Hydrodendrium Nutting, 1905

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

Genus Hydrodendrium Nutting, 1905

Hydrodendrium Nutting, 1905: 936 .

Nuttingia Stechow, 1909: 10 [invalid replacement name for Hydrodendrium Nutting, 1905 ].

Type species. Hydrodendrium gorgonoides Nutting, 1905 , by monotypy.

Diagnosis. Hydractiniid hydroids with erect colonies, comprising a massive, arborescent, spongy, branched hydrocaulus with a skeletal axis of intertwining and anastomosing chitinous fibres penetrated internally and invested externally by naked coenosarc; spines and hydrophores absent. Hydranths monomorphic, sessile, naked, anemone-like, arising from bands of coenosarc at surface, not embedded in skeleton; tentacles filiform, in a single distal whorl; hypostome flattened to dome-like.

Gonophores fixed sporosacs, arising as sac-shaped evaginations of hydranth body wall; colonies monoecious.

Remarks. Hydrodendrium Nutting, 1905 has seldom been used as a valid genus name. Stechow (1909) instituted Nuttingia as a replacement name for it after incorrectly considering Hydrodendrium a homonym of Hydrodendron Hincks, 1874 (see also Stechow 1923b). The two are not homonyms (ICZN Art. 56), and Nuttingia is an invalid junior objective synonym of Hydrodendrium . Both Hydrodendrium and Nuttingia were included in the synonymy of Hydractinia Van Beneden, 1844a by authors including Bouillon (1985), Bouillon et al. (1997) and Schuchert (2008a).

The genus Hydrodendrium is maintained here based on a unique combination of characters outlined in the diagnosis above. It is distinguished from Hydractinia in having monomorphic, stout hydranths with a low hypostome that arise from an erect and arborescent chitinous skeleton lacking spines, and gonophores that are fixed sporosacs arising as hernia-like protuberances from the walls of unmodified ( Nutting 1905) or only slightly shrunken hydranths. The growth form of the skeletal axis superficially resembles that of Solanderia Duchassaing & Michelin, 1846 or even an octocoral.

Hydrodendrium or its junior objective synonym Nuttingia have been assigned at various times to the families Hydrodendriidae Nutting, 1905 ( Nutting 1905), Podocorynidae Allman, 1864 ( Hickson & Gravely 1907), Bougainvilliidae Lütken, 1850 ( Stechow 1909) , Clathrozoidae Stechow, 1921a ( Stechow 1921a) , and Hydractiniidae L. Agassiz, 1862 ( Bouillon et al. 1997). Affinities of the genus appear to be with Hydractiniidae , as noted by authors such as Schuchert (2008a).