Mesamphisopus albidus, Gouws, 2008

Gouws, Gavin, 2008, New species of Mesamphisopus, an endemic South African freshwater isopod genus (Isopoda: Phreatoicidea: Mesamphisopidae), Zootaxa 1690, pp. 1-62 : 11-21

Mesamphisopus albidus n. sp.

Figures 1–4

Material examined. Holotype: SAM A45149, ♂ (7.2 mm), Franschhoek Pass , Franschhoek–Villiersdorp road, Western Cape, South Africa (33°55.73’S 19°09.57’E), coll. G. Gouws, 30 August 2001.

Paratypes: SAM A45150, 2 #m, 4 ♀, collection details as for holotype .

Other material: SAM A44933, 4 ♂, 5 ♀, collection locality as for holotype, coll. S . R. Daniels and G. Gouws, 17 April 2000 and 27 March 2001 . AM P67144, mounted SEM stubs of parts from 2 dissected ♂ (stubs AW450–458 and AW459–463, respectively) and a dissected preparatory ♀ ( AW461 ), collection locality as for holotype, coll. S . R. Daniels and G. Gouws, 17 April 2000 and 27 March 2001 .

Etymology. From the Latin “albidus”, meaning “white” or “light”, in reference to the light to absent pigmentation (gender masculine).

Diagnosis. Lightly pigmented or lacking pigmentation. Mandibular (genal or cheek) groove absent or weakly indented. Eyes small, maximum diameter 0.08 head depth. Pereon width in dorsal view near head width. Pleonites 1–4 width equal to composite length in dorsal view. Pleonites shallow; pleonites 1–5 depth:pereonite 7 depth respectively 1.09, 1.40, 1.53, 1.56 and 1.40. Pleotelson depth 1.33 pereonite 7 depth; ventral margin anterior to uropods with single row of 5 or 6 simple RS grading anteriorly to fine setae; lateral uropodal ridge curving strongly and extending posteriorly from uropods on pleotelson margin; posterior apex with 1 or 2 pairs of simple RS. Mandibular palp article 3 with 25 to 29 setulate setae. Maxilla lateral lobes with bidenticulate setae on distal tips and mesial margin; inner lateral lobe with 21 long bidenticulate setae. Maxilliped palp insertion on basis lateral margin with 1 PS; ventral surface with 1 subdistal SS, 3 SS more proximally; palp article 4 elongate-oval; article 5 length:width 1.67. Pereopod I propodal palm stout, with 4 or 5 denticulate bifid setae, 4 basally-inflated stout RS, 3–5 elongate broad-based setae. Pereopod II basis length approximately twice width. Pereopods V–VII basis dorsal ridge distinctly separated from basis shaft. Uropod rami (in cross-section) flattened dorsally and slightly ventrally; endopod RS on dorsal margin commence at midlength.

Descriptions. Male. Coloration. Off-white to cream, and light brown-grey to light slate-grey; most pigmentation occurs in longitudinal band dorsally with slight mottling of pigment laterally, particularly on pereonites. Pigmentation fades to light brown or completely to off-white or cream upon preservation, eyes remain black or fade to white in some individuals.

Head setae sparse, fine. Eyes approximately round; bulging dorsolaterally to projecting anteriorly.

Pereon setae on dorsal surface forming rows along posterior pereonite margins, length of setae 0.11 body depth. Pereonite 1 length:width in dorsal view 0.58. Pereonite 2 length:width 0.43. Pereonite 3 length:width 0.47. Pereonite 4 length:width 0.42. Pereonite 5 length:width 0.37. Pereonite 6 length:width 0.31. Pereonite 7 length:width 0.27.

Pleonites 2–4 respective lengths in dorsal view less than half the length of pleonite 5, 1–4 relative lengths unequal, increasing in length from anterior to posterior. Pleonites 1–5 dorsal length:maximum width of pleonites 1–5 respectively 0.19, 0.22, 0.30, 0.30 and 0.63.

Pleotelson dorsal surface in lateral view inflected ventrally, sparsely covered with fine setae, length equal to width; lateral length less than (0.75) depth.

Antennula ( Fig. 1) length 0.13–0.14 body length, with 7 or 8 articles. Articles 4 and 5 divisible into 1 large or 2 small articles. Article 3 with rudimentary antennular scale. Six fine aesthetascs below terminal article. Terminal article length:width 0.67. Penultimate article length approximately subequal to length of other articles. Distal articles circular in cross-section.

Antenna ( Fig. 1) length 0.49–0.55 body length. Flagellum length 0.58 total antenna length, generally with 19 or 20 (rarely 16–28) articles. Article 5 length subequal to article 4.

