Partial revision of the aduncus‐like species of Pleuroxus Baird, 1843 (Chydoridae, Cladocera) from the southern hemisphere with comments on subgeneric differentiation within the genus

The aim of this study is to revise the aduncus‐like species of the genus Pleuroxus Baird, 1843 (Chydoridae, Cladocera) from the southern hemisphere and test the accuracy of subgeneric differentiation within this genus according to Frey (1993a). Pleuroxus wittsteini Studer, 1878 from the Kerguelen archipelago is re‐examined with special attention to thoracic limbs. Forgotten P. carolinae (Methuen, 1910) from the mountains of South Africa is redescribed in detail; it is a species having nine to ten setae in filter plate II, and six to seven setae in filter plate IV, which is a plesiomorphic, non‐oligomerized state. Brief descriptions are also provided for P. scopuliferus (Ekman, 1900) and P. varidentatus Frey, 1993. A new species, P. hardingi sp. nov., is reported from high altitudes of the Bolivian Andes; this species was previously misidentified as P. aduncus by Harding (1955). A key for identification of aduncus‐like species in the southern hemisphere (except Australia and New Zealand) is proposed. Frey's (1993a) system for differentiation of the subgenera of Pleuroxus is found to be vulnerable in view of the new data. New studies, most probably using genetic methods, are needed for the construction of a consistent phylogenetic tree for the genus.


Introduction
Investigations of collections from the southern hemisphere revealed the presence of distinct species of the genus Pleuroxus Baird, 1843 characteristic of this part of the world (Frey 1991(Frey , 1993b. It was shown that, in contrast to all species of Pleuroxus from the northern hemisphere, some southern hemisphere species possess more numerous setae on the thoracic limbs, a plesiomorphic feature. In particular, all well-studied northern species are characterized by the gnathobasic filter plates of limbs II-III-IV-V consisting of 8-8-6-4 Type locality ''Den zahlreichen Teichen in der Nähe der Station an Betsy Cove an der NW.-Kü ste'', Island of Grande Terre, Kerguelen archipelago.

Short diagnosis
Body dark brown, not transparent, no medial keel on dorsum, postero-ventral angle without denticles. Rostrum does not reach apex of labral keel. Head shield posteriorly widely rounded, anteriorly produced as a blunt rostrum, PP51.4-1.8 ID. Head shield and valves with well-expressed reticulation, prominent under valve surface. All setae of valve ventral margin exactly marginal. Setules on posterior margin of valve exactly marginal. Postabdomen short and wide, strongly narrowing distally, with a widely rounded dorsodistal end, its anal margin clearly longer than preanal margin or postanal margin, anal teeth represented as series of small setules. Antenna I not reaching tip of rostrum, with a strong basal peg. On antenna II, one apical seta on both exopod and endopod shorter than other two setae. On limb I smallest ODL seta well-defined, distally with minute setules, two largest IDL setae subequal in size. On inner-distal portion of limb II scrapers 3-5 of subequal size, scrapers 6-7 of subequal size, setules on gnathobase plumose, filter plate II with eight setae. On exopodite III setae 1 and 3 subequal in length, filter plate III with eight setae. On inner portion of limb IV setae 2-4 with robust setules, filter plate IV with six setae. Exopodite V with two projections distally to distal seta 1.

