Ischnomesini Hansen, 1916: 54; Wolff, 1956: 86.
Ischnomesidae Gurjanova, 1932: 40; Menzies, 1962: 111; Wolff, 1962: 71 –73; Birstein, 1971: 198 –199; Menzies & George, 1972: 971; Chardy, 1974: 1537; Kussakin, 1988: 418; Kavanagh et al., 2006: 4 –5.
Diagnosis. Head deeply embedded in and fused with pereonite 1, without intersegmental articulation, without eyes or rostrum, dorsal surface sloping into frons and clypeus. Pereon thin, elongate, pereonites 4 and 5 elongate, subcylindrical, produced backwards and forwards, respectively, pereonite 5 longest; female spermathecal duct pores near dorsal midline of pereonite 5, not covered by posterior margin of pereonite 4; pereonites 5–7 narrowing posteriorly. Pleotelson shorter than pereonites 5–7; anus subterminal (at obtuse angle to ventral surface), external to pleopodal chamber and separated from it by broad cuticular bar, anal region projecting from anterior pleotelson. Antennula with 2 to 6 articles; article 1 short and rounded; article 2 tubular, much longer than wide; article 3 flagellar. Antenna much longer than anterior portion of body, article 3 without scale. Mandibles with distinct incisor and molar processes, dorsal condyle elongate and smoothly curled on posterior margin of mandibular body. Maxilliped basis broader than palp, endite shorter than palp, palp positioned in distal third of basis, articles 4–5 narrower than articles 1–3. Pereopod I prehensile between carpus and propodus; carpus with robust setae on ventral margin. Pereopods II–VII long, thin, paucisetose; dorsal (anterior) dactylar claw longer than posterior (ventral) claw, robust; posterior dactylar claw flattened, slightly curled. Pleopod III exopod shorter than endopod, thin, pointed, with 1 distal plumose seta; endopod quadrate, with 3 distal plumose setae. Uropod paucisetose, uniramous, inserting terminally lateral to anus. Sexual dimorphism common in nonsexual features: pereonites longer and thinner in males, spination of pereon and pleotelson more developed in males; antenna male flagellum more robust, with more setae and articles than in female; uropods longer in male than in female.
Haplomesus Richardson, 1908
Haplomesus Richardson, 1908: 81; Hansen, 1916: 59; Gurjanova, 1932: 42; Birstein, 1960: 6; 1963: 59; 1971: 209; Menzies, 1962: 117; Wolff, 1962: 86; Menzies & George, 1972: 973; Kussakin, 1988: 445; Kavanagh et al., 2006: 5 –7.
Not Haplomesus Merrin & Poore, 2003: 286.
Type species: Haplomesus quadrispinosus (Sars, 1879).
Species included: Haplomesus angustus Hansen, 1916; H. bifurcatus Menzies, 1962; H. biscayensis Chardy, 1975; H. brevispinis Birstein, 1960; H. celticensis Kavanagh, Wilson & Power, 2006; H. concinnus Birstein, 1971; H. consanguensis Mezhov, 1980; H. corniculatus Brökeland & Brandt, 2004; H. cornutus Birstein, 1960; H. formosus Mezhov, 1981; H. gigas Birstein, 1960; H. gorbunovi Gurjanova, 1946; H. hanseni Kavanagh, Wilson & Power, 2006; H. insignis Hansen, 1916; H. orientalis Birstein, 1960; H. modestatenuis Menzies & George, 1972; H. modestus Hansen, 1916; H. ornatus Menzies, 1962; H. profundicola Birstein, 1971; H. quadrispinosus (Sars, 1879); “ H. quadrispinosus” sensu Brandt, 1992; H. robustus Birstein, 1960; H. scabriusculus Birstein, 1960; H. tenuispinis Hansen, 1916; H. thomsoni (Beddard, 1886); H. tropicalis Menzies, 1962; H. zuluensis Kensley, 1984.
Excluded species: H. franklini Merrin & Poore, 2003 (incertae sedis).
Diagnosis. Pereonites 5–7, pleonite 1 and pleotelson lacking intersomite articulations. Pereonites 5–7 narrowing posteriorly. Antennulae with 5 or 6 articles, distal flagellar articles at least twice as long as wide. Pereopod I carpus without ventral expansion of palm. Maxilliped palp narrower than basal endite, articles 2 and 3 expanded. Uropods uniramous, single segmented, distally tapering. Mandible palp absent.