Syscenus Harger, 1880: 387.- Sars, 1899: 66.- Richardson, 1905: 212.- Stebbing, 1924: 9.- Wahrberg, 1930: 24. Nierstrasz & Schuurmans Steckhoven Jf., 1930: 77. Schultz, 1969: 196.- Menzies & George, 1972: 12.- Kussakin, 1979: 269.
Harponyx Sars, 1882: 60
(type species Harponyx pranizoides Sars, 1882).
Rocinela.- Bovallius, 1885: 4 (not Rocinela Leach, 1818).
Syscenus.- Stephensen, 1948: 41.
Type species. Syscenus infelix Harger, 1880, by monotypy. Gender neuter.
Diagnosis. Body elongate, about 3 to 4 times as long as wide. Cephalon without eyes, anterior margin forming median point. Coxae present on pereonites 2-7, those of pereonites 4-7 shorter than respective segment, not posteriorly produced. Pleon abruptly narrower than pleon, about 30-40% BL; pleonites all visible, becoming progressively wider towards posterior, with free lateral margins; pleonites 3 -5 lateral margins produced to form an acute point. Pleotelson large, as long or longer than pleon, usually with caudomedial point. Pleonal sternite present anterior to pleopod 1 peduncles.
Antennule peduncle 3-articulate, antenna peduncle 5- articulate; flagellae of both not extending to pleon.
Frontal lamina, clypeus and labrum present. Mandible with unicuspid incisor; molar process and spine row absent; palp 3-articulate. Maxillule slender, styliform, with 5 hooked, terminal spines; reduced medial lobe present. Maxilla lateral lobe present, with 3 recurved terminal spines. Maxilliped 4 articulate, article 3 with 2-3 recurved spines, article 4 with 1 recurved spine; endite absent.
Pereopods 1 -3 robust; distal margins of ischium to merus with long simple setae; dactylus prehensile, strongly recurved, longer than propodus. Pereopods 4- 7 becoming progressively longer; ischium to propodus provided with spines and abundant simple setae; dactylus shorter than propodus, weakly recurved.
Pleopods 1 and 2, both rami with PMS; pleopods 3 -5 endopods with reduced setae (pleopod 3) or naked, distinctly smaller than exopods; coupling hooks present on peduncles of pleopods 1 -5; pleopods not extending beyond pleotelson lateral margins. Uropods flat, lamellar; endopod longer than exopod.
Remarks. The genus is readily separated from all others of the family except Xenuraega by the pleon being abruptly narrower than the pereonite 7. Syscenus and Xenuraega are readily differentiated by Syscenus having a frontal lamina and lamellar uropods with both rami prominent, while Xenuraega lacks a frontal lamina and has a filamentous uropodal exopod and a stub-like endopod.
Several accounts illustrate the maxilliped as having only three articles (Harger 1883; Sars, 1899; Menzies & George, 1972; Monod, 1973; Nunomura, 1981). Examination of Atlantic specimens of Syscenus infelix (ZMUC CRU2073-2076) show that a small fourth maxilliped article (as illustrated here in Fig. 2G) is in fact present. The generic diagnosis given here is based on the type species and the new species described herein. The extent of character variation within the genus is not possible to assess as the remaining species are inadequately described.
Species of the genus have been recorded at depths between 146 and 3260 m.
Species included
Syscenus atlanticus Kononenko, 1988. This species has been recorded from a single North Atlantic location at a depth between 810-860 m. It is not possible to characterise this species against other congeneric species, although the original figures suggest that it differs from Syscenus infelix in having a broadly rounded pleotelson, and far longer antennal flagellae, supported by the examination of previously unreported material (ZMUC CRU2077).
Syscenus infelix Harger, 1880. The type species and the most widely recorded species of the genus. The type locality is " 15 miles northeast of Cape Cod," northwestern Atlantic (Harger 1880). The species has been reported from depths between 146 and 1117 m; records are from the northeastern and northwestern North Atlantic (Richardson, 1905), Japan (Richardson, 1909) the Philippines (Richardson, 1910), New Caledonia (Monod, 1973) and South Africa (Stebbing, 1924). Given the differences noted between Japanese and Atlantic specimens, the condition of the Philippine specimen, and lack of detail in the early records, confirmation of all these localities is required.
Syscenus intermedius Richardson, 1910. Known only from the Philippines, Albatross Station 5301.
Syscenus latus Richardson, 1909. Known only from Japan, taken at a depth of 742 m, southwest of Koshika Island, c. 31°39'N 129°20 -23'E.
Syscenus pacificus Nunomura, 1981. Known only from the East China Sea, 660- 990 m. Nunomura (1981, p. 16) apparently believed that he was describing only the second species of the genus, overlooking the publications of Richardson (1909, 1910) and Menzies & George (1972). The illustrations of Nunomura (1981) show striking points of resemblance to S. latus from the same area and the validity of S. pacificus is open to question.
Syscenus peruanus Menzies & George, 1972. The species is based on a juvenile with eyes, taken from the Peru-Chile Trench at a depth of 3254-3260 m.