Species-specific crab predation on the hydrozoan clinging jellyfish Gonionemus sp (Cnidaria, Hydrozoa), subsequent crab mortality, and possible ecological consequences

Here we report a unique trophic interaction between the cryptogenic and sometimes highly toxic hydrozoan clinging jellyfish Gonionemus sp. and spider crab libinia dubia. We assessed species-specific predation on the Gonionemus medusae by crabs found in eelgrass meadows in Massachusetts, USA. The native spider crab species. L. dubia consumed Gonionemus medusae, often enthusiastically, but the invasive green crab Carcinus maeuus avoided consumption. all trials. One out of two blue crabs (Calinectes sapidus) also consumed Gonionemus, but this species was too rare in our system study to evaluate further. Libinia crabs could consume up to 30 jellyfish, which was the maximum jellyfish density treatment in our experiments,. over a 24-hour period. Gonionemus consumption was associated with Libinia mortality. Spider crab mortality increased with Gonionemus consumption, and 100% of spider crabs tested died within 24 h of consuming jellyfish in our maximum jellyfish density containers. As the numbers of Gonionemus medusae used in our experiments likely underestimate the number of medusae that could be encountered by spider crabs over a 24-hour period in the field, we expect that Gonionemus may be having a negative effect on natural Libinia populations. Furthermore, given that Libinia overlaps in habitat and resource use with Carcinus, which avoids consumption, Carcinus populations could be indirectly benefiting from this unusual crab jellyfish trophic relationship.