ABSTRACT: At high population densities the green sea urchin Strongylocentrotus droebachiensis can affect subtidal community structure significantly by grazing down kelp beds to barren grounds. The parasitic nematode Echinomermella matsi has been suggested to reduce sea urchin population densities at barren ground localities, thereby giving rise to kelp recovery and possibly a cyclic behaviour between these states. Here, we evaluated the potential of E. matsi to affect host population dynamics through effects on host fertility. This was done by estimating the effect of infection on the host population gonad mass at 4 localities, ranging from barren ground to a kelp bed. The relationship between sea urchin gonad wet weight, sea urchin size and worm burden was modelled statistically. From this model we estimated the effect of infection at the host population level. We found a strong negative relationship between gonad wet weight of sea urchins and the wet weight of their nematode infrapopulations. In some animals this reduction in gonad development was estimated to be 100%, indicating functional castration. At the host population level, the relative reduction in gonad mass due to infection was less than 10% at all sample sites, and was in the same range both in the kelp forest and on the barren grounds. The main reason for this low population effect of infection is that most sea urchins carry low worm burdens, which will have a small effect on host gonads. Compared to the effect of inter-site differences in sea urchin size on mean gonad sizes, the effect of infection on mean gonad size was found to be small. We therefore suggest that processes that determine sea urchin size distributions, such as survival and growth, are likely to be more important in the determination of local reproductive potential than direct effects of infection on gonad development.

KEY WORDS: Kelp bed ecology · Parasitism · Sea urchin · Reproduction

Published in MEPS Vol. 163 (1998) on March 12
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998