Stephonyx Lowry & Stoddart, 1989

(Fig. 39)

Stephonyx Lowry & Stoddart, 1989: 521 .— Lowry & Stoddart, 2003: 284 (catalogue).— Diffenthal & Horton, 2007: 32.— Senna & Serejo, 2007: 8, 13 (key).—Narahara, Tomikawa & Torigoe: 2012 (key).

Type species. Euonyx biscayensis Chevreux, 1908, by original designation.

Included species. Stephonyx includes 13 species: S. arabiensis Diffenthal & Horton, 2007; S. biscayensis (Chevreux, 1908); S. carinatus Bellan-Santini, 1997; S. incertus Bellan-Santini, 1997; S. mytilus (Barnard & Ingram, 1990); S. laqueus (J.L. Barnard, 1967); S. normani (Stebbing, 1888); S. perexcavatus Narahara, Tomikawa & Torigoe, 2012; S. pirloti (Sheard, 1938); S. rafaeli sp. nov.; S. scutatus (Griffiths, 1977); S. talismani (Chevreux, 1919); S. uncinatus Senna & Serejo, 2007 .

Diagnostic description. Antenna 1 peduncle article 1 without anterodistal lobe; accessory flagellum not forming cap covering callynophore. Antenna 2 with weakly developed brush setae. Mandible molar a setose tongue with vestigial triturating area, a reduced column with a triturating surface or occasionally a raised weakly setose plate. Maxilla 1 outer plate a well developed 7/4 crown. Maxilla 2 inner plate slightly shorter than outer plate. Gnathopod 1 chelate; coxa 1 reduced, significantly shorter than coxa 2, subquadrate or tapering distally; ischium long (length 2 × to 4 × breadth) to very long (length 4 × to 6 × breadth); carpus very long (length more than 4 × breadth); propodus margins subparallel. Uropod 2 inner ramus not constricted. Telson deeply cleft .

Remarks. Only three uristid genera, Euonyx, Kyska and Stephonyx have chelate first gnathopods. Kyska differs from Euonyx and Stephonyx in having a fully developed gnathopod 1 coxa (significantly reduced in Euonyx and Stephonyx). Stephonyx is a genus of scavengers with fully developed 7/4 crown of setal-teeth on the outer plate of maxilla 1 whereas Euonyx is a genus of ectoparasitic amphipod with strongly reduced (small) setal-teeth on the outer plate of maxilla 1.