Description of thoracic limbs
Limb I (Figures 14, 15). Accessory seta, or corm seta in Frey (1993b), present (not represented in Figure 14, because located on other side of limb corm), ODL relatively small, bears a long seta with naked distal segment and a short seta with short, setulated distal segment. IDL larger than ODL, series of setules; first IDL seta short, naked, second and third IDL setae subequal in size and similarly armed distally with short, fine setules ( Figure 15). Endite 3 with three soft posterior setae (a-c) and stiff anterior seta 1 of similar length. Endite 2 with short seta anterior seta d, long setae e and f, and delicate posterior seta 2 armed with minute setules distally. Endite 1 with long, slender posterior setae g-i, a very short seta j, and anterior seta 3 similar to seta 2. Fascicles of thin setules on inner face of limb, plus bunches of longer thicker setules at ventral margin of limb. Two slender ejector hooks of remarkably different size. A short seta, a remnant of maxillar process, on limb base.
Limb II (Figures 16, 17). Exopodite subquadrangular, without a short seta. Inner portion of limb with eight scrapers, 1-2 specially long, 3-5 shorter, subequal in size, 3-8 shorter, also subequal in size. A series of small projections posteriorly to distal setae, and a small sensillum near scraper 4. Distal armature of gnathobase with a bunch of plumose setules, unique for the anomopods, and four setae (1-4). Filter plate II with eight setae, two distal members subequal in size and shorter than the rest; basalmost seta of filter plate with inflated basal segment.
Limb V (Figures 22, 23). Exopodite large, subovoid, with a single distal seta 1 and three lateral setae (2-4), distally to seta 1 there are two projections bearing long setules. Inner limb portion as elongated, flat lobe, with setulated inner margin, supplied with setulated setae 1 and 2, the latter bears specially robust setules. Distal armature of gnathobase as a single projection, filter plate V with four long setae.

Comments
The specimens investigated in the present case fully coincide with figures and description given for P. wittsteini from Kerguelen by Frey (1993b). In the key composed by Frey (1993b) there is a misleading formulation for P. wittsteini: its preanal margin is longer than the anal and postanal margins, which obviously contradicts his drawings (where the anal margin is the longest). In the same key couplet, there is a correct indication for brown P. paraplesius Frey, 1993 that ''both anal and postanal sections much longer than preanal''. So there is no clear difference in this character (except the fact that in P. wittsteini the anal margin is very wide). In P. paraplesius the postero-ventral angle of the valve is toothed, the shell is not pigmented brown. Our contribution to the study of P. wittsteini is the detailed redescription of its thoracic limbs.

Distribution
Kerguelen archipelago, Heard, Marion, Prince Edward Islands. Pugh et al. (2002) erroneously listed P. scopuliferus among junior synonyms of P. wittsteini and mistakenly referred localities of the former as belonging to the latter.

Neotype locality
Small rockpools (28u409430S, 28u509280E) at 2050 m a.s.l. near Fika Patso, Qwaqwa, Freestate, Republic of South Africa. The sample was collected on 8 November 1996 by K. Martens, and marked as RSA/96/111. The former type locality was ''Lake Chrissie, the lake lies in the Carolina district due east of Pretoria near the borders of Swaziland'' (Methuen 1910).

Material examined
Neotype: a parthenogenetic female in 96% alcohol, MGU Ml 56. Author's type material is apparently lost.

Short diagnosis
Body dark brown, not transparent, in anterior view dorsum as a smoothed angle, but true medial keel absent, postero-ventral angle without denticles. Rostrum does not reach apex of labral keel. Head shield posteriorly widely rounded, anteriorly makes a blunt rostrum, PP51.5-2 IP. Obscure striation expressed only in antero-ventral and postero-ventral portions of valve. All setae of valve ventral margin exactly marginal. Setules on posterior margin of valve situated on inner side of valve, relatively far from the margin, so their tips do not reach it. Postabdomen relatively long, slightly narrowing distally, with rounded and slightly prominent dorso-distal angle, its anal margin as long as preanal margin and clearly shorter than postanal margin, anal teeth thin and rather long, organized in successive series of two to three, rarely solitary, slightly increasing in size distally. Anntenna I not reaching tip of rostrum, with a strong basal peg. On antenna II, all apical setae subequal in size. On limb I smallest ODL seta rudimentary, two largest IDL setae unequal in size. On innerdistal portion of limb II, scrapers 1-3 with size decreasing basally, scraper 8 smaller than others; setules on gnathobase naked, filter plate II with nine, rarely 10 setae. On exopodite III seta 1 shorter than 3, filter plate III with eight setae. On inner portion of limb IV setae 2-4 with robust setules, filter plate IV with six to seven setae. Exopodite V with two projections distally to distal seta 1.

