.radiata in the field at the rate of ~1 g large ind.^-1 (diameter >40 mm) d^-1. Consumption by the sea urchins was not affected by variation in phlorotannin levels among laminae. The impact of feeding by H. purpurescens on E.radiata, measured as (1) changes in the biomass of the kelps and (2) changes in thallus elongation rates, was examined in field experiments done in 2 seasons in which different numbers and sizes of sea urchins were caged with individual E.radiata. In spring, all densities and sizes of H. purpurescens caused significant damage (biomass) to E.radiata after 4 wk, and higher densities (2 per kelp thallus) of large sea urchins resulted in kelp mortality. No measurable damage occurred in autumn, with all kelps losing large amounts of biomass. No significant effect on thallus elongation rates was seen in either season after 4 wk. However, elongation rates varied among treatments after 2 wk in autumn. Grazing by H. purpurescens did not result in compensatory growth by E.radiata, nor did levels of phlorotannins in the kelps increase in response to grazing (chemical induction).

KEY WORDS: Holopneustes purpurescens . Herbivory . Kelp . Echinoids . Mesograzers . Inducible defenses

Published in MEPS Vol. 127 (1995) on November 2
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995