Mouthfield. Clypeus width 0.72 head width. Labrum ( Fig. 1) roughly ventrally semicircular in anterior view, distal margin with fringe of fine setae. Paragnaths ( Fig. 1) with distolaterally rounded lobes; dense mats of fine setae mesially on lobes, longer setae laterally; setal row on thickened proximal mesial margins.

Mandible ( Figs 1, 2) palp length 1.10 mandible length; article 1 with group of elongate SS on dorsodistal margin; article 2 with longitudinal row of 16–23 elongate SS along dorsolateral margin, separate distal group of 4 or 5 elongate setae more mesially. Left spine row with 14 spines, 4 of which are bifurcate. Right spine row with 12 or 13 spines, 4 of which are bifurcate. Molar process length subequal to width.

Maxillula ( Fig. 2) mesial lobe length 0.54 lateral lobe length; width 0.38 lateral lobe width; with 2 simple accessory setae, 1 on distolateral margin and 1 between central PPS, or 1 between central PPS and 1 between mesial and central PPS; 1 short weakly setulate seta on distal tip. Lateral lobe distal margin with 5 smooth RS, 7 denticulate RS; with 4 RS forming distal setal row; ventral face with 2 widely spaced PS or PCS, additional shorter PS among distal RS.

Maxilla ( Fig. 2) mesial lobe substantially wider (1.86) than outer lateral lobe; proximal portion distinctly angled to distal portion; proximal and distal setal rows separated by gap; short fine setulate setae or PS becoming more elongate proximally in single dorsal basal row; 13–15 simple elongate setae with distinct base and smooth shaft in single ventral basal row; evenly spaced equally long distally setulate setae, numerous SS in distal rows. Outer lateral lobe longer than inner lateral lobe, wider than inner lateral lobe; with 16 long bidenticulate setae.

Maxilliped ( Fig. 2) epipod length:width 1.22. Endite length:total basis length 0.44; mesial margin with 3 coupling hooks on left side, 2 on right side; dorsal ridge with 16 large distally denticulate PS. Palp insertion on basis mesial margin with 1 SS; palp width across articles 2 and 3:endite width 1.31; article 4 length:width 1.11; article 5 length equal article 4 length.

Pereopod I ( Fig. 2) dactylus length subequal to palm, length:palm length 1.03; ventrodistal margin with thin scale-like spines along 0.05–0.24 total length; claw length:dactylus length 0.10; distal accessory claw length 0.20 primary claw length. Propodus length:width 0.92; dorsal margin with 6 setae in several groups between proximal and distal margin, 14 in distal group. Ischium dorsal margin with 3 SS. Propodal palm cuticular fringe weakly developed; low, cuticular process distally. Basis length:width 2.08; dorsal ridge with 4 setae positioned proximally and lateral to margin distally; ventrodistal margin with 4 elongate setae.

Pereopods II and III ( Fig. 3; Table 1). Pereopod II propodus with 4 broad-based setae, evenly spaced along ventral margin, setae 0.20–0.36 propodus length; carpus with 5 broad-based setae, setae 0.22–0.62 carpus length. Pereopod III propodus with 3 broad-based setae, evenly spaced along ventral margin, setae 0.18– 0.36 propodus length; carpus with 6 broad-based setae, setae 0.15–0.76 carpus length. Basis dorsal ridge in cross-section angular and produced but not forming distinct plate, with 12 elongate SS, along length or lateral to margin, and single PS distally on pereopod III. Pereopods II–IV ischium dorsal margin with 11 or 12 SS, including 1 RS.

Pereopod IV ( Fig. 3) with PNS on dorsal margin of basis. Dactylus length subequal to propodal palm; distal accessory claw length 0.25–0.33 primary claw length. Propodus length:pereopod length 0.12; length:width 1.46; distal width:palm width 0.69; with 6 broad-based setae on ventral margin, 3 distinctly larger than remainder; articular plate subequal in length to dactylar claw. Carpus length:pereopod length 0.13; with 5–7 broad-based setae on ventral margin. Ischium posterodistal margin with 10 setae. Basis length:width 2.30; dorsal ridge with 9 setae.

Pereopods V–VII ( Fig. 3; Table 2) with PNS on dorsal ridge of basis. Dactylus distal accessory claw ventral to primary claw. Propodus distal margins with 6 elongate RS. Pereopods V–VII ischium dorsal margin with 11–17 SS, including 4–6 robust. Basis dorsal ridge in cross-section angular on V, produced and forming distinct plate on VI and VII; lateral face with ventral ridge.

Penes length 0.38 body width at pereonite 7; with setae on shaft; distal tip rounded.

