How tiny species can be overlooked: the finding of Eudendrium capillaroides (Cnidaria, Hydrozoa) in the Strait of Gibraltar

Eudendrium capillaroides Schuchert, 2008 has not been reported since its original description. The new and abundant material found in Ceuta, southern coast of the Strait of Gibraltar, allows us to refine the morphological diagnosis of this species. Eudendrium capillaroides is characterized by small monosiphonic colonies, a dense ring at the base of the hydranth, and it seems to have a preference for growing on other hydroids. Male gonophores on atrophied polyps, two-chambered (occasionally one chamber), and female gonophores on reduced polyps in the initial stages of development but hydrants later completely atrophied. Nematocysts are heterotrichous microbasic euryteles of two size classes, a larger size densely distributed in a band on the hydrant body basally and a smaller size abundant mainly on the tentacles. Furthermore, we carried out molecular analyses to assess the status of E. capillaroides and its most similar congener E. capillare within the genus Eudendrium . The use of few morphological characters or incomplete descriptions may thus lead to an incorrect wide distribution of a nominal species that actually represents a species complex. This can particularly be the case in less conspicuous species, such as many hydroids, where the degree of diversity might be underestimated. The accurate description of tiny, inconspicuous and/or cryptic species is important in order to better estimate global marine diversity as well as to understand marine communities and the relationships between their components.