Fig. 67 A–C
See Peña Cantero & García Carrascosa (2002) for a complete synonymy.
Material examined. HCUS-S 0 74 (Hydrozoa Collection, University of Salento—fauna of the Salento Peninsula).
Description (based on our own observations; Peña Cantero & García Carrascosa 2002):
Hydroid. Hydrorhiza stolonal, colonies erect; hydrocauli very thin and delicate, monosiphonic, up to 20 mm high, irregularly branched, internodes long, in a zigzag arrangement, separated by transverse nodes; hydrocladia may originate from hydrothecal base; hydrothecae shallow, walls widening from base to top giving a wide aperture, rim everted and curved outwards, borne laterally at upper part of internodes, alternate and provided with a well developed hydrophore, secondary hydrothecae provided with hydrophore, may arise from primary hydrothecae. Gonothecae: male gonothecae ovate and flattened, female similar but slightly larger and broader.
Cnidome. Microbasic euryteles and mastigophores.
Habitat type. Eurybathic species that has been found from 0.5 to 200 m depth (Marinopoulos 1979; Boero & Fresi 1986).
Substrate. Algae, hydroids, concretions, Posidonia, bryozoans, polychaete tubes, anthozoans, sponge.
Seasonality. April (Picard 1955; Gili 1986), May (Medel et al. 1998), and October–March, July (Boero & Fresi 1986) in the western Mediterranean; June, July (De Vito 2006; this study) in Salento waters.
Reproductive period. April (Picard 1955; Gili 1986) and May (Medel et al. 1998) in the western Mediterranean.
Distribution. Cosmopolitan species present in the eastern and western Atlantic, Mediterranean, Indo-Pacific, Arctic, Antarctic (Broch 1918 in part as Halecium textum; Stepan’yants 1979; Ramil & Vervoort 1992; Medel & López-González 1996; Medel et al. 1998; Peña Cantero & García Carrascosa 2002; Bouillon et al. 2004; Gravili et al. 2008 a; Soto Ãngel & Peña Cantero 2013).
Records in Salento. Rare at Otranto (De Vito 2006; Gravili 2006; Gravili et al. 2008 a; this study).
Remarks. It is not always easy to distinguish the European Halecium species H. mediterraneum, H. labrosum, and H. tenellum (for details see Schuchert 2005 b). The distinctive characteristics of H. tenellum are that the colonies are more gracile, non-polysiphonic and smaller than those of the other two species, with internodes more elongate, strongly arranged in a zigzag manner.
References. Motz-Kossowska (1911), Broch (1912, 1933), Stechow (1919), Leloup (1934), Picard (1958 a), Rossi (1961), Patriti (1970), Boero (1981 a, b), Fresi et al. (1982), Boero & Fresi (1986), Gili (1986), Llobet et al. (1986, 1991), Llobet i Nadal (1987), Ramil & Vervoort (1992), Medel & López-González (1996), Migotto (1996), Medel et al. (1998), Piraino et al. (1999), Medel & Vervoort (2000), Peña Cantero & García Carrascosa (2002), Bouillon et al. (2004), De Vito (2006), Gravili (2006), Gravili et al. (2008 a), Puce et al. (2009).