Siphonolabrum Lang 1972

Genus Siphonolabrum Lang, 1972

Siphonolabrum Lang, 1971: 361, 365 (remarks), 367 (key to genera), as nomen nudum. Sieg, 1986b: 140 –142 (remarks on genus and key to species), 142–152, figs 95–99 (description of S. fastigatum).

Siphonolabrum Lang, 1972: 214 (generic diagnosis), 214–221, figs 1–4, plate 1 (description of S. mirabile). Sieg, 1983: 299 –300 (bibliography). Kudinova-Pasternak, 1984: 108 –109, figs 10–11 (description of S. langi). Gutu & Sieg (1999), 384 (classification). Dojiri & Sieg, 1997: 220 –224, figs 3.12–3.13 (description of S. californiensis). Larsen & Wilson, 2002: 12, 14 (classification derived from phylogenetic analysis). Bird, 2004: 2, 7–8 (remarks). Larsen, 2006: 141, 142 (remarks).

Type species: Siphonolabrum mirabile Lang, 1972, by monotypy.

Species included ( Japanese and trench species in bold): S. californiensis Dojiri & Sieg, 1997 (Santa Barbara Channel, California, 98 metres); S. fastigatum Sieg, 1986 (Joubin Islands, Antarctic Peninsula, 49–75 metres); S. langi, (Central Pacific, 1735–2430 metres); S. mirabile, (Guatemala Basin [Central East Pacific], 3534– 3563 metres, and the Azores [NE Atlantic], 4165 metres); S. tenebrosus n.sp .

Diagnosis (largely derived from Lang (1972) and Sieg (1986b) but with some amendments).

Female/neuter. Anarthrurid with pleon with five pleonites, about as wide as the pereonite 6 and pleotelson. Antennule of female four-articled. Antenna six-articled (but see below), terminal article very small. Labrum prominent, narrower than deep. Mandibles translucent and weak, incisor narrow, with denticulate tip, molar and lacinia mobilis lacking. Epignath with short terminal seta. Maxillule with eight unequal terminal setae. Maxilliped palp article 3 with three long setae; endite with one long seta. Cheliped propodus and fixed finger without fold, but with longitudinal ridge on latter; fixed finger with one ventral seta and two prominent teeth on incisive margin. Dactylus of pereopods 1–3 with long accessory seta. Carpus of pereopods 4–6 with three spiniform and one rod-like seta. Pleopods well-developed, biramous, with setae. Uropod exopod fused with basal article, short or as long as proximal article of endopod, with two terminal setae; endopod two-articled.

Male with enlarged pleon; antennule seven-articled (two-articled peduncle and five-articled flagellum), cheliped fixed finger with reduced teeth, pleopod with longer setae than female, uropod exopod articulated from basal article, endopod three-articled.

Remarks. Although the generic distinctions of Siphonolabrum from Anarthruropsis are said to be minor (Sieg 1986b; Larsen 2005), i.e. restricted to the number of maxillule endite spines and labrum shape, other characters appear to be valid, consistent and useful for making identifications: the probable existence of a mandibular molar in Anarthruropsis (for the type species A. galatheae at least), one long seta on each maxilliped endite in Siphonolabrum, rather than two small setae; one ventral cheliped fixed finger in Siphonolabrum – although A. edentula also has only one (possibly because only manca-II stages were described), and two prominent teeth on the fixed finger in Siphonolabrum. A more detailed revision of the two genera is desirable but is beyond the scope of this paper, as are analyses of phylogenetic relationships within and without the family Anarthruridae.

There may be an issue with the character-state given for the number of antennal articles, since the small ‘article’ figured by most authors and also in this paper (Figure 2 C) could be a basal peduncle, so that the actual article-number state could be ‘five’ rather than ‘six’. Ideally, scanning-electron-microscope study might resolve this problem.

Of the four previously described species, S. langi seems to be of dubious status within the genus, at least because the uropod exopod is shown by Kudinova-Pasternak (1981) as being discrete, not fused. In addition, the cheliped shape is more similar to those of some other taxa such as Robustochelia Kudinova-Pasternak, 1983 albeit having the anarthrurid or agathotanaid form of the attachment to the cephalothorax.

Apart from a dubious record (of a manca stage) from the North-East Atlantic (Azores) of S. mirabile (Lang 1972), the genus appears to be known so far only from the Pacific Ocean—the S. fastigatum records coming from the Pacific side of the Antarctic Peninsula.