Mesamphisopus setosus, Gouws, 2008

Gouws, Gavin, 2008, New species of Mesamphisopus, an endemic South African freshwater isopod genus (Isopoda: Phreatoicidea: Mesamphisopidae), Zootaxa 1690, pp. 1-62 : 43-51

Mesamphisopus setosus n. sp.

Figures 17–20

Material examined. Holotype: SAM A45155, ♂ (10.8 mm), small pool in river, below Guardian Peak , Jonkershoek Nature Reserve, South Africa (34°00.80’S 19°00.07’E), coll. G. Gouws, S. R. Daniels, A. Pardini and S. Willows-Munro, 18 April 2003.

Paratypes: SAM A45156, ♂ (11.4 mm, dissected) and ♀ (brooding 10.5 mm, dissected) parts slide mounted and in microvials, additional 3 ♂, 3 ♀, collection details as for holotype .

Other material: SAM A44939, ♂, ♀, collection locality as for holotype, coll. G. Gouws, 12 March 2001 .

Etymology. From the Latin “setosus”, in reference to the relatively abundant pereopod setation.

Diagnosis. Head lacking setae. Mandibular groove smoothly indented, more acutely indented anteriorly. Pereon setae short, length 0.05–0.08 body depth. Pleonites 1–4 width 1.21–1.24 composite length in dorsal view. Pleotelson dorsal surface with median ridge posteriorly between vaulted telson and apex; depth 1.77– 1.92 pereonite 7 depth; ventral margin anterior to uropods with single row of 4 or 5 simple RS, interspersed with fine, elongate setae; lateral uropodal ridge terminating at pleotelson margin above uropods; posterior apex with 1 pair of RS. Antennula short, length <0.13 body length. Mandibular palp article 3 with 27–32 setulate setae; additional setae on mesial surface. Maxillula mesial lobe with 4 accessory setae; lateral lobe distal margin with 4 smooth RS, 8 denticulate RS. Maxilla mesial lobe proximal and distal setal rows continuous; two parallel ventral basal setal rows present. Maxilliped palp article 4 shape elongate-oval; length:width 1.24. Pereopod I propodal palm with 9 stout serrate denticulate setae, 2 basally-inflated stout RS, 6 elongate broadbased setae; ischium dorsal margin with single RS. Pereopod III propodus length:width 2.20–2.25. Pereopods VI and VII length:body length> 0.50. Pereopods V–VII with PNS on dorsal ridge of basis of pereopod VII, dorsodistally on propodus of pereopods V–VII. Pleopodal endopod V shallowly cleft dorsomesially. Uropod total length 2.15 pleotelson length; protopod extending slightly posterior to pleotelson apex; endopod and exopod each with 10 or 11 RS.

Descriptions. Male. Coloration. Bodies lightly pigmented and light brown to orange-brown; less pigmented along pleotelson, lateral portions of pereonites, ventral parts of pleura of pleonites; darker pigmentation along dorsal longitudinal band, towards posterior margins of pereonites and pleonites; unpigmented patches along lateral portions of pereonites give body light mottled appearance; pereopods generally unpigmented and white, may have slight pigmentation on bases; some individuals wholly lacking pigmentation, white to off-white in colour; pigmentation fades to off-white to cream upon preservation, eyes remain black.

Head. Eyes projecting anteriorly; maximum diameter 0.10–0.12 head depth; approximately round.

Pereon width exceeding head width; setae on dorsal surface concentrated along posterior pereonite margins. Pereonite 1 length:width in dorsal view 0.41–0.45. Pereonite 2 length:width 0.42–0.54. Pereonite 3 length:width 0.49–0.59. Pereonite 4 length:width 0.48. Pereonite 5 length:width 0.41–0.45. Pereonite 6 length:width 0.39–0.43. Pereonite 7 length:width 0.21.

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 0.16, 0.19, 0.20, 0.25 and 0.58. Pleonites 1–5 depth:pereonite 7 depth respectively 1.35, 1.92, 2.16, 2.23 and 2.00.