Redescription
Parthenogenetic female. Body brown, not transparent. In lateral view body widely oval, high (body height/body length50.74-0.80 in adults), maximum height in middle (Figures 24,39,40). Dorsal margin evenly arched from tip of rostrum to postero-dorsal angle, which is well-defined, posterior margin straight, postero-ventral angle broadly rounded, without teeth, ventral margin with a slight prominence in middle. Obscure striation expressed only Head with long rostrum, protruding downward and posteriorly (Figures 25,(41)(42)(43). Eye only slightly smaller than ocellus, distance from tip of rostrum to ocellus greater than that between ocellus and eye. Head shield elongated, with maximum width immediately behind mandibular articulation, its posteriormost extremity widely rounded ( Figure 26). Two major head pores, PP51.5-2 IP. Lateral head pores minute, normally located asymmetrically to midline. Labrum with fleshy main body, small distal labral plate  (anterior plug in terms of Dumont and Silva-Briano 2000), and large medial labral keel, with well-defined apex projected significantly behind rostrum.
Valves large, ventral margin armed with numerous setae of different size in different regions, all plumose and located exactly marginally (Figures 27-29). A row of small setules situated on inner side of posterior valve margin, relatively far from the margin, so their tips do not reach it ( Figure 29).
Postabdomen elongated, wide, its ventral margin almost straight (Figures 30, 45). Preanal margin slightly concave (Figure 44), as long as anal margin, preanal and postanal angle well-defined, postanal margin clearly longer than anal margin, dorso-distal angle widely rounded and slightly prominent distally, inflated basis of claws bordered from postanal margin by a distinct depression (Figures 31, 32, 46). Each side of postanal portion provided with a row of thin and rather long postanal teeth, organized in successive series of two to three, rarely solitary, slightly increasing in size distally. Series of postanal denticles evenly grading into five to six series of marginal setules on anal margin. Laterally to marginal denticles, a row of fascicles consisting of short, fine setules. Postabdominal seta relatively short, with basal segment approximately as long as preanal margin and distal segment as long as basal one, supplied with delicate setules. Postabdominal claw short (significantly shorter than preanal or anal margin), massive, evenly curved, with setules along ventral margin, and two basal spines, proximal one being half size of distal (Figures 46,47), sometimes basalmost spine duplicated ( Figure 48).
Limbs very similar to those in P. wittsteini, but limb I with ODL smaller seta rudimentary, IDL second and third setae unequal in size (Figure 35), filter plate II ( Figure 36) with nine, and rarely 10, setae; scrapers 1-3 on limb II with size decreasing basally, scraper 8 smaller than others; seta 1 on exopodite III shorter than seta 3 ( Figure 37); filter plate IV with six to seven setae; seta 1 on exopodite V especially long, longer than seta 2 on inner limb portion and armed with rare setules (Figure 38).

Size
Parthenogenetic females 0.42-0.47 mm in available material.

Distribution
Known from four high-altitude localities, our two localities plus those of Methuen (1910) and Sars (1916), on the Sani Plateau and the Cape Flats.

Ecology
Our specimens were collected from pools, at pH 7.4-7.9, conductivity 142 mS cm 21 , 14.8-19.6uC, altitude 2050-2890 m a.s.l. The species occurs also in Lake Chrissie, a large and shallow high-altitude lake.

