Hydractiniidae L. Agassiz, 1862

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 31-32

Family Hydractiniidae L. Agassiz, 1862

Hydractinidae L. Agassiz, 1862: 339 [emended to Hydractiniidae by Hincks (1868)].

Diagnosis. Filiferan hydroid colonies with sessile zooids; hydrorhiza comprising either perisarc-covered stolons, or an encrusting mat of coalesced coenosarc that may be naked or perisarc-covered, with or without a calcareous skeleton; usually stolonal but in some species forming erect pillars, or branches, or arborescent colonies; chitinous or calcareous spines frequently present; protective tubes sometimes overgrowing hydranths. Zooids naked, either polymorphic, with gastrozooids, gonozooids, and sometimes dactylozooids, or less often monomorphic. Gastrozooids with filiform tentacles in a single whorl, or in two or more close whorls, or scattered over distal half of body, infrequently with just one or two filiform tentacles. Gonozooids with filiform tentacles in one or more whorls, or as blastostyles lacking both tentacles and mouth; dactylozooids, if present, lacking tentacles.

Gonophores fixed sporosacs, fixed or liberable eumedusoids, or free medusae, usually borne on gonozooids. Medusae, when present, bell-shaped; radial canals four, ring canal present; manubrium sac-shaped to tubular, not reaching beyond subumbrellar cavity, oral region with four unbranched or branched lips with terminal batteries of nematocysts; marginal tentacles solid, solitary; ocelli present or absent; gonads interradial, on manubrium, sometimes extending along proximal reaches of radial canals.

Remarks. The diagnosis of Hydractiniidae L. Agassiz, 1862 adopted here is essentially as given by Schuchert (2008a) in a review of recent genera and species of the family in European waters. His work is followed in regarding Hydrodendridae Nutting, 1905 [emended here to Hydrodendriidae] as a synonym of Hydractiniidae . The diagnosis of the family has been modified slightly to accommodate the genus Hydrodendrium Nutting, 1905 .

As for Hydractinia , type genus of the family, authorship and date of the name has been widely credited to Van Beneden (1841). However, the name therein was only the vernacular “Hydractinie,” and it was not until later ( Van Beneden 1844a) that it was made available as Hydractinia (ICZN Art. 11). Nomenclatural problems arise as a result, with Hydractinia Van Beneden, 1844a being a junior synonym of the virtually unused name Echinochorium Hassall, 1841 . In the interests of nomenclatural stability, the well-known name Hydractinia is designated here as valid and as a nomen protectum, while Echinochorium is relegated to a nomen oblitum by applying Reversal of Precedence provisions (ICZN Art. 23.9). First, Echinochorium has not been used as a valid name in zoology after 1899 (ICZN Art. 23.9.1.1). In contrast, Hydractinia has been used in more than 25 works by numerous authors (>10) in the past 50 years (ICZN Art. 23.9.1.2): (e.g. Millard 1975; Bouillon 1985; Hirohito 1988; Buss & Yund 1989; Bouillon et al. 1997, 2006; Brinckmann-Voss 1996; Schuchert 1996, 2008a; Boero et al. 1998; Cairns et al. 2002; Migotto et al. 2002; Peña Cantero & García Carrascosa 2002; Cunningham & Buss 1993; Watson 2003; Stampar et al. 2006; Miglietta 2006; Vervoort 2006; Xu & Huang 2006; Galea et al. 2007; Mills et al. 2007; Altuna 2008; Ferrell 2008; Puce et al. 2008; Miglietta et al. 2009; Calder et al. 2009). The generic name Dysmorphosa Philippi, 1842 also predates Hydractinia , but it is more likely a senior synonym of Podocoryna M. Sars, 1846 . Its type species, D. conchicola Philippi, 1842 , is of uncertain identity but possibly conspecific with Podocoryna exigua ( Haeckel, 1879) ( Schuchert 2008a) .

Hydractiniid hydroids can be difficult to delimit from those of anthoathecate families such as Oceaniidae Eschscholtz, 1829 , Cytaeididae L. Agassiz, 1862 , Bougainvilliidae Lütken, 1850 , Pandeidae Haeckel, 1879 , and Rhysiidae Brinckmann, 1965 . Moreover, concepts of genera within Hydractiniidae have varied widely, with as many as 14 ( Stechow 1923b) and as few as five ( Kramp 1932; Bouillon et al. 2006) recent ones being recognized in various classifications. Eleven genera were recognized as valid by Schuchert (2009), but he noted earlier ( Schuchert 2008a) that current subdivision of the family at the generic level is provisional and certain to change. Miglietta et al. (2009) observed that the current practice of collapsing Podocoryna M. Sars, 1846 and Stylactaria Stechow, 1921a into Hydractinia is impractical. Comprehensive taxonomic revision of the family, involving both molecular biology and alpha-taxonomy, is clearly needed. The sometimes-overlooked genus Hydrodendrium Nutting, 1905 , represented in Hawaii by H. gorgonoides Nutting, 1905 , is included here in Hydractiniidae , but its family affinities need to be more clearly resolved.

Stampar et al. (2006) listed nearly 100 species in a broadly defined genus Hydractinia that also encompassed Podocoryna and Stylactaria , and Daly et al. (2007) reported that about 100 species of hydractiniids were known overall. In light of the degree of crypsis now thought to occur in the family, and in hydrozoans generally, that number will likely prove to be conservative. Indeed, additional new nominal species of hydractiniids continue to be described (e.g. Lin et al. 2010).