THE FUNCTIONAL ROLE OF ASTERIAS VULGARIS VERRILL (1866) IN THREE SUBTIDAL COMMUNITIES
The functional role of Asterias vulgaris has been investigated in three subtidal communities at the Isles of Shoals, New Hampshire, U.S.A., from 1975 to 1980. The three communities are located at depths of 8 m, 18 m, and 30 m along a transect from the intertidal to a depth of 35 meters on rocky substrate of an exposed shore. Components of the study based on in-situ observations included an analysis of population structures, feeding biology, flux rates, predators and natural history of Asterias vulgaris in the 3 communities. The feeding data shows a series of prey specializations by size such that a progression of specialization on increasingly larger prey seems to be required for the species to grow larger (i.e. ectoprocts to hydroids to small gastropods to echinoids . . . to bivalves for the largest individuals). The communities at 8 m and 30 m contain a variety of prey that are utilized, and normally distributed population structures of asteroids are found. The middle community at 18 m is anomalous in several ways; fewer asteroids are feeding at 18 m, a small species of Leptasterias is present in high densities which may be a direct competitor for food as well as a predator upon Asterias vulgaris, especially at the most critical small sizes, and the resulting population structure is strongly skewed to very small individuals, 90% of which are Leptasterias sp. The giant Asterias vulgaris, found only at 18 m feed almost exclusively on large Modiolus modiolus, and are probably migrants into the community from deeper, soft-substrate communities. Asterias vulgaris is a generalist species found in a wide range of communities, consuming the prey available in a community in relation to its size-limited abilities. Its role in the 3 communities is not that of a major structuring or controlling factor, but rather it is itself controlled by the characteristics of the community. Most of the species in the low diversity Gulf of Maine are opportunistic in nature and not competitive dominants. Therefore concordance to recent community models is in terms of which community is more (8 m) or less (30 m) opportunistic. The 8 m community is a shallow subtidal continuation of the intertidal into a highly productive algal zone. The 30 m community is much more physically stable, but has lower productivity than the shallower areas. The 18 m community is transitional between the geographically more extensive 8 m and 30 m communities and is an area of ecological release for some species and stress for others. The functional role of Asterias vulgaris in three adjacent subtidal communities showed differences in the feeding biology, population structure, and rates of movement of this species in each community. The observed differences in the populations can be related to differences in ecological aspects of each community. The important potentially controlling factors for Asterias vulgaris appear to be: (1) physical disturbance at 8 m due to temperature changes and wave action; (2) decreased food availability at 18 m due to a Leptasterias sp. predator and competitor exaggerating low food potential; and (3) decreased primary productivity at 30 m. Asterias vulgaris is a foraging predator which takes prey as encountered and its population structure and feeding biology reflect a dynamic equilibria with the community.
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