Corydendrium flabellatum Fraser, 1938a

Calder, Dale R., Carlton, James T., Keith, Inti, Ashton, Gail V., Larson, Kristen, Ruiz, Gregory M., Herrera, Esteban & Golfin, Geiner, 2022, Biofouling hydroids (Cnidaria: Hydrozoa) from a Tropical Eastern Pacific island, with remarks on their biogeography, Journal of Natural History 56 (9 - 12), pp. 565-606 : 573-576

Corydendrium flabellatum Fraser, 1938a

( Figures 2b , 3 )

? Corydendrium flabellatum Fraser, 1938a: 11 , pl. 1, fig. 2a, b.

Type locality

Panama: Secas Islands , 7,965278°N, 82.00833°W, 46 m (lectotype, Calder et al. 2009) . Material examined

Chatham Bay, dock 004, 1 colony, 3.2 cm high, without gonophores, coll. G. Ashton, #179822. –Chatham Bay, dock 004, 2 colonies, to 3 cm high, 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, 1938a was based on sterile material from the Tropical Eastern Pacific. Uncertainty was expressed at the time about its generic affinities ( Fraser 1938a). Hydroids of Corydendrium are 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. flabellatum corresponds with these characters except for the gonophores, which were, and remain, undescribed.

Corydendrium flabellatum has 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. flabellatum from C. parasiticum . The cnidome of C. flabellatum was 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, 1767 from Bermuda ( Calder 1988). Sizes of the two categories in C. parasiticum and in Cocos material are close, particularly those of the euryteles. Desmonemes of C. parasiticum from Bermuda were slightly larger than those in hydroids from the present collection. The essentially identical morphology and cnidome might thus suggest that C. flabellatum represents introduced populations of C. parasiticum , which is considered introduced in the Hawaiian Islands ( Carlton and Eldredge 2015). Alternatively, C. flabellatum may represent an endemic Tropical Eastern Pacific clade of the globally distributed hydroid referred to as C. parasiticum (a likely species complex). With C. parasiticum having originally been described from the Mediterranean Sea, its type locality is geographically remote from that of C. flabellatum , located on the Pacific coast of Panama. Moreover, the type localities 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. flabellatum as 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.

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 Corydendrium and C. flabellatum ) to ‘ Filifera IV’. According to the phylogenetic analyses of Bentlage and Collins (2021), components of Filifera III and Filifera IV have a closer relationship to siphonophores than to Capitata Kühn, 1913 , Filifera I, Filifera II and Leptothecata Cornelius, 1992 . In terms of classification, C. flabellatum might 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 Eudendrium cf. certicaule (‘ Filifera I’) as in traditional taxonomic accounts.

Corydendrium flabellatum has been reported from the Pacific coasts of Panama (Secas Islands, 07°57 ʹ 55″N, 82°00 ʹ 30″W) and Mexico (east of islands off Navidad Head, 19°12 ʹ 50″ N, 104°49 ʹ 48″W; off Isabel Island, 21°51 ʹ 35″N, 105°54 ʹ 30″W) ( Fraser 1938a, 1943).

Reported distribution

Cocos Island: first record.

Elsewhere: Pacific coasts of Panama and Mexico ( Fraser 1938a; Calder et al. 2009).

Figures

Figure 2. Anthoathecata: Pennariidae, Oceaniidae and Eudendriidae. (a) Pennaria disticha: hydranth, pedicel and part of hydrocaulus, #266339. (b) Corydendrium flabellatum: two hydranths and distal end of hydrocaulus, #266340. (c) Eudendrium cf. certicaule: hydranth and part of pedicel, #240637. (d) Eudendrium cf. certicaule: pedicel and hydranth with female gonophores, #240634. (e) Eudendrium cf. certicaule: pedicel and hydranth with male gonophores, #240636. (f) Eudendrium cf. certicaule: base of hydrocaulus with a branch, #240636. Scale bars: a, b, f = 0.2 mm; c–e = 0.1 mm.

Figure 3. Oceaniidae. Corydendrium flabellatum, nematocysts. (a) desmoneme, #266339. (b) microbasic eurytele, #266340. (c) microbasic eurytele, discharged, #266340.