Pleopods ( Fig. 4; Table 3). Pleopodal endopod margins with SS and PS on I–IV, SS on V. Protopod mesial margins/epipods I–IV with 4, 2, 2 and 1 coupling hooks, respectively; II–V with 2, 5, 7 and 6 elongate SS, respectively; III–V with additional 2, 2 and 8 fine short SS, respectively. Protopods lateral epipods III–V with 17, 17 and 16 elongate SS on margins, respectively; epipod III margin with 2 PS. Pleopod I exopod broadest proximally, mesial margin straight; protopod length subequal to that of other pleopods, width subequal length. Pleopod II endopod appendix masculina basal musculature pronounced; with 33 or 34 setae on margin; length 0.47 pleopod length; distal tip extending beyond distal margin of endopod.

Pereopod III 2.25 2.20 2.64 2.84 2.00 1.75 Carpus length:pereopod length

Pereopod II 0.14 0.12 0.15 0.14 0.15 0.11 Pereopod III 0.13 0.12 0.14 0.13 0.13 0.10 Carpus length:width

Pereopod II 1.81 1.69 1.90 2.00 1.75 1.18 Pereopod III 1.67 1.67 1.84 1.92 1.50 1.33 Basis length:pereopod length

Pereopod II 0.27 0.28 0.25 0.26 0.24 0.30 Pereopod III 0.26 0.29 0.25 0.26 0.28 0.27 Basis length:width

Pereopod II 2.15 2.26 2.29 2.46 1.92 2.04 Pereopod III 2.11 2.40 2.30 2.67 2.36 1.74 Pereopod VI 0.50 0.49 0.48 0.43 n.d. 0.38 Pereopod VII 0.53 0.49 0.42–0.43 0.44 n.d. 0.42 Dactylar claw length:dactylus length

Pereopod V 0.33 0.34 0.29 0.47 0.26 0.25 Pereopod VI 0.28 0.28 0.33 0.32 0.24 0.24 Pereopod VII 0.30 0.39 0.33 0.37 n.d. n.d. Propodus length:pereopod length

Pereopod V 0.14 0.15 0.13 0.14 0.13 0.17 Pereopod VI 0.15 0.15 0.13 0.14 0.15 0.15 Pereopod VII 0.15 0.13 0.14 0.15 n.d. 0.14 Carpus length:pereopod length

Pereopod V 0.14 0.15 0.16 0.14 0.16 0.13 Pereopod VI 0.16 0.15 0.17 0.15 0.17 0.15 Pereopod VII 0.16 0.17 0.15–0.16 0.14 n.d. 0.14 Basis length:width

Pereopod V 1.72 1.63 2.07 1.95 1.63 1.30 Pereopod VI 1.56 1.72 1.73 1.90 1.50 1.19 Pereopod VII 1.59 1.84 1.61 1.76 1.45 1.55

M. baccatus n. sp. M. kensleyi n. sp. M. paludosus n. sp.

M F M F M F

Pleopod length:body length

Pleopod I 0.17 0.13 0.16 0.14 0.19–0.21 0.20

Pleopod II 0.19 0.16–0.17 0.18 0.16 0.19–0.24 0.23

Pleopod III 0.17 0.15–0.18 0.18 0.17 0.20–0.24 0.23

Pleopod IV 0.16 0.15–0.16 0.16 0.17 0.18–0.22 0.20–0.21

Pleopod V 0.13 0.13–0.16 0.15 0.13 0.18 0.18

Exopod length:width

Pleopod I 2.99 2.68 2.59 2.64 2.90–2.91 2.51

Pleopod II 1.95 1.90–2.10 1.93 2.13 2.18–2.25 2.02

Pleopod III 1.44 1.64–1.83 1.50 1.64 1.68–1.91 1.62

Pleopod IV 1.35 1.38–1.40 1.45–1.53 1.46 1.58–1.86 1.41

Pleopod V 1.40 1.16–1.50 1.21 1.14 1.15–1.29 1.21

Exopod distal article length:exopod length

Pleopod II 0.32 0.26–0.27 0.37 0.31 0.28–0.29 0.23

Pleopod III 0.30 0.30–0.31 0.32 0.32 0.28–0.30 0.25

Pleopod IV 0.32 0.28–0.30 0.36 0.35 0.33–0.34 0.30

Pleopod V 0.35 0.27–0.34 0.39 0.32 0.34–0.35 0.29

Endopod length:width

Pleopod I 2.68 3.05 2.42 3.04 2.51–2.94 2.73

Pleopod II 2.21 1.57–2.69 1.92 2.00 2.53–2.61 2.15

Pleopod III 1.76 1.59–1.89 1.55 1.22 2.13–2.18 2.00 Pleopod IV 1.51 1.53–1.67 1.29–1.40 1.39 1.87–1.93 1.83 Pleopod V 1.06 1.45–1.47 1.24 1.13 1.51–1.70 1.46 Endopod length:exopod length

