The targeting of large-sized benthic macrofauna by an invasive portunid predator: evidence from a caging study

Abstract

The Portunid crab Charybdis japonica was first found in Waitemata Harbour, New Zealand, in 2000. It has established breeding populations and has been spreading, yet information on its dietary preferences in New Zealand are unknown. We conducted field caging experiments to elucidate prey choices and potential impacts of Charybdis on benthic communities. We tested the hypothesis that Charybdis would reduce the previously demonstrated positive influence of native pinnid bivalves, Atrina zelandica, on the abundance and richness of surrounding soft-sediment macrofauna. Adult male Charybdis were introduced to cages with and without Atrina that included soft-sediment macrofaunal communities of ambient composition and abundance. After leaving the crabs to feed overnight, changes in community structure (relative to sediments without crabs) were determined by coring the sediment and analysing the resident macrofauna. Prey choices were verified by extracting taxa from the stomachs of crabs collected from the cages in which they had been feeding. The abundance of large taxa including burrowing urchins, bivalves and native crabs was lower in the presence of Charybdis compared to areas without this invader. The stomach contents of Charybdis were dominated by these same three taxa, constituting 85 % of the prey abundance when using stomach fullness as a weighting factor. Our hypothesis was supported with the greatest net losses occurring in cages with Charybdis and Atrina. Reduction in the abundance of Echinocardium cordatum by Charybdis could have cascading ecological effects, as these urchins play a critical role in benthic soft-sediment ecosystems in New Zealand via bioturbation and biogenic disturbance.