POPULATION-DYNAMICS OF THE ECHINOMERMELLA-MATSI (NEMATODA) - STRONGYLOCENTROTUS-DROEBACHIENSIS (ECHINOIDA) SYSTEM - EFFECTS ON HOST FECUNDITY

At high population densities the green sea urchin Strongylocentrotus d roebachiensis can affect subtidal community structure significantly by grazing down kelp beds to barren grounds. The parasitic nematode Echi nomermella matsi has been suggested to reduce sea urchin population de nsities at barren ground localities, thereby giving rise to kelp recov ery and possibly a cyclic behaviour between these states. Here, we eva luated the potential of E. matsi to affect host population dynamics th rough effects on host fertility. This was done by estimating the effec t of infection on the host population gonad mass at 4 localities, rang ing from barren ground to a kelp bed. The relationship between sea urc hin gonad wet weight, sea urchin size and worm burden was modelled sta tistically. From this model we estimated the effect of infection at th e host population level. We found a strong negative relationship betwe en gonad wet weight of sea urchins and the wet weight of their nematod e infrapopulations. In some animals this reduction in gonad developmen t was estimated to be 100%, indicating functional castration. At the h ost population level, the relative reduction in gonad mass due to infe ction 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 fo r this low population effect of infection is that most sea urchins car ry low worm burdens, which will have a small effect on host gonads. Co mpared to the effect of inter-site differences in sea urchin size on m ean gonad sizes, the effect of infection on mean gonad size was found to be small. We therefore suggest that processes that determine sea ur chin size distributions, such as survival and growth, are likely to be more important in the determination of local reproductive potential t han direct effects of infection on gonad development.