Mesamphisopus paludosus, Gouws, 2008

Gouws, Gavin, 2008, New species of Mesamphisopus, an endemic South African freshwater isopod genus (Isopoda: Phreatoicidea: Mesamphisopidae), Zootaxa 1690, pp. 1-62 : 36-43

Mesamphisopus paludosus n. sp.

Figures 13–16

Material examined. Syntypes: SAM A45157, ♂ (11.6 mm, dissected) and ♀ (brooding 12.0 mm, dissected) parts slide mounted and in microvials, additional 6 ♂, 6 ♀, temporary wetland along Elim–Struisbaai road, opposite “Crane’s Nest” guest farm, Agulhas Plain, South Africa (34°38.45’S 19°52.08’E), coll. S . R. Daniels and G. Gouws, 5 December 2001 .

Other material: SAM A45158, ♂, 2 ♀, collection locality as for syntypes, coll. G. Gouws and H. Endemann, 24 November 2001 . SAM A45159, temporary wetland to the east of vlei, in kraal of “ Ratel’s River ” farm, Agulhas Plain, South Africa (34°44.50’S 19°40.80’E) coll. S . R. Daniels and G. Gouws, 5 December 2001 . University of Cape Town , Freshwater Research Unit WCW B13, Rattelrivier, on Agulhas Plain, South Africa (34°44.47’S 19°40.70’E) (further collection details unavailable) .

Etymology. The epithet is the Latin adjective “paludosus”, meaning “marshy” or “swampy”, in reference to the temporary wetlands of the type locality.

Diagnosis. Mandibular groove with acute indentation. Pereonites with sparse, fine setae, setae length 0.05–0.06 body depth. Pleonites 2–4 respective lengths in dorsal view more than half the length of pleonite 5. Pleonites 1–4 width 0.95–0.98 composite length in dorsal view. Pleotelson dorsal surface in lateral view evenly curving; lateral length subequal (0.94–0.98) to depth; depth 1.64–1.69 pereonite 7 depth; ventral margin anterior to uropods with single row of 3–5 simple RS grading anteriorly to fine setae; posterior apex not reflexed, with 1 pair of RS, additional pair towards ventrolateral margin and subapical pair dorsally. Antennula long, length 0.23 body length, with 9 or 10 long slender articles. Antenna long, length 0.78 body length. Mandibular palp article 3 with 32–38 finely setulate setae. Maxillula lateral lobe distal margin with 3 smooth RS, 9 denticulate RS. Maxilliped palp article 5 length:width 1.85. Pereopod I propodal palm with 4 stout denticulate serrate setae, 3 basally-inflated stout RS, and 4 elongate broad-based setae. Pereopods II and III propodus length:width 2.36–2.60. Pereopod IV propodus slender (length:width 1.89 in male, 2.43 in female). Pereopods V–VII basis dorsal ridge in cross-section angular on V and VI, produced and forming distinct plate on VII. Only pleopod I and II endopod margins with setae in male and pleopod I in female. Pleopod I protopod longer (1.29) than wide. Pleopod II appendix masculina short, length 0.29–0.34 pleopod length; distal tip not reaching distal margin of endopod. Uropod endopod with 11 RS along length of dorsal margin and apically; exopod with 5 (male) or 6 (female) RS. Pleopods long; length:body length> 0.19 for pleopods I–III and> 0.18 for pleopods IV and V.

Descriptions. Male. Coloration. Body strongly pigmented, dark-brown to black-brown dorsally and laterally; off-white (living specimens) to yellowish-white (preserved specimens) where not pigmented; more heavily pigmented patches or spots formed laterally on pleonites, pigmentation forms mottlings on pereopods and dendritic patterns laterally on cephalon and on pereopods.

Head setae sparse, fine. Eyes projecting anteriorly; maximum diameter 0.15–0.18 head depth; approximately round. Antennal notch posterior extension reaches just below anterior portion of eye.

