Bythotiara Gunther 1903

Bythotiara Günther, 1903

Bythotiara n. g. Günther, 1903: 424; Schuchert, 2009: 443. Type species: Bythotiara murrayi Günther, 1903.

Diagnosis. Medusa with four simple or bifurcated radial canals, without centripetal canals; with or without secondary (rudimentary) tentacles, these entirely covered with nematocysts; gonads interradial, with transverse furrows; no ocelli. Remarks. Only three Bythotiara species occurs in South China Sea.

Bythotiara depressa Naumov, 1960 (Figs 1–3, 5–6)

Bythotiara depressa Naumov, 1960: 191 –192, fig. 77; Kramp, 1968: 54, fig. 144; Arai & Brinckmann-Voss, 1980: 64; Brinckmann-Voss & Arai, 1998: 40 –42, fig. 1a, 1c; Xu et al., 2014: 281, fig. 140.

Material examined. Three specimens from the southern South China Sea, one specimen, 28 August 2011, station NT11012 (04°00’N, 113°00’E), depth 42.5 m; the other two specimens, 30 August 2011, station NT11015 (17°45’N, 111°15’E), depth 126 m. One specimen from the southern Taiwan Strait, station TD07101 (21°14’N, 116°27’E), depth 329 m, 31 January 2007; coll. Xiang Peng.

Description. Umbrella laterally flattened, 5–7 mm in height, 4.5–5.5 mm in width, with thick mesoglea, especially at apex; manubrium half length of bell cavity or less; gonad interradial, with varied degree of folding, usually with two or four transverse folds; eight tentacles thick at base, becoming very thin before terminal nematocyst clusters; without secondary tentacles; terminal nematocyst clusters of adult specimens mostly spherical, rarely ovate but still arising abruptly from a narrow portion of the tentacle; four unbranched radial canals, without centripetal canal.

Distribution. Southern of Taiwan Strait and southern of South China Sea; sea of Okhotsk, Kurile Islands, Bering Sea (Naumov, 1960), northeast and northwest Pacific (Arai & Brinckmann-Voss, 1980).

Remarks. Bythotiara depressa has frequently been confused with Protiaropsis anonyma (Maas, 1905) (= Heterotiara anonyma). Based on the literatures description of B. depressa and his own abundant material of P. anonyma from Papua New Guinea, Bouillon (1980) suggested this two species might be conspecific. However, Arai & Brinckmann-Voss (1980) recommended that they should be kept separate after comparing 232 specimens of B. depressa from the northeast and northwest Pacific to 30 specimens of P. anonyma from Papua New Guinea.

The two species are very similar. Each of them has four unbranched radial canals and tentacles (normally eight) with terminal nematocyst thickening. Although they are placed in different genera based on degree of gonad folding, the two species are very similar on it. Both of them may be smooth or show loose folding as the manubrium contracts.

However, Bythotiara depressa can be distinguished from the Protiaropsis anonyma by: 1) the terminal nematocyst clusters of the tentacles are usually spherical in adult, and always arise abruptly from a very thin portion of tentacle in the former, whereas they are more elongate in the latter; 2) radial canals of the former are twice the thickness of comparably size latter, but B. depressa does not show any thickening at the junction of radial and circular canals, whereas P. anonyma shows a slight thickening at that point; 3) the vacuolated portion of endodermal cells in the adnate tentacle base is thicker in B. depressa than in P. anonyma (Figs 2, 4).