Larvae which fall far short of their potential: highly localized recruitment in an alpheid shrimp with extended larval development

Many marine invertebrates which are sessile or sedentary as adults have larvae which swim actively for days or weeks in the laboratory. In such species it is generally assumed that the larvae disperse widely, and that settlement in the immediate vicinity of parents is a rare event. Here we present the results of a series of experiments whose purpose was to test this assumption. The subjects were three sibling species of alpheid shrimp which live symbiotically with the Caribbean sea anemone, Bartholomea annulata. The shrimp have larvae which in the laboratory are phototactic and swim actively for up to one week. On an isolated patch reef we placed anemones and either no adult shrimp, fertile pairs of Alpheus immaculatus, or pairs of A. immaculatus which were rendered infertile surgically. We monitored recruitment by all three alpheid species into anemones placed from 0–70 m from the reef. The three treatments allowed us to distinguish the effect of adult conspecifics in general (gregarious settlement) from the effect of conspecifics releasing young. Recruitment patterns of the three species were similar except in the presence of fertile pairs of A. immaculatus. During this treatment, there was a significant difference between A. immaculatus and the other two species, with the former showing enhanced recruitment into anemones placed within 6 m of the reef. This suggests that a detectable proportion of the successful recruits settled extraordinarily close to their parents.