Rhizorhagium arenosum (Alder, 1862),

Gravili, Cinzia, Vito, Doris De, Camillo, Cristina Gioia Di, Martell, Luis, Piraino, Stefano & Boero, Ferdinando, 2015, The non-Siphonophoran Hydrozoa (Cnidaria) of Salento, Italy with notes on their life-cycles: an illustrated guide, Zootaxa 3908 (1), pp. 1-187: 16-17

publication ID


publication LSID


persistent identifier


treatment provided by


scientific name

Rhizorhagium arenosum (Alder, 1862)


Rhizorhagium arenosum (Alder, 1862) 

Fig. 8View FIGURE 8

See Schuchert (2007) for a complete synonymy.

Material examined. HCUS-S 0 0 8 (Hydrozoa Collection, University of Salento—fauna of the Salento Peninsula).

Description (based on our own observations; Hincks 1868; Schuchert 2007, 2012):

Hydroid. Hydrorhiza as creeping ramified stolons covered by perisarc; colony stolonal; hydrocaulus monosiphonic, short, perisarc generally covered with minute sand grains or mud; hydranth fusiform, covered by a pseudohydrotheca not enveloping tentacles; hypostome nipple-shaped, with one distal whorl of 6–12 long, slender, amphicoronate, filiform tentacles. Gonophores as fixed sporosacs, pyriform, 1–2 per hydrocaulus, on short pedicels on lower half of hydrocaulus. Female sporosacs oblong, covered by filmy perisarc, mature ones with many eggs (> 30), embryonic development takes place in a gelatinous mass (acrocyst) to the planula stage. Male sporosacs similar to female ones. Colour: milky white.

Cnidome. Desmonemes and microbasic euryteles.

Habitat type. Intertidal (see Schuchert 2007, 2012), here found in the subtidal.

Substrate. Algae, stones and other substrates.

Seasonality. November in Salento waters (De Vito 2006; this study).

Reproductive period. In northwestern European waters ( Schuchert 2012), reproductive colonies occur from early summer to autumn.

Distribution. Atlantic, Mediterranean (see Bouillon et al. 2004; Schuchert 2007; Gravili et al. 2008 a).

Records in Salento. Rare in the following localities: Grotta del Ciolo (Denitto et al. 2007); Otranto (De Vito 2006; Gravili 2006; Gravili et al. 2008 a; this study).

Remarks. Schuchert (2004, 2007) considers the Mediterranean record of this species by Lo Bianco (1909) as a misidentification of Pachycordyle pusilla ( Motz-Kossowska 1905)  .

References. Hincks (1868), Berrill (1948), Picard (1958 a), Riedl (1959), Bouillon et al. (2004), Schuchert (2004, 2007, 2012), De Vito (2006), Gravili (2006), Denitto et al. (2007). Gravili et al. (2008 a).