Systellaspis pellucida

Systellaspis pellucida (Filhol, 1884)

[New Japanese name: Sukashi-toge-hiodoshi-ebi]

Figs. 4, 5

Restricted synonymy:

Acanthephyra pellucida Filhol, 1884: 144, 162 [type locality: North-East Atlantic, 26°20’N, 14°53’W, 782 m; lectotype designated by Crosnier & Forest (1973)].

Acanthephyra affinis Faxon, 1896: 162, pl. 2, figs. 1–3 [type locality: off Granada, 286 m].

Systellaspis lanceocaudata.— Balss 1925: 243, figs. 12, 13. Not Systellaspis lanceocaudata Spence Bate, 1888.

Systellaspis affinis.— Calman 1939: 190.— Dennel 1940: 345, figs. 19–28, pl. 25, figs. 4, 5, pl. 26, figs. 1–6; Chace 1947: 39, fig. 3.

Syetellaspis pellucida.— Crosnier & Forest 1973: 92, figs. 26c, 27c.— Chace 1986: 67, figs. 34m –o, 35g, h.— Chan & Yu 1986:— Crosnier 1987: 720, figs. 12, 13, 14a, b, 15.— Hanamura 1987: 25, fig. 9c–e; Hanamura & Evans 1994: 52.— Cardoso & Young 2005: 70, figs. 54–58.— Pequegnat & Wicksten 2006: 102.— Hayashi 2007: 77.— Poupin & Corbari 2016: 17, fig. 4g.

Material examined. T/ RV “ Toyoshio-maru ”, 2005-04 cruise, stn 5, W of Yoron Island, bottom depth 1005–1014 m, 23 May 2005, ORI net oblique tow, 1 ovigerous female (cl 13.5 mm), CBM-ZC 11281.

Colouration in fresh condition (Fig. 4). Body generally semitransparent with scattered red chromatophores, cephalothorax inside reddish; carapace with purple streaks becoming longer posteriorly (= photophores) aligned adjacent to ventrolateral margin; each pleomere with narrow red line posteriorly. Cornea gray-brown. Antennular and antennal peduncles and flagella semitransparent. Maxilliped 3 generally transparent, distal part of ultimate article reddish, grooming setae on inner surface of distal 2 articles yellow-brown. Pereopods 1 and 2 generally semitransparent, fingers reddish. Pereopods 3 and 4 also generally semitransparent, but proximal parts of carpi dark red. Pereopod 5 propodus and carpus generally dark red except for white distal part of propodus; merus purplish. Pleopods 1–5 generally semitransparent, each with small black spot at base. Telson and uropods transluscent

Distribution. Known from low latitudinal areas in the Indo-West Pacific and Atlantic Oceans; mesopelagic to near bottom, at depths of 291–3292 m, but commonly found between 300 and 600 m (Crosnier & Forest, 1973; Chace 1986; Chan & Yu, 1986; Crosnier, 1987; Cardoso & Young, 2005).

Remarks. The genus Systellaspis Spence Bate, 1888 is represented globally by 11 species (De Grave & Fransen 2011; Sha and Wang, 2015), of which S. debilis (A. Milne-Edwards, 1881), S. lanceocaudata Spence Bate, 1888 and S. paucispinosa Crosnier, 1987 have been reported from Japanese waters (Hayashi, 2007; Komai & Komatsu, 2009). The present specimen is identified as S. pellucida because of the following features (cf. Chace, 1986): rostrum elongate, distinctly overreaching antennal scale (Fig. 5A); carapace not carinate on posterior half of dorsal midline, without lateral ridge (Fig. 5A); branchiostegal spine shortly buttressed (Fig. 5A); pleomere 3 distinctly carinate dorsally, with non-recurved posterodorsal spine (Fig. 5B); pleomeres 4 and 5 with posterior margin of tergum entire, not spinulose, between posteromedian spine and junction with pleuron (Fig. 5B); pleuron 5 without spine on posterior margin (Fig. 5B); pleomere 6 dorsally flattened but not distinctly sulcate in midline; telson with 3 pairs of small dorsolateral spiniform setae, posteromedian projection with 3 lateral pairs and 1 terminal pair of spiniform setae (Fig. 5C). The distribution and structure of the dermal photophores were extensively described by Calman [1939; as S. affinis (Faxon, 1893)] and Dennel (1940; as S. affinis).

Among the three species previously known from Japanese waters, S. lanceocaudata is most similar to S. pellucida. The former is differentiated from the latter by the presence of a pair of conspicuous spines on the posterodorsal margin of the pleomere 5 (versus no spines), the dorsally sulcate pleomere 6 (versus dorsally rounded) and the posterior margin of the telson drawn out into an elongate, acuminate process (versus drawn out into a terminally blunt process). Calman (1939) clarified that specimens from off Zanzibar, East Africa, identified with S. lanceocaudata by Balss (1925), actually represented S. pellucida (as S. affinis).

Systellaspis pellucida has been widely reported from the Indo-West Pacific and Atlantic, including Taiwan (Chan & Yu, 1986), and thus the occurrence of the species in Japanese waters could be expected. The present study confirms for the first time the presence of S. pellucida in Japanese waters.

Crosnier (1987) recognized two forms within S. pellucida (S. pellucida “forme typica ” and S. pellucida “forme longirostris ”), but the name “ longirostris ” is an infrasubspecific name, and thus is not available (Article 10.2, ICZN, 1999). The two forms are differentiated by the proportional length of the rostrum: the Atlantic and Indo- West Pacific specimens were referred to S. pellucida “forme typica ” (rostrum length/carapace length 1.1–1.6) and S. pellucida “forme longirostris ” (rostrum length/carapace length 1.5–2.3), respectively (Crosnier, 1987). Our specimen has the rostrum being about 1.3 times as long as the carapace, rather agreeing with the Atlantic population in this regard, and not consistent with the division of Crosnier (1987). Nevertheless, the general tendency in the rostral length difference between specimens from the Atlantic and Indo-West Pacific may indicate the existence of population structure.

Chace (1986) noted that this species is “usually found on or near bottom in 291–3292 m ”, but our specimen was collected from the mesopelagic zone with an oblique tow of an ORI net.