Obelia dichotoma Linnaeus 1758

Obelia dichotoma (Linnaeus, 1758)

Fig. 108 A–D

See Peña Cantero & García Carrascosa (2002) for a complete synonymy.

Material examined. HCUS-S 115p and HCUS-S 115m (Hydrozoa Collection, University of Salento—fauna of the Salento Peninsula)—polyp and medusa stages.

Description (based on our own observations; Stechow 1919; Ramil & Vervoort 1992; Cornelius 1995):

Hydroid. Hydrorhiza as creeping stolon system; colonies erect, very varied in size and shape; hydrocauli monosiphonic, up to 35 cm high, branched, flexuose to straight, thickened in old colonies, internodes with several annuli at their base, perisarc of older tubes usually conspicuously darker than those of younger tubes; hydrothecae bell-shaped, usually not very deep, thin walled, borne alternately and laterally on completely annulated pedicels at the upper part of the internodes, rim even to crenate, slightly flaring, diaphragm transverse to oblique. Colours: in preserved specimens horn-coloured to dark red-brown; perisarc of older tubes usually conspicuously darker than those of younger tubes. Gonothecae usually inverted-conical on annulated pedicels, truncated at the distal end, mature ones with a short distal neck.

Habitat type. Eurybathic species that in the Mediterranean has been found from the tidal level to about 200 m depth (García Carrascosa 1981; Boero & Fresi 1986; Ramil & Vervoort 1992). Hydroid stage recorded in water from normal salinity down to 12‰ (Cornelius 1995).

Substrate. Epibiontic on other hydroids, Pecten, algae, mussels, bryozoans, ascidians, barnacles, phanerogams, Crustacea, gorgonaceans, worm-tubes, sponge, rock and on recuperated instruments.

Seasonality. Present all the year (Boero & Fresi 1986; Llobet et al. 1991; De Vito 2006; Puce et al. 2009; this study) in several localities of the Mediterranean Sea.

Reproductive period. January and February (Broch 1933; Boero & Fresi 1986; Gili 1986; Puce et al. 2009), March (Boero & Fresi 1986; Gili 1986; Puce et al. 2009), April (Stechow 1919; Broch 1933; Boero & Fresi 1986; Gili 1986; Puce et al. 2009), May (Boero & Fresi 1986; Puce et al. 2009), June (Broch 1933; Puce et al. 2009), July (Broch 1933; Peña Cantero & García Carrascosa 2002), September (Broch 1933; Morri & Bianchi 1999; Puce et al. 2009), November (Broch 1933; Boero & Fresi 1986; Denitto 1996), December (Broch 1933; Boero & Fresi 1986; Gili 1986; Denitto 1996; Puce et al. 2009), and January–August (De Vito 2006; this study) in the Mediterranean Sea.

Medusa. Umbrella circular, flat, 2.5– 6 mm wide, mesoglea very thin; without gastric peduncle; mouth with 4 simple lips; 4 radial canals; gonads spherical to ovoid, sac-like, hanging from middle to end of the radial canals; numerous short, chordal, not extensile marginal tentacles with short endodermal roots extending into bell mesoglea; 8 statocysts situated on underside of basal bulbs of some marginal tentacles.

Cnidome. b-mastigophores 2 types (polyp); isorhiza haploneme, the ‘I D -' and ‘I d ’ types (medusa).

Distribution. Cosmopolitan (Ramil & Vervoort 1992; Medel & López-González 1996; Schuchert 2001a; Peña Cantero & García Carrascosa 2002; Bouillon et al. 2004; Gravili et al. 2008a).

Records in Salento. Common at: Marina di Corsano, Torre Vado, Ponte del Ciolo (Presicce 1991); Gulf of Taranto, Brindisi (Denitto 1996; Miglietta et al. 2000); Otranto (Denitto 1996; Miglietta et al. 2000; Fraschetti et al. 2002; De Vito 2006; Gravili 2006; Gravili et al. 2008a; this study); La Strea, Porto Cesareo (Faucci & Boero 2000); Costa Merlata (Fraschetti et al. 2002; Andreano 2007); Other Apulian records: Gargano (Fraschetti et al. 2002).

Remarks. The whole cycle was examined in the present study. O. dichotoma is among the most phenotypically varied of all hydroids; however one of the nematocyst types is diagnostic of the species, which is especially useful in distinguishing it from the somewhat similar O. longissima (for details see Östman 1982, 1987, 1999; Cornelius 1995). The medusae of Obelia are not distinguishable at the species level, whereas they are very distinct from all other hydromedusae (Boero et al. 1996). Boero et. al. (2007) showed that the medusae of O. dichotoma are active filter feeders of microbes.

References. Cavolini (1785), Schneider (1898) as Campanularia plicata, Babic (1910, 1913b), Broch (1912, 1933), Stechow (1919), Vatova (1928), Leloup (1934), Billard (1936), Picard (1951a, 1952, 1958 a), Riedl (1959, 1966), Rossi (1961, 1971), Fey (1970), Schmidt (1973), Chimenz Gusso & Rivosecchi Taramelli (1975), Relini & Romairone (1976), Montanari & Morri (1977), Repetto et al. (1977), Morri (1979b, 1980b, 1981a, b,c, 1985), Boero (1981a, b), García Carrascosa (1981), Morri & Martini (1981), Morri & Bianchi (1982, 1983, 1999), Boero et al. (1985), Isasi (1985), Boero & Fresi (1986), Llobet et al. (1986, 1991), Roca (1986), García-Rubies (1987), Llobet i Nidal (1987), Ramil (1988), Morri et al. (1991), Bianchi et al. (1993b), Boero & Fresi (1986), Ramil & Vervoort (1992), Vervoort (1993), Medel & López-González (1996), Migotto (1996), Morri & Bianchi (1999), Piraino et al. (1999), Faucci & Boero (2000), Medel & Vervoort (2000), Miglietta et al. (2000); Schuchert (2001a), Fraschetti et al. (2002), Peña Cantero & García Carrascosa (2002), Bouillon et al. (2004), De Vito (2006), Gravili (2006), Cinar et al. (2008), Gravili et al. (2008a), Morri et al. (2009), Puce et al. (2009), Bianchi et al. (2011).