Impact of introduced seastars Asterias amurensis on survivorship of juvenile commercial bivalves Fulvia tenuicostata

The introduction and establishment of the predatory seastar Asterias amurensis in southern Australia is considered a major threat to benthic marine communities and commercial bivalves. The impact of A. amurensis on a soft sediment assemblage in SE Tasmania was quantified, with particular attention to effects of seastar predation on the survivorship of recently settled juveniles of the commercial bivalve Fulvia tenuicostata. In a manipulative experiment, densities of F tenuicostata juveniles were reduced by ca. 15 fold (from 580 to 35 m(-2)) in the presence of seastars at background densities relative to the treatment without seastars. In a feeding survey, A. amurensis exhibited preference for F tenuicostata following settlement of the bivalve over the period from December 1997 to January 1998, which comprised 80 and 50% of the seastar's stomach items in February 1998 and April 1998, respectively. However, A. amurensis preferred a variety of other prey taxa when the abundance of F tenuicostata was low, particularly other bivalves, gastropods and the echinoid Echinocardium cordatum. This indicates that the seastar may potentially affect the abundance of other prey taxa. The results provide further support to the hypothesis that predation by A. amurensis is largely responsible for the recent decline and subsequent rarity of large bivalves in its current distribution in Tasmania. The potential of seastar predation to have wider ecosystem level effects than the short-term direct effects reported here is a major concern.