Pleotelson dorsal surface in lateral view inflected ventrally, sparsely covered with fine setae; length 1.04– 1.10 width; lateral length less (0.78–0.79) than depth.

Antennula ( Fig. 17) with 7 articles. Article 3 with antennal scale. 6 Or 7 tiny aesthetascs, 4 sensory setae on terminal article and along distal margin of subterminal article. Terminal article length:width 0.50–0.58. Penultimate article distinctly longer than other articles. Distal articles circular in cross-section.

Antenna ( Fig. 17) length 0.65 body length; flagellum length 0.68 total antenna length, with 29–35 articles. Article 5 longer than or subequal to article 4.

Mouthfield. Clypeus widening and broadly triangular laterally; width 0.72 head width. Labrum ( Fig. 17) ventrally broad and truncate, with fringe of fine setae on distal margin laterally to slight apex; dorsal margin same width as clypeus. Paragnaths ( Fig. 18) with distolaterally rounded lobes; with dense mat of fine setae from distal extent of lobes inwards along mesial margins; SS scattered along proximal mesial margins; lateral margin with dense mat of fine setae, discontinuous with apical setal rows.

Mandible ( Fig. 18) palp length 1.08–1.25 mandible length; article 1 with elongate SS distoventrally; article 2 with elongate SS along length ventrolaterally and ventromesially, separate distal groups of elongate setae laterally and ventrally; setae greater than half respective article length. Left spine row with 12 spines, 6 of which are bifurcate. Right spine row with 8 spines, 3 of which are bifurcate. Molar process length subequal to width or longer than wide.

Maxillula ( Fig. 18) mesial lobe length 0.50 lateral lobe length; width 0.54 lateral lobe width; accessory setae simple on distolateral margin, distally denticulate between central PPS and 2 distally denticulate between distomesial setae; 1 short weakly setulate seta on distal tip. Lateral lobe distal margin with 3 RS in distal setal row; ventral face with 2 widely spaced PS.

Maxilla ( Fig. 18) mesial lobe width 0.74 outer lateral lobe width; proximal portion distinctly angled to distal portion; 24 elongate, finely serrate setae in ventral basal setal rows; 36 closely-set elongate setae with distinct bases and thick shafts in dorsal basal row; 30 SS, PS and PCS, of which 4 are more robust PCS, in multiple distal rows. Outer lateral lobe longer than inner lateral lobe, wider than inner lateral lobe; distal margin with 19 long bidenticulate setae. Inner lateral lobe distal margin with 14 long bidenticulate setae.

Maxilliped ( Fig. 18) epipod length:width 1.15. Endite length:total basis length 0.37–0.41; mesial margin with 2 coupling hooks on left side, 4 on right side; dorsal ridge with 20 large distally denticulate PS. Palp insertion on basis mesial margin with 1 SS; ventral surface with 1 elongate distally plumose or pectinate seta subdistally towards mesial margin; palp width across articles 2 and 3:endite width 1.81; article 5 length:width 1.57, article 5 length:article 4 length 0.71.

Pereopod I ( Fig. 19) length:body length 0.48. Dactylus length subequal to or longer than palm, length:palm length 1.57; ventrodistal margin with thin scale-like spines along 0.16 total length; claw length:dactylus length 0.11; distal accessory claw length 0.25 primary claw length. Propodus length:width 1.18–1.22; dorsal margin with 6 setae confined to single distal group. Propodal palm cuticular fringe well developed; with low stout cuticular projection distally. Merus distodorsal margin includes 1 RS. Basis length:width 2.05; dorsal margin with 4 setae, mostly proximally; ventrodistal margin with 2 or 3 elongate setae.

Pereopods II and III ( Fig. 19; Table 1) dactylus with few fine setae; distal accessory claw length 0.30– 0.50 primary claw length. Pereopod II propodus and carpus each with 5 broad-based setae, evenly spaced along ventral margins, setae 0.08–0.36 respective propodus and carpus lengths. Pereopod III propodus with 4 broad-based setae on ventral margin, distal seta separated by gap from evenly spaced proximal setae, setae 0.15–0.28 propodus length, 3 shorter (0.06 propodus length) broad-based setae lateral to margin; carpus with 6 broad-based setae, evenly spaced along ventral margin with distal pair more closely set, setae 0.14–0.36 carpus length. Basis dorsal ridge in cross-section angular and produced but not forming distinct plate, with 8–12 elongate SS along margin. Pereopods II–IV ischium dorsal margin with 9 SS, including 2 or 3 RS. Pereopod III with single PNS on basis dorsal ridge.

