A Comparison of Recent and Historical Populations of the Colonial Hydroid Hydractinia

Abstract

The hydroid Hydractinia sp. nov. occurs as an epibiont on shells occupied by the hermit crab Pagurus longicarpus. There is a seasonal pattern of abundance, with maximum reproduction, recruitment, and competition occurring during the summer months. Recruitment occurs in stereotypic positions on the undersurface of shells. Since recruits are clustered, intraspecific competitive encounters occur most frequently when hydroid colonies are small. The hydroid size—frequency distribution is bimodal, suggesting high mortality at small colony size and low mortality at large colony size. A refuge from intraspecific competition exists on small shells. Interspecific competition between the bryozoan Alcyonidium polyoum and H. sp. nov. is also seasonal, and static observations of overgrowth interactions imply that A. polyoum is the dominant competitor. Predation on H. sp. nov. is rare in Long Island Sound. Samples of five museum collections dating from 1875 to 1916, supplemented by a limited collection of Miocene material, display patterns of site—specific recruitment, size—frequency distribution, and shell—size dependence of recruitment that are largely congruent with patterns observed in modern samples.