Stylactaria Stechow, 1921a

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 34-35

Genus Stylactaria Stechow, 1921a

Stylactaria Stechow, 1921a: 250 .

Type species. Stylactis inermis Allman, 1872 [ Stylactaria inermis ], by monotypy.

Diagnosis. Hydractiniid hydroids with colonies exclusively stolonal. Polyps sessile, naked, polymorphic, with gastrozooids, gonozooids, and sometimes dactylozooids, arising from a creeping hydrorhiza comprising hydrorhizal stolons covered with perisarc; hydrorhiza not encrusting, calcareous, or covered with naked perisarc; spines present or absent. Gastrozooids, gonozooids, and dactylozooids as for family.

Gonophores fixed sporosacs or fixed or free eumedusoids, developing on gonozooids proximal to whorl of tentacles. Fully developed medusoids sac-shaped; manubrium simple, tubular, lacking mouth, oral lips, and oral tentacles; marginal tentacles rudimentary, 8–10 in number; radial canals four; ocelli absent. Gonad surrounding manubrium.

Remarks. As noted above, classification of Hydractiniidae L. Agassiz, 1862 at the generic level remains unsettled. The genus Stylactaria Stechow, 1921a has recently been treated both as a synonym of Hydractinia Van Beneden, 1844a (e.g. Bouillon et al. 1997, 2006; Boero et al. 1998; Cairns et al. 2002; Stampar et al. 2006; Schuchert 2008a) and as distinct ( Calder 1988; Namikawa 1991; Miglietta et al. 2009). In this work, Stylactaria is once again recognized as valid pending a more detailed taxonomic reappraisal of the family. Investigations on the nature of stolonal versus mat growth forms in Hydractiniidae , and the influence of genetics and environmental effects on them ( Ferrell 2008), may prove relevant in taxonomy as well as ecology.

The taxonomy and nomenclature of Stylactaria (including Stylactis auct.) has been reviewed in a previous work ( Calder 1988). Earlier detailed accounts of the genus include those of Iwasa (1934), Bouillon (1971), and Hirohito (1988). Subsequent reviews have been provided by Namikawa (1991) and Bouillon et al. (1997), who summarized the characters of 29 nominal species and another five unnamed species assigned to the genus. Schuchert (2008a) reviewed species in European waters under the genus Hydractinia . The greatest diversity within the genus is known from Japan, with 14 species reported from that country. The most frequent substrates of known species are shells of gastropods, with or without pagurid crabs ( Bouillon et al. 1997).

Species assigned to Stylactaria are morphologically quite homogeneous and difficult to distinguish. Characters utilized in the taxonomy of the genus were recently reviewed and evaluated by Bouillon et al. (1997). Understanding of the group is likely to advance once more molecular work has been completed, as in the related genus Hydractinia . Greater attention to nematocyst complement and distribution may also aid in species differentiation, as in the genus Eudendrium Ehrenberg, 1834 .

Medusoids in certain species of Stylactaria are known to be liberated at dusk or soon after dark ( Sigerfoos 1899; Calder 1971, 1988), as in Pennaria disticha Goldfuss, 1820 .