Pereon width exceeding head width. Pereonite 1 length:width in dorsal view 0.30–0.39. Pereonite 2 length:width 0.39–0.41. Pereonite 3 length:width 0.43–0.50. Pereonite 4 length:width 0.42–0.50. Pereonite 5 length:width 0.47. Pereonite 6 length:width 0.43. Pereonite 7 length:width 0.36.

Pleonites 1–4 relative lengths unequal, pleonite 4 length greater than or subequal to pleonites 1–3. Pleonites 1–5 dorsal length:maximum width of pleonites 1–5 respectively 0.25, 0.25, 0.25, 0.28 and 0.41. Pleonites 1–5 depth:pereonite 7 depth respectively 1.37, 1.99, 2.13, 2.16 and 1.91.

Pleotelson dorsal surface sparsely covered with fine setae; dorsal length 1.04–1.08 width.

Antennula ( Fig. 13) article 5 divisible into 1 large or 2 small articles. Four aesthetascs on terminal article; 3 aesthetascs and 1 SS peripherally on distal margin of subterminal article. Terminal article length:width 0.73–0.80. Penultimate article subequal in length to other articles. Distal articles circular in cross-section.

Antenna ( Fig. 13) flagellum length 0.69 total antenna length, with 28–36 articles, with abundant fine setae along distal margins. Article 5 longer than article 4.

Mouthfield. Clypeus not widening laterally; width 0.89 head width. Labrum ( Fig. 13) ventrally semicircular in anterior view, with fringe of fine setae along distal margin; dorsal margin wider than clypeus. Paragnaths ( Fig. 13) with distolaterally rounded lobes; with dense mats of fine setae distomesially along lobes, becoming shorter proximally; longer SS scattered apically and distolaterally.

Mandible ( Fig. 14) palp length 1.07 mandible length; article 1 with elongate SS at distoventral margin; article 2 with longitudinal row of elongate SS along ventral margin, separate row of 6 elongate SS along distolateral surface near dorsal margin; setae more than half respective article lengths. Left spine row with 13 spines, 3 of which are bifurcate. Right spine row with 11 spines, 2 of which are bifurcate. Molar process wider than long.

Maxillula ( Fig. 14) mesial lobe length 0.49 lateral lobe length; width 0.61 lateral lobe width; with 2 accessory setae (1 distally denticulate, 1 simple), 1 on distolateral margin and 1 at base of the mesial of two central PPS. Lateral lobe distal margin with 4 RS forming distal setal row; ventral face with 2 widely spaced PS.

Maxilla ( Fig. 14) mesial lobe width 0.71 outer lateral lobe width; proximal portion smoothly continuous with distal portion; proximal and distal setal rows separated by gap; 8 fairly robust serrate setae and PCS in single ventral basal row; 36 closely-set elongate PS in dorsal basal row; 18 SS and PS and 3 PCS in multiple distal rows. Outer lateral lobe length subequal to inner lateral lobe, wider than inner lateral lobe; distal margin with 18 long bidenticulate setae. Inner lateral lobe distal margin with 13 long bidenticulate setae.

Maxilliped ( Fig. 14) epipod length:width 1.14; distal margin setae fine and in fringe. Endite length:total basis length 0.41; mesial margin with 2 coupling hooks on left and right sides; dorsal ridge with 15 large distally denticulate PS. Palp insertion on basis mesial margin with 1 SS; palp width across articles 2 and 3:endite width 1.33–1.40; article 4 subcircular, length:width 0.96; article 5 length:article 4 length 0.92.

Pereopod I ( Fig. 15) length:body length 0.44. Dactylus length subequal to palm length or slightly longer, length:palm length 1.36; ventrodistal margin with thin scale-like spines along 0.08 total length; claw length:dactylus length 0.10; distal accessory claw length 0.27 primary claw length. Propodus length:width 1.23; dorsal margin with 10 or 11 setae distributed singularly between proximal and distal margin, including 5 in distal group. Propodal palm cuticular fringe well developed, continuous along proximal third of palm, intermittent towards distal portion. Ischium dorsal margin with 3–5 SS. Basis length:width 1.93; dorsal margin with 12 SS positioned along ridge; ventrodistal margin with 3 elongate setae.

