Species identification of bivalve-inhabiting marine hydrozoans of the genus Eugymnanthea

Species-level identification is difficult in the symbiotic bivalve-inhabiting hydrozoans of the genus Eugymnanthea (Cnidaria, Hydrozoa). Morphological differences are detected only in the adult medusoid stage. Eugymnanthea is known only from the Mediterranean and the western Pacific, and doubt persists over whether the two localities are inhabited by different species. Because the bivalve host, Mytilus galloprovincialis, is thought to have been introduced by humans from the Mediterranean to the western Pacific, there has been speculation that the Mediterranean Eugymnanthea was also introduced along with its host. Here, we evaluate the species status of the two hydrozoan forms with breeding experiments, morphology, and two recently developed tools for discrimination: a mesoglea cell adhesion and spreading test, and 16S rDNA comparison. Reciprocal crosses of the two forms failed to produce normal offspring, providing evidence that they are indeed different species according to the biological species concept, and suggesting that the Pacific form is not an invasion of the Mediterranean form. The tissue-grafting test failed to distinguish between the two forms, while the morphological and genetic evidence corroborated the breeding results.