Recruitment and habitat selection in the intertidal sea anemones, Anthopleura elegantissima (Brandt) and A. xanthogrammica (Brandt)

Abstract

Juvenile sea anemones, Anthopleura elegantissima (Brandt) and A. xanthogrammica (Brandt), are common in intertidal mussel beds ( Mytilus californianus Conrad) on the outer coast of Washington state. Settlement of Anthopleura elegantissima also has been observed in rock crevices at sites in the San Juan Islands where mussel beds are absent. A massive settlement of A. elegantissima in 1972–1973 was monitored until 1976 by quadrat sampling at 42 sites on Tatoosh Island, Washington. The settlement was most dense in high areas of the mussel beds and in beds composed of large mussels. There was no evidence of recruitment in subsequent years. Mean size of individuals in the mussel beds increased over the 3-yr period and density doubled suddenly in 1975, probably by fission of existing individuals. Removal of anemones and patches of mussel bed at Tatoosh Island demonstrates that A. elegantissima can move out of the mussel bed and colonize cleared space below it. Settlement of A. elegantissima in crevices in the San Juan Islands occurred both in 1974 and in 1975 although not every monitored area received recruits. Two years' growth was sufficient for individuals to reach mean size of anemones in the local population. Newly settled A. xanthogrammica were not observed during 1972–1977. However, some mussel beds contained dense populations of very small juveniles, of a size (≤ 2 cm) not commonly observed in other intertidal habitats. Experimental areas cleared of all A. xanthogrammica were recolonized primarily by juveniles (<6.5 cm diameter, 81% of immigrants). Cleared areas adjacent to mussel beds received far more recruits than did those several meters distant. Thus the mussel bed serves as an important juvenile habitat for both species. Recruitment of A. xanthogrammica into mussel beds, and subsequent downward migration into tidepools and surge channels, may be a mechanism by which offspring select habitats with sufficient prey resource (mussels) for adult growth and survival.