Shallow water hydroids (Cnidaria, Hydrozoa) from the 2002 NOWRAMP cruise to the Northwestern Hawaiian Islands Author Calder, Dale R. Author Faucci, Anuschka 0000-0001-9002-8987 anuschka@hawaii.edu text Zootaxa 2021 2021-12-24 5085 1 1 73 journal article 10.11646/zootaxa.5085.1.1 1175-5326 5802920 12FC3342-F2A0-4EE1-9853-9C5855076A10 Phialellidae (undetermined) Fig. 2f Voucher material. Gardner Pinnacles , on Halopteris longibrachia , nom. nov. , 14.ix.2002 , one colony, 0.35 mm high, without gonothecae, coll. A. Faucci , ROMIZ B5406 . Remarks. The single, sterile, stolonal colony examined here occurred as a tiny epizoite on the hydroid Halopteris longibrachia , nom. nov. Although referable to the family Phialellidae Russell, 1953 , and likely to either Opercularella Hincks, 1869 or Phialella Browne, 1902 , neither the specific nor generic identity of the hydroid could be categorically established. Given a paucity of species-specific characters, and in the absence of gonophores, distinguishing hydroids of these two genera is difficult. The specimen from Gardner Pinnacles has therefore been identified here simply as Phialellidae (undetermined). The trophosome of this hydroid resembles those of species such as? Phialella quadrata ( Forbes, 1848 ) from the Seychelles ( Millard & Bouillon 1973 ) and Opercularella sp. from the coast of Ecuador ( Calder et al . 2021 ). Some 10 species are currently assigned to Phialella in WoRMS ( Schuchert 2021b ). Distinguished by characters of the medusa stage, their trophosomes are likely all much alike. Notably, hydroids assigned to three species of Phialella from California were found to be essentially indistinguishable ( Boero 1987 ), and Watson (1994b) reported that colonies of P. quadrata and P. fragilis ( Uchida, 1938 ) were identical. A hydroid tentatively assigned by Galea et al . (2014) to P. falklandica Browne, 1902 , the type species of Phialella , resembles that of Opercularella lacerata Johnston, 1847 , but it differs in having gonothecae with rounded longitudinal ridges rather than smooth to wavy walls. Morphologically similar to P. quadrata , P. falklandica , and related species are hydroids included in Opercularella , with some four species recognized in WoRMS ( Schuchert 2021a ). Indeed, molecular data in Maronna et al . (2016) indicate that P. quadrata is genetically close to Campanularia lacerata , the type species of Opercularella . Even closer to P. quadrata in their phylogram is Opercularella pumila S.F. Clark, 1875 , misassigned to Campanulina Van Beneden, 1847 in that work and certain others (e.g., Cornelius 1995; Bouillon et al . 2006 ; Schuchert 2021a ). As presently defined, hydroids assigned to Opercularella are said to have fixed gonophores while those referred to Phialella liberate a medusa (Cornelius 1995; Bouillon et al . 2006 ). Should the two genera prove to be identical, the name Opercularella would have nomenclatural priority. Reported Distribution. Hawaiian archipelago. First record.