Centromedon Sars, 1891

Horton, Tammy & Thurston, Michael, 2011, Centromedon zoe (Crustacea: Amphipoda: Lysianassoidea: Uristidae), a new deep-water scavenger species from the North Atlantic, with a key to the genus Centromedon, Zootaxa 2869, pp. 54-62 : 55-56

Genus Centromedon Sars, 1891

Centromedon Sars, 1891: 99 –101, pl. 34, fig. 2. — Barnard, 1962: 35. — Olerod, 1980: 50, 51. — Barnard & Karaman, 1991: 474. — Lowry & Stoddart, 1995: 13.

Type species. Anonyx pumilus Lilljeborg, 1865 , original designation.

Diagnosis. Lateral cephalic lobes triangular, subacute to acute. Eyes apparently absent. Callynophore well-developed in both males and females. Epistome flattened and separated from upper lip by a sinus or notch; upper lip rounded and projecting in front of epistome. Inner lobes of lower lip weak or absent. Mandibular molar conicolaminate with vestigial triturative area, palp attached level with molar. Maxilla 1, inner plate stout with two pappose setae on apex (one in C. pavor ); outer plate with 7–11 setal-teeth in 7–4 crown arrangement (reduced to 4–3 crown in C. pavor and 6–3 crown in C. pumilus ); palp two–articulate, reaching beyond outer plate. Maxilla 2, outer plate longer than inner plate. Maxilliped, outer plate not reaching article 3 of palp, suboval, with long robust setae apically, small and sparsely placed robust nodular setae on straight inner margin; article 4 of palp normal to vestigial. Gnathopod 1 variable from subchelate to simple; coxa 1 variable from strongly to weakly tapered, (not tapered in C. typhlops ); ischium short (1: <1.13). Gnathopod 2 subchelate to chelate. Pereopod 7 with distal articles shortened. Epimeron 3 posteroventrally produced into an acute tooth. Urosomite 1 with dorsal carina. Outer ramus of uropod 3 two-articulate, article 2 long (>50% article 1 length). Telson longer than wide, deeply cleft.

Species composition. Centromedon pumilus ( Lilljeborg, 1865) ; Centromedon productus ( Goës, 1866) ; Centromedon typhlops ( Sars, 1879) ; Centromedon calcaratus ( Sars, 1879) ; Centromedon laevis ( Bonnier, 1896) (new combination); Centromedon mediator ( Barnard, 1962) (new combination); Centromedon pavor Barnard 1966 ; and Centromedon zoe sp. nov.

Remarks. Uristes typhlops mediator Barnard, 1962 was originally described as a subspecies of Centromedon typhlops ( Sars, 1879) . An examination of the type material of both Uristes typhlops mediator and Centromedon typhlops shows that they differ by characters of the gnathopod 2, epimeron 3 and telson. There is also a clear size difference between the two species, with mature females of C. typhlops reaching 17 mm in length while those of C. mediator are 6 mm. Uristes typhlops mediator conforms to the diagnosis of Centromedon and is hereby transferred to this genus as C. mediator .

Orchomenella laevis Bonnier, 1896 was described from a single adult male (5 mm) taken in the Bay of Biscay (44°17' N, 02°18' W) at 950 m. It was transferred to Tryphosella by Barnard, 1964 but it has a non–dominant epistome, reduced distal articles of pereopod 7 and otherwise conforms to the new diagnosis of Centromedon , except for the outer plate of the maxilliped, which does reach article 3 of the palp. It is hereby transferred to this genus as C. laevis .

The distinctions between the genus Uristes and Centromedon are unclear but clarification awaits a thorough redescription of Uristes gigas Dana, 1849 , the type species of Uristes and a re-examination of the species currently included in that genus, an analysis which is beyond the scope of this paper. The location of Dana’s material is unknown. It is very likely that the type material was part of a collection on loan from the Smithsonian Institution to William Stimpson, at the Chicago Academy of Science, at the time of the Chicago fire in 1871. All of Stimpson’s material was lost in the fire and it is known that he had on loan 10,000 jars of Crustacea, many of which contained type material described by James Dana ( Evans 1967). Type material is not held at the Natural History Museum, London ( Thurston & Allen 1969) and is presumed to be no longer extant. It is generally accepted, however, that Tryphosa antennipotens Stebbing, 1888 is synonymous with Uristes gigas (see Stebbing 1899). A full redescription of this species and designation of a neotype is needed to clarify the status of the genus. It is probable that a large number of the species currently included in the genus (22+) will be transferred to other genera following a critical analysis.

Centromedon should be retained for those species characterised by: sub-acute to acute head lobes; tapered coxa 1; laminar, unridged molar with vestigial triturative area; maxilla 1 outer plate with 7-4 crown setal-tooth arrangement; shortened distal articles of pereopod 7 and acutely produced epimeron 3.

The similarity of Centromedon to Galathella Barnard & Karaman, 1987 was noted by Lowry & Stoddart (1995), who pointed out that these are the only two genera in the Uristidae with a tapering first coxa and 7-4 crown setal-tooth arrangement. It was also noted that both genera show a range of gnathopod 1 morphologies with species characterised by a simple, weak structure through to others with a strongly subchelate gnathopod 1. The only differences between the two genera appear to be the sub-acute lateral cephalic lobe (rounded in Galathella ), the lack of proximal A3 setae on the mandibular palp, the less well-developed molar, and the acutely produced posteroventral corner of epimeron 3 in Centromedon . Contrary to Lowry & Stoddart (1995), most species of Centromedon ( C. productus , C. typhlops , C. zoe and C. mediator ) have callynophores in the female. There is a degree of variability within the genus in that C. pumilus and C. pavor have a reduced number of setal-teeth on the outer plate of maxilla 1, a reduced article 4 of the maxilliped palp and less well-developed callynophore and gnathopod 1 ( Olerod, 1980).