Ischnomesidae Hansen 1916

Ischnomesidae Hansen, 1916

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.

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.

Remarks. The family Ischnomesidae is easily distinguished by an elongate fifth pereonite, which occurs uncommonly in other families, and certainly not as pronounced as seen in this family. Ischnomesids have other unique combination of characters such as cephalon embedded in the first pereonite with an unexpressed articulation, completely dorsal and exposed spermathecal duct pores in the female, a subterminal anus, subcylindrical pereonites 4 and 5, short and highly modified antennulae and uniramous uropods. Although the mandibles are relatively normal for Janiroidea, they have an unusual curled dorsal condylar region that is confluent with the posteromedial margin of the mandibular body, and the mandibular palp is generally absent (although a few species have a palp, such as Ischnomesus roseus Wolff, 1962). Although we don't have sufficient data on all described species, most species have a brittle calcified cuticle, which contributes to their fragmentation in deep­sea samples.