Mesamphisopus Nicholls 1943

Gouws, Gavin, 2008, New species of Mesamphisopus, an endemic South African freshwater isopod genus (Isopoda: Phreatoicidea: Mesamphisopidae), Zootaxa 1690, pp. 1-62 : 10-11

Genus Mesamphisopus Nicholls 1943

Type species. By original designation, Phreatoicus capensis Barnard 1914: 233–240 , plates 23 & 24. (Type locality: Table Mountain , South Africa) .

Synonymy. Phreatoicus: Barnard 1913 , Barnard 1927, Sheppard 1927, Barnard 1940

Mesamphisopus: Nicholls 1943 , Kensley 2001

Species list. Mesamphisopus abbreviatus ( Barnard 1927)

Mesamphisopus albidus n. sp.

Mesamphisopus baccatus n. sp.

Mesamphisopus capensis ( Barnard 1914)

Mesamphisopus depressus ( Barnard 1927)

Mesamphisopus kensleyi n. sp.

Mesamphisopus paludosus n. sp.

Mesamphisopus penicillatus ( Barnard 1940)

Mesamphisopus setosus n. sp.

Mesamphisopus tsitsikamma n. sp.

Generic description. Head length shorter than width in dorsal view; width 0.76–0.89 pereonite 1 width; lateral profile of dorsal surface smoothly curved; surface shiny, appears granular below cuticle. Eyes with ocelli distinguishable as clusters of units, pigmentation dark. Cervical groove smoothly curved to straight; extending nearly to dorsal margin of head. Antennal notch shallow. Pereon with fine setae on dorsal surface. Pereonite 1 dorsal margin in lateral view shorter than pereonite 2. Pereonites 1–7 in dorsal view wider than long. Pleonites in lateral view deeper (> 1.09 pereonite depth) than pereonites; pleonite 1 pleura distinctly shallower (0.81 pleurae 2–5 depth) than pleurae of pleonites 2–5. Pleotelson vaulted (lateral fields vertical); dorsal surface smooth; lateral length 0.13–0.15 body length; ventral surface anterior to uropods strongly concave; posterior setae along ventral margin anterior to uropods longer than adjacent anterior setae. Antennula article 4 shorter than article 3. Terminal article length:antennular length 0.01–0.02. Penultimate article width approximately subequal to ante-penultimate article width. Antenna. Article 6 shorter or subequal in length to articles 4 and 5 combined. Mouthfield ventrally angled; mandibular insertion axis in lateral view nearly level (parallel to longitudinal axis of body); adjacent to posterior margin of head and anterior margin of pereonite 1. Clypeus broad bar, slightly rounded at mandibular fossae. Labrum asymmetrical, with notch along right margin. Mandible palp article 1 easily visible; article 3 relatively linear, mesial surface naked. Incisor processes broad, width greater than thickness. Left and right incisor processes with 4 distal cusps. Left lacinia mobilis with 3 cusps. Right lacinia mobilis large, well separated and distinct from remainder of spine row. Spine rows uninterrupted (first spine not separated from remainder); basal insertions crossing dorsally and then abruptly angling posteriorly. Molar process stout, heavily keratinised; triturating surface heavily ridged, with 1 tooth. Maxillula mesial lobe width less than lateral lobe; with 4 PPS. Lateral lobe distal margin narrower than midsection. Maxilliped epipod distal tip truncate to rounded; palp length:basis length 0.88–1.03. Pereopod I. Dactylus lateral surface with row of fine setae along axis; with distal accessory claw ventrolateral to primary claw. Propodus length:pereopod length 0.25–0.27 (0.19–0.21 in females). Propodal palm convex to straight in male; stout denticulate setae and basally-inflated RS occur distally; elongate broad-based setae occur along palm, mostly proximally. Merus dorsal margin projecting dorsodistally, U-shaped in cross-section (dorsal surface facing carpus flattened). Basis ventrodistal margin setae shorter than ischium. Pereopods II and III dactylus shorter than propodus; with 1 distal accessory claw ventrolateral to primary claw. In male, pereopod II length:body length 0.41–0.44; pereopod III length:body length 0.39–0.42. Pereopod II and III propodus length 0.13–0.16 respective pereopod length in male, 0.12–0.15 in female; carpus length 0.12–0.15 respective pereopod length in male; basis length 0.24–0.28 respective pereopod length in male. Pereopod IV length:body length 0.33–0.36. Dactylus with distal accessory claw. Propodus distal width in male less than palm width. Basis dorsal ridge in cross-section angular and produced but not forming plate, setae occurring along ridge. Pereopods V–VII progressively increasing in length. Dactylus with distal accessory claw. Basis lateral face with central groove or ridge. Pereopod VII basis dorsal ridge distal margin indented. Penes curved posteriorly; extending to midline; cuticle smooth; distally tubular. Pleopods. Exopod I uniarticulate, exopods II–V biarticulate; II–V proximal article distolateral lobes shorter than distal article. Endopods unilobed. Protopod mesial margins with elongate, inflexible SS. Pleopod I exopod distal and lateral margins rounded, ventral surface flat. Pleopod II endopod distal margin rounded; exopod distal segment longer than wide, lateral margin proximally rounded. Uropod protopod dorsomesial margin smooth, in lateral view nearly straight, in dorsal view parallel to ventral margin, dorsomesial and dorsolateral margins with RS and SS. Endopod longer than protopod; subequal-longer than exopod; straight-curving dorsally; ventral margin convex-straight proximally. Exopod shorter than pleotelson.

Remarks. Consideration of all species in the genus reveals that Nicholls’ (1943) generic diagnosis is too broad, including features known to occur in some, and in some cases all, phreatoicid taxa ( Nicholls 1943, 1944). These include the occurrence of a fifth oostegite in the females, the presence of coupling hooks on the pleopod protopods and simple robust setae distoventrally on the uropodal protopod (see above). Importantly, the diagnosis is inaccurate with regard to the character he regarded as having particular diagnostic importance. Nicholls (1943) considered the presence of setae on the endopods of all five pleopods as being diagnostic for Mesamphisopus . This was true for the species known at the time, but the examination of more individuals has provided an exception ( Mesamphisopus paludosus n. sp. does not bear setae on all five endopods). The presence of setae on all five pleopodal endopods has also been documented in recently described taxa (e.g., Eremisopus: Wilson & Keable 2002a ). Likewise, the presence of plumose setae on these endopods is not unique to Mesamphisopus ( Sayce 1902; Nicholls 1943, 1944). In the absence of a more detailed examination of all species in order to identify key synapomorphies of the genus (and the identification of these in related genera), Mesamphisopus is best defined by a unique combination of characters. Interestingly, Nicholls (1943) also appeared to stress a combination of characters, rather than individual synapomorphies, to define the genus. However, several features appear to be unique to Mesamphisopus , some of which are discussed above. Foremost among these are the minute and vestigial terminal segment of the antennula, the shape and form of the telson (including the apex and posterolateral margins), and the degree of fusion of the coxae of pereonites 2 to 4. These are largely free; fusion has not extensively limited mobility, sutures are still clearly visible and coxae and epimera are exposed. The utility of these characters will become more apparent following a more comprehensive revision of the species included in the genus and the description of more taxa.