Aethozooides uraniae, a new deep-sea genus and species of solitary bryozoan from the Mediterranean Sea, with a revision of the Aethozoidae

Bryozoa is a phylum of about 6000 extant species that are almost exclusively colonial. Few species of the uncalcified Gymnolaemata, the ctenostomes, however, show solitary forms that essentially consist of single zooids. Recently, several specimens of a solitary ctenostome bryozoan were encountered for the first time in the deep Mediterranean Sea, at the edge of an anoxic brine lake. Differences in size, tentacle number, and in the variability of cystid appendages set these specimens apart from all other known solitary species. Moreover, additional morphological autapomorphic traits suggest the erection of a novel genus to allocate the new species. Consequently, the new taxon Aethozooides gen. nov. is proposed in virtue of the general resemblance of the Mediterranean specimens with those of the genus Aethozoon Hayward, 1978. Aethozooides uraniae gen. et sp. nov. shows significant variability in the number and location of cystid appendages that range from two on the basal side to one or two on the zooid mid-peristomial position and/or, rarely, on the terminal frontal side. The polypide possesses a distinct, long tentacle crown always carrying 10 tentacles. The prominent retractor muscle consists of numerous bundles that, in contrast to other known gymnolaemates, attach not only to the lophophoral base but also to various parts of the gut. Distally, the aperture shows a set of four apertural muscles including four parieto-vaginal bands. Reviewing the state and diversity of solitary ctenostomes, we propose a revision of the family Aethozoidae to include the genera Franzenella d’Hondt, 1983, Aethozoon, Aethozooides, and two species currently affiliated to the genus Franzenella (F. monniotae and F. radicans) for which we erected the new taxon Solella gen. nov.