Obelia dichotoma ( Linnaeus, 1758 )

Gil, Marta & Ramil, Fran, 2025, Hydroids (Cnidaria, Hydrozoa) from Guinea Bissau, Zootaxa 5577 (1), pp. 1-89 : 35-36

Obelia dichotoma ( Linnaeus, 1758)

Sertularia dichotoma Linnaeus, 1758: 812 .

Laomedea dichotoma : Broch 1914: 37.— Leloup, 1937: 100.— Buchanan, 1957: 359.

Obelia dichotoma : Picard, 1951a: 111.— Rees & Thursfield, 1965: 98–99.—Millard, 1975: 229–230, fig. 75A–B.— Gili et al., 1989: 107–108, fig. 32A–B.— Ramil & Vervoort, 1992a: 243–244, fig. 68C.— Cornelius, 1995b: 296–300, fig. 69.— Medel & Vervoort, 2000: 49–53, fig. 10C–D.— Peña Cantero & García Carrascosa, 2002: 161–162, fig. 31B–C.

Laomedea (Obelia) dichotoma : Vervoort, 1959: 315–316.

Obelia hyalina : Billard, 1931a: 675.

Clytia longicyatha : Billard, 1906b: 168.

Material examined. BISSAU-0810, stn BS191, 10º18'25"– 10º19'20"N, 16º08'47"– 16º08'47"W, 24–25 m, 3-XI-2008: nine colonies (one with gonothecae), up to 10 mm high, of which six growing on crustaceans, one on Diphasia digitalis and one on Clytia gracilis .

BISSAU-0810, stn BS193, 10º21'13"– 10º21'51"N, 16º10'17"– 16º10'56"W, 20–21 m, 3-XI-2008: 10 colonies without gonothecae, 5–12 mm high, of which seven growing on bryozoans and one on Idiellana pristis .

BISSAU-0810, stn BS195, 10º22'44"– 10º23'22"N, 16º15'37"– 16º16'18"W, 29 m, 3-XI-2008: two colonies without gonothecae growing on bryozoans.

Remarks. The campanulate hydrothecae with a slightly polygonal margin (in apical view), marked by indistinct longitudinal plicae, and the presence of gonothecae allowed an accurate identification of this species.

Biology. This species has been found growing on rocks, algae, seagrasses, hydroids and a wide range of other invertebrates, such as sponges, anthozoans, bryozoans, polychaetes, mollusks, and crustaceans ( Medel & Vervoort 2000; Peña Cantero & García Carrascosa 2002). It has also been found on swimming animals, such as fish, turtles and even parasitic copepods on fish ( Cornelius 1995b). In the northern hemisphere, fertile material has been found throughout the year ( Peña Cantero & García Carrascosa 2002).

In our material, colonies were found growing on the hydroids C. gracilis , D. digitalis and I. pristis , as well as on crustaceans and bryozoans. Gonothecae were found in November.

Distribution. Obelia dichotoma is a species with a circumglobal distribution ( Gil & Ramil 2021b). In West Africa, it has been recorded from Morocco ( Billard 1906b, as Clytia longicyatha ; Patriti 1970; Ramil & Vervoort 1992a), Mauritania ( Billard 1906b, as C. longicyatha ; Billard 1931b, as Obelia hyalina ; Medel & Vervoort 2000; Gil & Ramil 2017a), Western Sahara ( Leloup 1937, as L. dichotoma ), the Cape Verde Islands ( Rees & Thursfield 1965; Medel & Vervoort 2000), Senegal ( Picard 1951a), Gambia [ Vervoort 1959, as L. (Obelia) dichotoma ], Guinea-Bissau [ Vervoort 1959, as L. (Obelia) dichotoma ; Gili et al. 1989], Senegal, Angola [ Vervoort 1959, as L. (Obelia) dichotoma ], Ghana ( Buchanan 1957), São Thomé ( Broch 1914, as L. dichotoma ) and the Vema Seamount ( Gil & Ramil 2021b). Its bathymetric distribution ranges from the intertidal ( Cornelius 1995b; Medel & Vervoort 2000) to 540 m ( Vervoort 2006).

Our colonies were collected at depths between 24–29 m.