New ecological insight on two invasive species: Craspedacusta sowerbii (Coelenterata: Limnomedusae) and Dreissenia polymorpha (Bivalvia: Dreissenidae)

The ecology of polyps of Craspedacusta sowerbii has been poorly investigated. It is known that they live attached to hard surfaces. They have never been linked with any animal, in particular not with Dreissena polymorpha (zebra mussel), which is known to have only endosymbionts. This study was conducted in March 2004 in Cingi-Lingi Lake. The lake is an oligo-mesotrophic man-made gravel pit in northeast Croatia. During this study, polyps were found singly or in colonies of two to five on willow twigs and on the shells of D. polymorpha collected from 8 m depth and on metal barrels. Brown hydra (Pelmatohydra oligactis) individuals were also present next to polyps of Craspedacusta on zebra mussel shells, collected from metal barrels. The presence of polyps of Craspedacusta and brown hydras together on live bivalve shells is significant, as we suggest that it marks the first record of commensalism of this kind.