LAUSR

The Chinese mitten crab Eriocheir sinensis is an invasive species outside native range and represents a substantial threat to recipient ecosystems. It is regarded as an invasive species in Japan and has recently been found in Tokyo Bay, but little is known about its possible impacts on native species, especially on the congener Japanese mitten crab Eriocheir japonica. This study examined shelter competition between E. sinensis and E. japonica under laboratory conditions. Shelter competition trials were conducted in pairs of native and invasive males in different size combinations. Our results indicated that competition between two species was size-dependent: E. japonica always successfully defended shelter when competing with size-matched or slightly larger E. sinensis; E. sinensis always won shelter when its carapace or chela were at least 10 or 7% larger, respectively, than those of E. japonica. The two species exhibited different behavioural patterns: compared with E. sinensis, E. japonica frequently displayed positive behaviour (approach, walking leg contact, chela contact) and seldomly displayed negative behaviour (retreat). Our results highlight the important role of strong native competitors in controlling invasion success and suggest E. sinensis may be unlikely to exclude native E. japonica from its habitat by direct competition for shelter.