This study compared the individual and combined effects of two introduced marine species in SE Tasmania - the northern Pacific seastar (Asterias amurensis) and the European green crab (Carcinus maenas) - and investigated their impact on native invertebrate fauna using in situ caging experiments. Both species predate upon bivalves, and this study assessed the biological interaction between these introduced species and native bivalve species - allowing the impact of multiple exotic predator species to be investigated in one system. The cage experiments have 5 treatment groups, including all combinations of presence (single animal) and absence of seastars and crabs, and a control with neither. Predator activity (number and type of bivalves consumed) was recorded after 8 weeks by suction-sampling each cage and counting and identifying fauna.
Five treatment groups contained all combinations of presence (single animal) and absence of crabs and seastars in cages. There was also an unmanipulated plot (1m2) without cage or animals added.
Cages were constructed of (1mx1 m base x 0.7m high) steel frame with legs (0.5m long) to anchor cage into sediment with 100-150mm of the cage side was buried. The cage top and sides were covered in plastic mesh (6mm).
To control for patchiness of infauna in the analysis of treatment effects, we used a randomised complete-block design.
The experiment ran for 8 weeks before cages and predators were removed. Two sampling techniques were used. Treatment plots were sampled with cores (150mm diameter, 100mm deep) to estimate abundance of infaunal and epifaunal organisms (>1mm). Three replicate cores were extracted at random positions for each plot. Samples were not taken within 0.1m of the cage perimeter to avoid edge effects.
The entire contents of the plots were sampled to a 0.1m depth using a diver-operated air-driven suction device. To do this, an open square frame (1mx 1m) was inserted into the sediment to isolate the plot, and all contents vacuumed into a 1mm meshbag.
Samples were sieved (1.0mm mesh) before fixing in 5-10% buffered formalin with Rose Bengal stain, and then rinsed in freshwater before storing in 100% ethanol. Infaunal and epifaunal organisms (>1mm) in core samples were sorted and identified to the lowest possible taxon. Suction samples were sieved again (2.0mm mesh), and all bivalves and the echinoid Echinocardium cordatum were sorted and identified to species.
The number of clams (Fulvia tenuicostata and Katelysia rhytiphora) eaten by each predator was counted in suction samples. Undamaged, empty shells with gaping valves identified bivalves that were eaten by seastars. Bivalve hinges with only a fraction of the shell remaining were identified as prey eaten by crabs. The lengths of live and undamaged empty bivalves were measured in all treatments.