Pereopods II and III ( Fig. 15; Table 1) with PNS scattered along dorsal margin of basis. Dactylus distal accessory claw length 0.35–0.50 primary claw length. Pereopod II propodus with 8 broad-based setae, evenly spaced along ventral margin, setae 0.10–0.18 propodus length; carpus with 10 or 11 broad-based setae, evenly spaced along and lateral to ventral margin, setae 0.09–0.34 carpus length. Pereopod III propodus with 6 broadbased setae, evenly spaced along ventral margin, setae 0.11–0.17 propodus length; carpus with 18 broadbased setae, evenly spaced along ventral margin and series of 4 distolateral to margin, setae 0.10–0.31 carpus length. Basis dorsal ridge in cross-section angular and produced without forming distinct plate, with 10–13 elongate SS along dorsal ridge, 1 or 2 more robust than others. Pereopods II–IV ischium dorsal margin with 11 or 12 SS, including 2 RS.

Pereopod IV ( Fig. 15) with PNS on dorsal margin of basis. Dactylus length subequal to palm; distal accessory claw length 0.25 primary claw length. Propodus length:pereopod length 0.12; distal width:palm width 0.82; with 4 broad-based setae on ventral margin; articular plate subequal in length to dactylar claw. Carpus length:pereopod length 0.14; ventral margin with 7 broad-based setae, 5 distinctly larger than others. Ischium posterodistal margin with 5 or 6 setae. Basis length:width 2.63; dorsal ridge with 9 setae.

Pereopods V–VII ( Fig. 15; Table 2) with PNS on dorsal ridge of basis. Dactylus distal accessory claw ventral to and separated from primary claw, length 0.33 primary claw length. Propodus distal margins with 3–5 elongate RS. Pereopods V–VII ischium dorsal margin with 5–19 SS, including 1–3 RS. Basis dorsal ridge with elongate fine setae along entire margin.

Penes length 0.44 body width at pereonite 7; with setae on shaft; distal tip rounded to truncate.

Pleopods ( Fig. 16; Table 3). Endopod margin with SS and PS on I, single PS only on II. Protopods mesial margins/epipods I–IV with 6, 3, 3 and 2 coupling hooks, respectively; II–V with 4, 7, 8 and 9 elongate inflexible SS, respectively. Protopods lateral epipod III length 2.04–2.17 width, lateral epipod V length 1.88–1.94 width. Protopod lateral margin of pleopod I with 4 fine elongate setae; lateral epipods III–V with 23, 22 and 22 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. Pleopod II endopod appendix masculina with 25 setae on margin.

Uropod ( Fig. 16) total length 1.82 pleotelson length. Protopod length:width 3.90; length 0.45 uropod total length; extending posteriorly subequal to pleotelson apex; dorsomesial ridge produced, plate-like, ridge length:endopod length 0.52. Rami (in cross-section) flattened on dorsal surface only. Exopod length 0.80 endopod length.

Female. Head. Cervical groove straighter than in male.

Pereon. Pereonite 2 length:width 0.44–0.52. Pereonite 5 (length:width 0.43), pereonite 6 (0.37) and pereonite 7 (0.29) relatively shorter and wider than in male.

Antennula with additional aesthetasc on subterminal article.

Pereopod I length:body length 0.38. Dactylus ventrodistal margin with thin scale-like spines along 0.17 total length; primary claw length:dactylus length 0.13; distal accessory claw length 0.41 primary claw length. Propodus length:width 1.31. Propodal palm straight; cuticular fringe less than half palm length, intermittent distally; with 10 serrate and bifid denticulate stout setae; single stout basally-inflated RS, 3 or 4 elongate broad-based setae. Basis with 7 or 8 dorsal setae positioned along or lateral to ridge; ventrodistal margin with 4 elongate setae, 2 shorter than others.

