Corydendrium flabellatum Fraser, 1938a: 11 Biofouling hydroids (Cnidaria: Hydrozoa) from a Tropical Eastern Pacific island, with remarks on their biogeography Calder, Dale R. Carlton, James T. Keith, Inti Ashton, Gail V. Larson, Kristen Ruiz, Gregory M. Herrera, Esteban Golfin, Geiner Journal of Natural History 2022 2022-07-11 56 9 - 12 565 606 YPSV Fraser, 1938 a Fraser 1938 [396,919,1511,1538] Hydrozoa Oceaniidae Corydendrium Animalia Anthoathecata 8 573 Cnidaria species flabellatum   ( Figures 2b, 3)   ?   Corydendrium flabellatum Fraser, 1938a: 11, pl. 1, fig. 2a, b.    Typelocality   Panama: Secas Islands, 7,965278°N, 82.00833°W,  46 m( lectotype, Calder et al. 2009). Material examined   Figure 2. Anthoathecata: Pennariidae, Oceaniidaeand Eudendriidae. (a)  Pennaria disticha: hydranth, pedicel and part of hydrocaulus, #266339. (b)  Corydendrium flabellatum: two hydranths and distal end of hydrocaulus, #266340. (c)  Eudendriumcf. certicaule: hydranth and part of pedicel, #240637. (d)  Eudendriumcf. certicaule: pedicel and hydranth with female gonophores, #240634. (e)  Eudendriumcf. certicaule: pedicel and hydranth with male gonophores, #240636. (f)  Eudendriumcf. certicaule: base of hydrocaulus with a branch, #240636. Scale bars: a, b, f = 0.2 mm; c–e = 0.1 mm. Chatham Bay, dock 004, 1 colony, 3.2 cmhigh, without gonophores, coll. G. Ashton, #179822. –Chatham Bay, dock 004, 2 colonies, to 3 cmhigh, without gonophores, coll. G. Ashton, #266340.  Cnidome Nematocysts of hydranth ( Figure 3a–c): desmonemes (n = 10): 4.7–5.2 μm long × 3.2–3.4 μm wide; heterotrichous microbasic euryteles (n = 10): 8.3–9.0 μm long × 3.8–4.1 μm wide. Remarks The original description of  Corydendrium flabellatum Fraser, 1938awas based on sterile material from the Tropical Eastern Pacific. Uncertainty was expressed at the time about its generic affinities ( Fraser 1938a). Hydroids of  Corydendriumare defined as oceaniids having colonies that are either erect, polysiphonic and irregularly branched, or exceptionally stolonal, perisarc tubes of hydrocaulus and branches that are adnate over all or a considerable part of their lengths and that terminate near the hydranth bases, hydranths that are elongate and cylindrical, with scattered filiform tentacles, and gonophores that are elongate, fixed and either located within perisarc tubes beneath the hydranths or appearing as external outgrowths of the stem and branches ( Calder 1988; Schuchert 2004; Bouillon et al. 2006). Fraser’s (1938a)account of  C. flabellatumcorresponds with these characters except for the gonophores, which were, and remain, undescribed.   Corydendrium flabellatumhas been considered a synonym of the circumglobal  C. parasiticum Linnaeus, 1767( Vervoort, 1946; Calder 1988), although it is retained as a distinct species by Schuchert (2004). There appears to be little in terms of morphology to distinguish  C. flabellatumfrom  C. parasiticum. The cnidome of  C. flabellatumwas not described by Fraser (1938a), and was previously unknown. That of material examined here, comprising desmonemes and euryteles, is identical to that of  C. parasiticum Linnaeus, 1767from Bermuda( Calder 1988). Sizes of the two categories in  C. parasiticumand in Cocos material are close, particularly those of the euryteles. Desmonemes of  C. parasiticumfrom Bermudawere slightly larger than those in hydroids from the present collection. The essentially identical morphology and cnidome might thus suggest that  C. flabellatumrepresents introduced populations of  C. parasiticum, which is considered introduced in the Hawaiian Islands ( Carlton and Eldredge 2015). Alternatively,  C. flabellatummay represent an endemic Tropical Eastern Pacific clade of the globally distributed hydroid referred to as  C. parasiticum(a likely species complex). With  C. parasiticumhaving originally been described from the Mediterranean Sea, its typelocality is geographically remote from that of  C. flabellatum, located on the Pacific coast of Panama. Moreover, the typelocalities of the two species are separated by the Central American Isthmus, a major biogeographic barrier. Few hydroid species are known to have naturally crossed that barrier ( Moura et al. 2019). For now, we recognise  C. flabellatumas valid, pending genetic comparisons, and tentatively native to the Eastern Pacific. Further complicating this species’ status is, as we note above, a lack of knowledge of the gonophores; additional study may suggest that the species should eventually be treated as a species inquirenda.   Figure 3. Oceaniidae.  Corydendrium flabellatum, nematocysts. (a) desmoneme, #266339. (b) microbasic eurytele, #266340. (c) microbasic eurytele, discharged, #266340. Within the suborder Filifera Kühn, 1913, four separate clades were recognised by Cartwright et al. (2008). Utilising grey nomenclature ( Minelli 2017), they assigned the family Oceaniidae Eschscholtz 1829(inclusive of  Corydendriumand C. flabellatum) to ‘ FiliferaIV’. According to the phylogenetic analyses of Bentlage and Collins (2021), components of FiliferaIII and FiliferaIV have a closer relationship to siphonophores than to Capitata Kühn, 1913, FiliferaI, FiliferaII and Leptothecata Cornelius, 1992. In terms of classification, C. flabellatummight thus have been discussed at the end of this work rather than near the beginning. For now, however, it has been placed between  Pennaria disticha(Capitata)and  Eudendriumcf. certicaule(‘ FiliferaI’) as in traditional taxonomic accounts.   Corydendrium flabellatumhas been reported from the Pacific coasts of Panama(Secas Islands, 07°57 ʹ55″N, 82°00 ʹ30″W) and Mexico(east of islands off Navidad Head, 19°12 ʹ50″ N, 104°49 ʹ48″W; off Isabel Island, 21°51 ʹ35″N, 105°54 ʹ30″W) ( Fraser 1938a, 1943).  Reported distribution Cocos Island: first record. Elsewhere: Pacific coasts of Panamaand Mexico( Fraser 1938a; Calder et al. 2009). 3891616336 [160,1102,1703,1728] Panama 46 7.965278 Secas Islands 1 -82.00833 8 573 1 lectotype