Short-distance dispersal in a fouling community amphipod crustacean, Jassa marmorata Holmes

Amphipod crustaceans lack a larval dispersal stage and their means of dispersal has not been studied adequately. We investigated short-distance dispersal in the fouling community amphipod Jassa marmorata Holmes, utilizing artificial collectors in the form of replicated tufts of polypropylene rope wired into the fouling crust. Collectors were replaced at 2-wk intervals for 1 yr. The population in the fouling crust was assayed by means of monthly scrape samples removed from the habitat adjacent to collectors. Recruitment into collectors was limited almost entirely to a period from May through July. Dispersing individuals were mostly juveniles < 1.5 mm (newly released from brood pouches). Adult recruits (primarily, gravid females and nonthumbed males) were most prevalent in May and were, on average, smaller than adults in the crust population at that time. Small adults may be less competitive for tube sites and mates and, thus, more likely to disperse. Recruitment into collectors closely paralleled abundance patterns in the crust, with a major decline occurring in August. Recruitment of 1-mm juveniles was significantly correlated with the number of adults in the crust, suggesting that juvenile recruitment is density-dependent. In a species with overlapping summer cohorts, active juvenile recruitment may be a means of reducing competition with conspecifics and efficiently exploiting a continuously changing environment.