Pereopods II and III ( Table 1). Pereopod II propodus with 6 broad-based setae, setae 0.20–0.33 propodus length; carpus with 6 broad-based setae along ventral margin, setae up to 0.43 carpus length. Pereopod III propodus with 8 broad-based setae along ventral margin and one lateral, setae up to 0.22 propodus length; carpus with 8 broad-based setae, generally evenly spaced along ventral margin, second to fourth setae more closely set, setae up to 0.44 carpus length.

Pereopod IV dactylus distal accessory claw length 0.33 primary claw length. Ischium posterodistal margin with 6 or 7 setae.

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

Pleopods ( Table 3). Endopod I margin with single PS. Protopods mesial epipod IV with 4 coupling hooks; mesial epipods II–V with 5, 8, 8 and 7 elongate inflexible SS, respectively. Protopod lateral margin of pleopod I with fine elongate seta; lateral epipods III–V with 21, 26 and 26 elongate inflexible SS on margins, respectively.

Uropod protopod dorsomesial ridge length:endopod length 0.61.

Distribution. Known only from the above localities, possibly distributed across the temporary wetlands of the Agulhas Plain. The type locality is a fairly extensive wetland, now inundated pastural land, along the Elim-Struisbaai Road. The water was relatively deep (40 cm) and tannin-stained. The area was dominated by grass and Restio . Isopods were primarily taken directly beneath shrubs.

Remarks. Mesamphisopus paludosus is distinguished from all known species in the genus by the shape of the pleotelson. Mesamphisopus paludosus has a characteristic, elongate, shallow pleotelson, with a dorsal margin that curves smoothly or extends almost linearly, rather than being sharply inflected ventrally (see Barnard 1927; Nicholls 1943; Kensley 2001). The posterior apex of the pleotelson is thus not reflexed or upturned. The apex in M. abbreviatus has been reported as not being up-turned ( Nicholls 1943), but the sharp ventral inflection and downward slope of the dorsal margin (see Nicholls 1943: Fig. 10.3s; Kensley 2001: Fig. 3.8a) gives the pleotelson of this species a short, deep or stubby (rather than elongate and shallow) appearance ( Nicholls 1943).

The setation of the pleopodal endopods of M. paludosus is also unique within Mesamphisopus , although the pleopods of M. penicillatus have yet to be examined. In M. paludosus , setae are present only on pleopods I and II (plumose on both) in the male and pleopod I (also plumose) in the female. The occurrence of setae on all five pleopodal endopods has been regarded as a key diagnostic feature of the genus ( Nicholls 1943; but see Discussion). No further evidence, at this stage, suggests that M. paludosus be excluded from the genus. This rather suggests that the pattern of pleopodal setation may not be a valid generic character and argues for a reexamination of the characters believed to be of systematic importance within the genus.

Further distinctive features of M. paludosus include an antennula with ten articles, reaching one-quarter the body length, and an antenna reaching four-fifths the body length. These are the longest observed within Mesamphisopus . The appendix masculina is, relatively, the shortest within the genus, not extending to the distal margin of the endopod. In other species, the appendix maculina is described or figured as reaching the distal margin of the endopod ( M. abbreviatus , M. baccatus , M. depressus , M. tsitsikamma ) or extending beyond it ( M. albidus , M. capensis , M. kensleyi , M. setosus ). More broad-based setae are also encountered on the carpus and propodus of pereopods II–IV than in other species. These setae are, however, not as stout as those in other species, and are scarcely stouter than other setae on the limbs.

Mesamphisopus paludosus is most similar to M. penicillatus , both sharing the sparse, short, fine setation of the head, pereon and pleotelson, the abundant and diagnostic fine setation of the ventral margin of the antennal basal articles, and the presence of a pair of sub-apical dorsal robust setae on the pleotelson ( Barnard 1940; Kensley 2001). The difference in pleotelson shape should, however, enable the species to be distinguished relatively unambiguously.

A completely developed appendix masculina was observed arising from the mesial margin of the endopod of the right pleopod I in one dissected male: a developmental abnormality rather than a feature of the species.