(Fig. 6 A–C)
Hydractinia clavata Jäderholm, 1905: 5 –6, pl. 3 figs 6–8; Stepanjants, 1979: 15, pl. 1 fig. 9; Blanco, 1994 a: 152; 1994 b: 184; Peña Cantero, 2004: 768; Stampar et al., 2006: 58.
Stylactella clavata — Stechow, 1925: 401; Iwasa, 1934: 262, 269.
Cytaeis clavata — Bouillon et al., 2006: 145.
Perarella clavata — Rees, 1962: 394, fig. 11; Peña Cantero, 2008: 453.
Material examined. Antarctic Swedish South Polar Expedition. 1901 -03: Type (SMNH— 7946), Stn 8, 11.II.1902, 64°03’S, 56 ° 37 ’W (Erebus and Terror Gulf, Graham Region), 360 m, soft clay, colony with numerous polyps and gonophores, on gastropod shell.
Description. Stolonal colony growing on gastropod shell. Hydrorhiza consisting of a mesh of anastomosing tubes of perisarc, not covered by a coenosarc layer. With gastrozooids and gonophores. Neither spines nor dactylozooids. Gastrozooids relatively large (Fig. 6 A) [6–8 mm long and 0.3 to 1 mm wide, according to Jäderholm (1905)], with conical hypostome and a distal crown of 15–17 filiform tentacles (Fig. 6 A, B). Gonozooids absent; gonophores directly arising from hydrorhiza, club-shaped (Fig. 6 C), about 2 mm long and 0.50–0.75 mm wide. Both gastrozooids and gonophores with a short, basal cup of perisarc (Fig. 6 A, C). Female gonophores with a very large number of eggs.
Measurements (in µm). Cnidome: microbasic mastigophore [range 8.0– 9.5 x 3.5–4.5, mean 9.2 ± 0.5 x 4.2 ± 0.3 (n= 10); ratio, range 2.0– 2.4, mean 2.2 ± 0.1 (n= 10)], desmonemes [range 5.5–6.5 x 3.5–4.5, mean 6.1 ± 0.3 x 4.2 ± 0.3 (n= 10); ratio, range 1.3–1.6, mean 1.5 ± 0.1 (n= 10)].
Remarks. Rees (1962: 394–395, fig. 11) also provided a description of the species after examining the type material. Perarella clavata is a morphologically well-characterized species, with gonophores arising directly from the hydrorhiza. The only missing relevant information concerned the cnidome. I have tried to fill this gap with the reevaluation of the type material. The presence of a basal perisarc cup also supports that this species belong to the family Cytaeidae.
Rees (1956, 1962) re-established the genus Perarella for species with fixed sporosacs or degenerated medusae. Bouillon et al. (2004, 2006), following the idea that genera should not be delimited based exclusively on medusa reduction, considered, however, this genus congeneric with Cytaeis. Nevertheless, Schuchert (2007) indicated that this approach is valid in cases where a hydroid can be attributed unambiguously to a certain family, but not in the case of Perarella, which could even belong to the families Bougainvilliidae or Pandeidae. He concluded that it is advisable to retain the genus Perarella until its affinities are clearly revealed by molecular phylogenetic analyses. I agree with his opinion.
Ecology and distribution. Perarella clavata has been collected at depths from 220 m (Peña Cantero 2008) to 360 m (Jäderholm 1905), epibiotic on gastropod shells (Aforia magnifica) (Jäderholm 1905; Peña Cantero 2008). It has been found with gonophores in January (Peña Cantero 2008) and February (Jäderholm 1905).
Perarella clavata is endemic to West Antarctica, being only known from the Erebus and Terror Gulf, Graham Land (Jäderholm 1905) and off the south of Livingston Island, in the South Shetland Islands (Peña Cantero 2008).