Comments
The taxon was reported by Sars (1916) as P. inermis from a pond in the Cape Flats; he said that his ''P. inermis'' was ''without any obvious denticles'' at postero-ventral angle, had reticulation only at anterior and posterior margins, and strong postanal spines. Then, Methuen (1910) established Chydorus carolinae as a new species. His too small and inadequate description was accompanied with quite realistic illustrations (Methuen 1910, Plate 16: Figure 44a, b). Keeping in mind that the type material was apparently lost, and that aduncus-like species of Pleuroxus from the southern hemisphere posed a difficult taxonomic problem, we selected the neotype of this species ''to define the nominal taxon objectively'' (International Commission on Zoological Nomenclature 2000, Article 75).

Material examined
A single female, the lectotype. The diagnosis below is based predominantly on Frey's (1993b) redescription.

Diagnosis
Body dark brown, not transparent, dorsum with medial keel, postero-ventral angle with one to two denticles. Rostrum long (Figure 49), projecting behind apex of labral keel. Head shield pointed at posterior end, anteriorly makes a long rostrum, PP53.7-4.8 IP. Wellexpressed prominent striation on head shield and valve. Setae at posterior portion of valve ventral margin situated submarginally. Setules on posterior margin of valve situated exactly marginally. Postabdomen wide, significantly narrowing distally, with rounded and slightly prominent dorso-distal angle, its anal margin roughly as long as postanal margin and longer than preanal margin, anal teeth strong, solitary at dorso-distal angle, represented as series of two to four strong denticles in distal half of postanal margin, and then as series of fine setules in its basal portion. Antenna I not reaching tip of rostrum, with short, rounded basal peg. On antenna II, one apical seta on each branch shorter than two other setae. On limb I smallest ODL seta short, two largest IDL setae subequal in size. On inner-distal portion of limb II, scrapers 1-3 with size decreasing basally; setules on gnathobase naked, filter plate II with eight setae. On exopodite III setae 1 and 3 subequal in length, filter plate III with nine setae. Filter plate IV with six setae. Exopodite V with two projections distally to distal seta 1, distalmost projection especially large. Frey (1993b).

Distribution
Southern portions of Chile and Argentina, see Frey (1993b

Short diagnosis
Body (Figure 50) relatively light and transparent, dorsum without a medial keel, posteroventral angle with one to three, rarely four denticles. Rostrum distinctly shorter than labral keel ( Figure 51). Head shield with rounded posterior end, anteriorly makes a blunt rostrum, PP53.0-3.6 IP. Striation on head shield and valve ill-defined. Setae at valve ventral margin situated exactly marginally. Setules on posterior margin of valve situated on inner face of valve submarginally. Postabdomen (Figure 52) long, slightly narrowing distally, with rounded and slightly prominent dorso-distal angle, its anal, preanal, and postanal portions roughly equal in length, anal teeth strong, solitary or in series of two to three in distal half of postanal margin, and as series of fine setules in its basal portion. Anntenna I with a welldefined basal peg. On antenna II, one apical seta on each branch shorter than other two setae. On limb I smallest ODL seta short, two largest IDL setae greatly unequal in size. On innerdistal portion of limb II, scrapers 1-3 with size decreasing basally; setules on gnathobase delicate, naked, filter plate II with eight setae. On exopodite III setae 1 and 3 unequal in length, filter plate III with eight setae. Filter plate IV with six setae. Exopodite V with a single projection distally to distal seta 1 (according to Frey 1993b, Figure 164). Frey (1993b).

Distribution
Our new findings change ideas on the distributional range of this species in South America, now, in addition to Frey's (1993b) single locality in the Province of Santa Cruz, it is found in Jujuy Province, in the northern part of the Argentinian Andes.

Type locality
Pools of the bofedal system in the cordillera del Tunari (part of the Cordillera Oriental) near the city of Cochabamba, Cercado Province, Bolivia (4000-4400 m a.s.l., 17u109560S-17u179190S, 66u079620-66u229990W). The type series was collected in February 2004 by J. S. Coronel, S. Declerck, and G. Crespo.