Pleopod I 0.98 1.03 0.89 1.15 0.92–0.98 1.03 Pleopod II 0.82 0.76–0.94 0.77 0.80 0.86 0.85 Pleopod III 0.96 0.88–0.94 0.88 0.66 0.83–0.99 0.90 Pleopod IV 0.81 0.80–0.92 0.63–0.71 0.81 0.79–0.87 0.83 Pleopod V 0.51 0.71–0.73 0.62 0.68 0.72–0.76 0.71

Uropod ( Fig. 3) total length 1.86 pleotelson length. Protopod length:width 3.27; length 0.41 uropod total length; extending posteriorly subequal to pleotelson apex; dorsomesial ridge produced, plate-like, ridge length:endopod length 0.41. Endopod dorsal margin with 9 or 10 RS. Exopod length 0.83 endopod length; dorsal margin with 7 or 8 RS.

Female. Pereonites in dorsal view relatively shorter and wider than in male.

Pereopod I dactylus ventrodistal margin with thin scale-like spines along half its length; claw length:dactylus length 0.17. Propodus length:width 1.24. Propodal palm concave; cuticular fringe well developed; without distal low, cuticular process of male; with 1 bifid, 3 or 4 denticulate stout setae and 9 or 10 elongate broadbased SS. Basis length:width 2.32; dorsal margin with 9 setae along ridge.

Pereopods II and III ( Table 1). Pereopod II carpus with 4 broad-based setae. Pereopod III propodus with 4 broad-based setae; carpus with 5 broad-based setae.

Pereopod IV propodus lacks marginal broad-based seta; articular plate shorter than dactylar claw. Carpus length 0.11 pereopod length; with 3 broad-based setae on ventral margin. Ischium posterodistal margin with 7 setae. Basis dorsal ridge with 5 setae.

Pereopods V–VII. Consult Table 2 for comparison with male ratios.

Pleopods ( Table 3). Endopod I–V margins with PS. Coupling hooks present on protopod mesial epipods I–III only, respective counts of 3, 1 and 2. Protopod mesial epipods II–V with 4, 5, 6 and 6 elongate SS, respectively; IV and V with 2 and 4 additional finer SS, respectively.

Uropod total length 1.68 pleotelson length. Protopod dorsomesial ridge length:endopod length 0.68. Endopod with 6 or 7 RS; exopod with 3 RS.

Distribution. Known only from type locality, near Franschhoek, in the Hottentots Holland Mountains. The type locality was a narrow (20 to 30 cm in width), slow-flowing and muddy stream, almost completely obscured by schlerophyllous heathland vegetation. Specimens were found within the channel under a dense “floor” of fine, matted roots.

Remarks. Mesamphisopus albidus can be immediately distinguished by the weak or complete absence of pigmentation. This feature is not entirely diagnostic, because individuals of two species, M. setosus n. sp. and M. tsitsikamma n. sp., may occasionally lack pigmentation. Barnard (1927) also documented the absence of pigmentation in several populations he identified as M. capensis collected in the Hottentots Holland Mountains and Langeberg Mountains.

The extent of the setation of the head, pereon and pleotelson, M. albidus approaches that shown by M. capensis , M. tsitsikamma , and perhaps M. abbreviatus , although individual setae are longer in M. tsitsikamma . Mesamphisopus albidus is more setose than M. penicillatus and M. paludosus n. sp., and less setose than M. depressus , M. baccatus n. sp. and M. kensleyi n. sp., particularly with regard to the pleotelson.

The eyes of M. albidus are remarkably small and are the smallest within Mesamphisopus . This character state, in combination with the lack of pigmentation, suggests an early adaptation to hypogean lifestyle: individuals of this species were collected and dug out of the sandy bottom of the small seepage stream in which they occurred, beneath a considerable depth of matted root fibres, through which light is unlikely to penetrate.

The species is unique in regard the occurrence of robust setae midway along the uropodal exopod and endopod (in other species these setae occur along the length of the exopod and endopod). The setation of the pleopodal endopods is generally typical of Mesamphisopus (plumose on I–IV, simple on V), but M. albidus is the only species within Mesamphisopus where plumose setae occur laterally on the pleopodal epipods. While lacking the pair of dorsal sub-apical robust setae on the pleotelson, as described by Barnard (1927) for some individuals and used as a diagnostic characteristic for certain species by Kensley (2001), the setation of the posterior apex of the pleotelson does vary. One or two pairs of robust setae are common on the apex (or one pair on the apex, with one pair more ventrally), although five setae have been observed in one individual.