Pereopod IV ( Fig. 19) with PNS on dorsal and ventral margins of basis. Dactylus longer than propodal palm; distal accessory claw length 0.25 primary claw length. Propodus length:pereopod length 0.15; length:width 1.59; 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 0.11; with 5 broad-based setae on ventral margin, 3 distinctly larger than others. Ischium posterodistal margin with 7–9 setae. Basis length:width 2.11; dorsal ridge with 15 or 16 setae.

Pereopods V–VII ( Fig. 19; Table 2) dactylus distal accessory claw ventral or ventrolateral to primary claw, length 0.20–0.42 primary claw length. Propodus distal margins with 6 or 7 RS, including 3 or 4 elongate RS. Pereopods V–VII ischium dorsal margin with 2–4 SS, including 2 or 3 RS. Basis dorsal ridge in cross-section angular on V, produced and forming distinct plate on VI and VII, with elongate fine setae along and lateral to margin; lateral face with ventral ridge.

Penes length 0.39 body width at pereonite 7; with setae on shaft; distal tip rounded.

Pleopods ( Fig. 20; Table 3). Endopod I–IV margins with PS and SS, V with SS only. Protopod mesial margins/epipods I–IV with 5, 4, 3 and 2 coupling hooks, respectively; II–V with 1, 5, 6 and 9 or 10 elongate inflexible SS, respectively. Protopod lateral epipod III length 1.93–2.13 width, lateral epipod V length 1.73– 1.81 width. Protopods lateral margin of pleopod I with 3 or 4 elongate inflexible SS; lateral epipods III–V with 25, 31 and 30 or 31 elongate inflexible SS on margins, respectively. Pleopod I exopod broadest proximally, mesial margin straight, dorsal surface with setae; protopod length subequal to that of other pleopods, width subequal length. Pleopod II endopod appendix masculina basal musculature pronounced; with 34 setae on margin; length 0.47 pleopod length; distal tip extending beyond distal margin of endopod.

Uropod ( Fig. 17) protopod length:width 3.18; length 0.40 uropod total length; dorsomesial ridge weakly produced, plate-like, ridge length:endopod length 0.50. Rami (in cross-section) flattened on dorsal surface only. Endopod dorsal margin with 10 RS along length and apically. Exopod length 0.82 endopod length; with 11 RS along dorsal margins and apically.

Female. Head. Cervical groove more curved than in male.

Pereon. Pereonite 1 length:width in dorsal view 0.35–0.38. Pereonite 2 length:width 0.51–0.54. Pereonite 4 length:width 0.43. Pereonite 5 length:width 0.33. Pereonite 6 length:width 0.35.

Pleonites 2 and 3 respective lengths in dorsal view less than half the length of pleonite 5, pleonite 4 equal to or more than half the length of pleonite 5.

Pleotelson ventral margin anterior to uropods with single row of 5 or 6 simple RS, interspersed with elongate, fine setae, posterior seta generally longer than anterior adjacent setae with shorter RS placed between 2 most posterior setae.

Antennula. Terminal article with 4 aesthetascs. Penultimate article length subequal to or less than length of other articles.

Antenna. Article 5 length subequal to or shorter than article 4.

Pereopod I length:body length 0.39. Dactylus ventrodistal margin with thin scale-like spines along 0.38 total length; claw length:dactylus length 0.15. Propodus length:width 1.25–1.27; dorsal margin with setae along length. Propodal palm concave; without low, cuticular projection/process of male; with 10 serrate and bifid denticulate stout setae, 5 elongate broad-based setae. Merus distodorsal margin includes 1–3 RS. Ischium dorsal margin without setae. Basis length:width 2.07; dorsal margin with 3 setae along ridge; ventrodistal margin with 4 elongate setae, 2 more elongate than others.