Short diagnosis
Body brown, not transparent, dorsum without a medial keel, postero-ventral angle without denticles. Rostrum not reaching apex of labral keel. Head shield with triangular-rounded posterior end, anteriorly makes a blunt rostrum, PP52.5-3 IP. Striation on head shield and valve ill-defined. Setae at posterior half of valve ventral margin situated submarginally. Setules on posterior margin of valve situated on inner face of valve submarginally, but projecting behind the valve edge. Postabdomen long, slightly narrowing distally, with rounded and slightly prominent dorso-distal angle, its anal and preanal portion roughly equal in length, shorter than postanal portion, anal teeth represented in adults as series of setules. Antenna I with a rudimentary basal peg, sensory seta arises from a low prominence on antennular body. On antenna II, one apical seta on each branch shorter than two other setae. On limb I smallest ODL seta thin and armed with long setules, two largest IDL setae subequal in size. On limb II, scrapers 1-3 with size decreasing basally; setules on gnathobase naked, filter plate II with eight setae. On exopodite III setae 1 and 3 unequal in length, filter plate III with eight setae. Filter plate IV with six setae. Limb V with seta 1 of exopodite short and three projections distal to it.

Description
Parthenogenetic female. Body brown, not transparent. In lateral view body widely oval, high (body height/body length50.72-0.76 in adults), maximum height somewhat anteriorly to middle (Figures 53, 56). Dorsal margin evenly arched from tip of rostrum to postero-dorsal angle, which is well-defined, posterior margin almost straight, postero-ventral angle welldefined, without teeth, ventral margin prominent in anterior half. Obscure striation expressed only in antero-ventral and postero-ventral portions of valve, while entire valve punctuate, these dots smaller and sparsely located as in P. carolinae (Figure 56). In anterior view body subovoid, without traces of a keel. Head with long rostrum, protruding downward and slightly posteriorly ( Figure 57). Eye markedly smaller than ocellus, distance from tip of rostrum to ocellus greater than that between ocellus and eye. Head shield elongated, with maximum width anteriorly to mandibular articulation, its posteriormost extremity triangular-rounded ( Figure 58). Two major head pores, PP52.5-3 IP. Lateral head pores minute. Medial labral keel large, with well-defined apex projecting significantly behind rostrum.
Valves large, with ventral margin armed with numerous setae of different size in its different regions, located exactly marginally in anterior half of the margin (Figure 59), and slightly submarginally in its posterior half (Figures 60-62). A row of fine, relatively long setules situated on inner side of posterior valve margin, close to the margin, their tips projected behind it (Figures 55,(61)(62).
Postabdomen elongated, distinctly narrowing distally, its ventral margin slightly convex ( Figure 63). Preanal margin slightly concave, as long as anal margin or somewhat shorter than the latter, preanal and postanal angles well-defined, postanal margin clearly longer than anal margin, dorso-distal angle distinct, slightly prominent distally, inflated basis of claws bordered from postanal margin by a shallow depression (Figures 64, 65). Each side of postanal portion provided with successive series of marginal setules, size of setules increasing distally in each series. Series of setules continues in anal margin, but the size of setules in each series subequal. Laterally to marginal setules, a row of fascicles consisting of short, fine setules. Postabdominal seta relatively short ( Figure 56). Postabdominal claw as long as anal margin, massive, slightly curved, with setules along concave margin, and two basal spines, proximal one being less than half length of distal (Figures 54,64,65). In juveniles postabdomen ( Figure 66) with more pronounced angles, with shorter postanal portion and thin, sole postanal teeth instead of groups of setules in adults.
Antenna I not reaching tip of rostrum (Figure 57), slightly narrowing distally, with a rudimentary basal peg (Figures 67, 68). Antennular sensory seta slender, longer than half the antennule, arising at one-third of antennular length from distal end on a small projection of the antennular body. Nine short aesthetascs of slightly differing size. Antenna II relatively short (Figure 56), similar to than of P. carolinae, but among endopod apical swimming setae, one seta shorter than two others, and chitinous insertions present within distal segments of all setae (Figures 69, 70).
Limbs very similar to those in P. wittsteini, but smaller seta on ODL of limb I thin and armed with long setules, IDL first seta especially small and thin ( Figure 71); limb III with especially thick, bottle-shaped sensillae near setae 2 and 3 ( Figure 72); limb IV with thin setules on setae 2-4 and rare setules on basal segment of seta 2 of gnathobase distal armature (Figures 73, 74); limb V with seta 1 of exopodite short and three projections distal to it (Figure 75), instead of two projections in other species.

