Three’s a crowd: Habitat interaction between native (Carcinus maenas) and invasive (Hemigrapsus sanguineus and Hemigrapsus takanoi) brachyuran crabs.
Supervisor:  David W. Thieltges (Royal Netherlands Institute for Sea Research, NIOZ).
Two shore crabs from Asia, the Asian shore crab Hemigrapsus sanguineus and the brush-clawed shore crab H. takanoi, were introduced in the European coasts in the mid-90s. H. sanguineus was first observed outside its native range in the east coast of the United States during the ’80s, which shares the same rocky intertidal as their native habitat. Consequently, it had been reported to caused population declines and displaced in some areas the established invader Carcinus maenas, which is native to European waters. However, the effects of H. sanguineus on C. maenas is yet to be fully understood. In the Wadden Sea, H. sanguineus was first reported in Bremerhaven in 1993. H. sanguineus and H. takanoi were first reported in the Dutch Wadden Sea in the mid-2000s and were able to successfully invade the whole Wadden Sea. Both species commonly inhabits underneath boulders in their native range. The Wadden Sea is a UNESCO world heritage site that had undergone severe habitat modifications through anthropogenic pressure in the past decades. The pacific oyster Magallana gigas was initially introduced for aquaculture but had successfully established in the wild. Also, it had undergone fortification through coastal defenses, which created rocky/boulder habitats. In this study, the habitat interactions between the native C. maenas and the invasive H. sanguineus and H. takanoi were assessed through a series of laboratory experiments: habitat preference overlaps, differences in aggregation behavior, and shelter use of C. maenas in the presence of conspecific, H. sanguineus, and H. takanoi competitors. Further, a field assessment was conducted in two sites in Texel, The Netherlands to check the current abundance and distribution of the three-crab species in oyster and boulder habitats. Results showed that native and invaders have a strong preference for structurally complex habitats. In terms of aggregation behavior, H. takanoi was found to be highly gregarious. Shelter competition experiments confirm the superiority of both H. sanguineus and H. takanoi over C. maenas, regardless of its size and prior residence advantage. Based on the field assessment, higher crab abundance was observed in oyster beds than less complex boulder habitats. The invaders were observed to be abundant in the preferred oyster beds while C. maenas was abundant in boulders with algae habitats. Based on the results, C. maenas will probably be relegated to marginal habitats considering both H. sanguineus and H. takanoi are still in its expansion phase. Displacement and close to local extinction decrease of C. maenas population, as observed in the east coast of the United States, is unlikely considering the vast stretch of alternative boulder habitat from coastal defenses.