Recruitment and Postrecruitment Interactions in a Colonial Hydroid

Abstract

The colonial athecate hydroid Hydractinia echinata encrusts gastropod shells inhabited by hermit crabs of the genus Pagurus. In field samples recruits were found clustered in specific locations on the undersurface of Urosalpinx cinerea shells. Assays of the behavior of larvae indicated that the bacteria that induce metamorphosis coat the entire shell. A series of laboratory experiments demonstrated that site—specific recruitment reflected the interplay between initial sites of contact with the shell, movement prior to metamorphosis, and differential mortality after metamorphosis. In situ experiments demonstrated that growth rates were high in the areas most frequently occupied by recruits. Recruitment sites determined the ontogenetic timing of subsequent intraspecific encounters between colonies, and thus influenced both the cost and outcome of competition. These results show that bacterial induction of metamorphosis is associated with extensive juvenile mortality, that field distribution of recruitment represents differential mortality rather than adaptive patterns of habitat choice, and suggest that environmental variation in recruitment position acts to maintain genetic variation in competitive ability.