Size
Parthenogenetic females 0.33-0.55 mm in available material.

Etymology
The species is named after J. P. Harding who recorded it from South America (as P. aduncus).

Distribution
It is known from the type locality (pools in the cordillera del Tunari), and from Lagunillas Pond (near Lake Titicaca), both from high altitudes in Bolivian Andes.

Ecology
The pools where the type series was collected are small, shallow with a layer of organic matter about 20-25 cm thick. About 30% of the pool surface was covered with aquatic plants and the rest was uncovered except for some algal mats. The physical parameters from the type locality at the time of collection were as follows: depth 18 cm, temperature 22.0uC, dissolved oxygen 8.4 mg l 21 , pH 6.87. Other information in Coronel et al. (2005).

Comments
This taxon was first found by Harding (1955) and determined as P. aduncus (see Discussion). Then Uéno (1967) described specimens of brown colour, lacking denticles at the postero-ventral angle of the valve and with a peg on antenna I, apparently belonging to the same taxon. At the same time, quite typical P. aduncus was found in the Lake Titicaca region (Harding 1955, Figures 87-90;Smirnov 1996, Figures 179-185).
As our key below does not deal with Australian and New Zealand species, i.e. those described by Frey (1991), it is necessary to report their differences from a new species for accurate confirmation of its independent status. Pleuroxus harding sp. nov. differs from P. helvenacus Frey, 1991 in having postanal teeth on the postabdomen represented as a series of small setules, and having no tubercles on the valves; from P. hastirostris Sars, 1903, P. foveatus Frey, 1991, and P. inermis Sars, 1896 in having no teeth on the postero-ventral angle of the valve (the latter pair also bear strong postanal teeth). In addition, antenna I in all Australian forms are supplied with a peg.

Discussion
Mainly with reference to the material from the northern hemisphere it was supposed for a long time (Richard 1897;Jenkin 1934;Harding 1955) that Pleuroxus aduncus is a highly variable taxon, and that there is a group of closely allied populations of this species. While partly this is probably so, investigations of a wider range of material, based on a wider assortment of characters, demonstrated that there are numerous, in part already -described species of this group. Harding (1955) was probably the last to use the concept of Pleuroxus aduncus sensu lato, as he did not separate the new species from vicinities of Lake Titicaca into a new taxon. In his Figures 83-92 he combined all aduncus-like forms, both with a basal peg on antenna I and without it, into P. aduncus sensu lato. More characters made separations of such different forms necessary. His specimen without a basal peg of antenna I from Lagunillas pond seems to be identical to our material. Harding found specimens without a peg at the base of antenna I also in four other water bodies. In contrast, some other animals represented in his illustrations apparently belong to other aduncus-like species occurring in this region, see also illustrations of Rey (1993) and Smirnov (1996). Our examination of a relatively limited material from the southern hemisphere resulted in the discovery of a new species, redescription of forgotten P. carolinae, as well as new information on the distribution of P. varidentatus. It is possible that the fauna of this group is more diverse in the southern hemisphere. Particularly important regions for the discovery of new species are the highlands of South America and Africa, where, for example, several endemic ilyocryptids were found recently (Kotov and Š tifter 2006).
Remarkably, our study of P. carolinae and P. hardingi sp. nov. revealed earlier unknown combinations of taxonomically important characters. As a result of this observation we