Pereopods II and III ( Table 1). Single PNS present on ventral margin of basis of pereopod II. Pereopod II propodus with 4 broad-based setae; carpus broad-based setae up to 0.41 carpus length. Pereopod III propodus with 3 broad-based setae, closely set pair near ventral margin midpoint, third between proximal margin and midpoint, setae 0.09–0.25 propodus length; carpus with 4 broad-based setae evenly spaced on ventral margin and 3 closely set distolaterally, setae up to 0.43 carpus length.

Pereopod IV approaching prehensile condition. PNS on dorsal margin of basis and anterodorsal margin of carpus. Dactylus distal accessory claw length 0.33 primary claw length. Propodus length:pereopod length 0.11; length:width 1.83. Propodus with 1 broad-based seta on ventral margin. Ischium posterodistal margin with 4 setae ventrolaterally. Basis length:width 2.37; dorsal ridge with 11 setae.

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

Pleopods ( Table 3). Protopod mesial margins/epipods I–IV with 2, 1, 1 and 1 coupling hooks, respectively; II–V with 3, 6, 6 and 9 elongate inflexible SS, respectively. Protopod lateral margin of pleopod I with 3 elongate inflexible SS; lateral epipods III–V with 21, 29 and 23 elongate inflexible SS on margins, respectively.

Uropod protopod length:width 3.54. Endopod with 11 RS; exopod with 10 RS.

Distribution. Known only from the type locality, below Guardian Peak, Jonkershoek Nature Reserve. The type locality is a small pool (approximately 0.5 m in diameter) at high altitude (1088 m) at the top of the catchment, where the valley is broad and not yet incised. Individuals were found in the acidic (pH 4.5), tannin-stained water, among matted plant roots and coarse gravel along the margins of the pool.

Remarks. Mesamphisopus setosus is characterized by being lightly pigmented, and by having an abundance of robust setae on the pereopods. The relative length of the uropod, being twice as long as the pleotelson, is diagnostic for this species and is the longest within Mesamphisopus .

Further features of the mouthparts, particularly the maxillula and maxilla, are also diagnostic for this species. Four accessory setae are found amongst the four pappose setae on the distal and distomesial margin of the mesial lobe of the maxillula of M. setosus . The remaining five species described herein possess only two accessory setae. Two accessory setae also occur in M. abbreviatus and M. depressus , and two or three (among four or five pappose setae) occur in M. capensis ( Barnard 1914; Nicholls 1943). The ventral basal setal row on the maxilla mesial lobe is represented by a double row of setae in M. setosus , as in M. depressus ( Nicholls 1943) . Only a single row is present in other species of the genus. More setae (24) are included in these ventral basal setal rows than in those of the other species. The proximal and distal setal rows of the mesial margin are also continuous, whereas they are separated by a gap in other species.

The setation of uropodal rami is generally variable and is regarded as an inconsistent character ( Nicholls 1943). Nontheless, this too appears to distinguish M. setosus : up to ten robust setae occur on the dorsal margins of both the endopod and exopod. This number of setae is approached only in M. albidus and M. paludosus , where a similar number of robust setae is found on the endopod only.

Mesamphisopus setosus shows its closest morphological affinity to M. albidus . Both species are lightly pigmented or unpigmented and are similarly setose along the pereon and pleotelson. Both have a weak median ridge posteriorly on the pleotelson (just anterior to the apex) and lack the dorsal sub-apical robust setal pair. They also have a well developed, low, stout cuticular projection distally along the propodal palm of pereopod I (a feature seen too in M. tsitsikamma ), and have a similar extension of the appendix masculina. Mesamphisopus setosus can, however, be distinguished from M. albidus by the larger eyes, the sparser setation of the head, the longer antenna, the heavier setation of the limbs, and the greater relative length of the posterior pereopod (V–VII) series.

Although the distal margin of the pleopod II endopod figured (Fig. 40B) was truncate and concave, the distal margin of the right pleopod II endopod was rounded, approaching the normal condition seen in the remaining endopods of both the male and female.