The hydroid of the medusa Lizzia blondina Forbes, 1848

A tiny, filiferan hydroid living within the sponge Haliclona simulans (Johnston, 1842) could be identified as the so far unknown polyp stage of the hydromedusa Lizzia blondina Forbes, 1848. This finding is based on two lines of evidence: (i) the direct observation that sponge pieces with the hydroid release young Lizzia blondina medusae, and (ii) 16S rRNA gene sequences obtained from DNA samples extracted from mixed sponge–hydroid samples were identifiable as either Lizzia blondina or sponge-related. Histological examination of the hydroid showed that it is colonial, and the individual polyps are connected through stolons which penetrate deeply into the sponge tissue. The polyps only protrude temporarily and partially for the purpose of feeding. The hydroid can retract its tentacles and the hypostome in an introvert-like pouch, becoming thus almost invisible on the sponge surface. The association of the Lizzia blondina hydroid with the sponges of the genus Haliclona Grant, 1841 is likely a rather specific relationship.