Sousa, Francisco Diogo R., Elmoor-Loureiro, Lourdes M. A. & Panarelli, Eliana A., 2017, The amazing diversity of the genus Monospilus Sars, 1862 (Crustacea: Branchiopoda: Aloninae) in South America, Zootaxa 4242 (3), pp. 467-492: 480-485

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Monospilus  sp.

( Figs. 7–8View FIGURE 7View FIGURE 8)

Material examined. A single parthenogenetic female dissected ( CLLA 208-213) from Salto Grande Reservoir (3115’31.41”S 5755’33.66”W), Lower Uruguai River  , Uruguay. Material leg Danilo Naliato.

Diagnosis. Parthenogenetic female. Body spherical, brownish, dorsal margin strongly arched. Head with a large ocellus. Head Shield triangular, its posterior portion elongated, rostrum short, apex sinuous, in lateral view slightly pointed downward. Single major head pore with a protruded rim. Labrum with a labral keel having a single row of setulae, apex obtuse. Carapace with incomplete moulting, ventral margin rounded armed with 55–58 plumose setae located exactly at the valve margin, clearly not articulated. Antennule short, antennular sensory seta slender, nine aesthetascs of similar length and longer than antennular body. Antenna with branches/basal segment ratio about 2, segments armed with thin spinulae, apical spines naked. Abdomen as long as thorax. Postabdomen short, anal margin concave, with two distinct angles; postanal margin constricted and armed with three thick spines. Postabdominal claw slender, longer than preanal margin, with two pectens, proximalmost basal spine about half size of distalmost basal spine, distalmost basal spine about 1/3 of length of the postabdominal claw. First limb with seta 1 of endite 3 laterally armed with short denticles; endite 2 with seta (e) long. Second limb with a single seta on exopodite; anterior soft seta inserted near base of first scraper; scrapers 3, 6, 7–8 armed with thick denticles; gnathobasic brush armed with thin setulae. Third limb with exopodite bearing seven setae, first seta longer than half length of the second seta. Fourth limb with exopodite bearing six setae; distal endite with second flaming-torch seta relatively thick (3). Fifth limb with exopodite not divided into lobes; first seta with a thick base; gnathobase armed with one element and two setae on filter comb. Sixth limb present. Ephippial female, ephippium and male unknown.

Description. Adult parthenogenetic female. Habitus ( Fig. 7View FIGURE 7 A). Body spherical, up 0.32 mm, body height/ length ratio about 0.84, brownish, dorsal margin strongly arched, without a dorsal keel or a lateral projection.

Head ( Fig. 7View FIGURE 7 A, C). Compound eye absent, large ocellus. Head shield ( Fig. 7View FIGURE 7 D –E) triangular; posterior portion elongated. Rostrum short, apex sinuous; a single incision on antero-lateral margin; in lateral view slightly pointed downward. Single major head pore with a evident rim, apparently inserted in slight depression without ornamentation; PP/body length about 0.29; lateral head pores absent.

Labrum ( Fig. 7View FIGURE 7 F). Labral keel short, with a row of setulae; margin sinuous with two lobes; apex obtuse.

Maxilla not studied.

Carapace ( Fig. 7View FIGURE 7 A –B). Moulting incomplete, accumulating five valves; ornamentation as hexagons each with a projected edge; ventral margin rounded, armed with 55–58 plumose setae, longer at middle portion; setae are located exactly at the valve margin, clearly not articulated. Inner spinulae on posterior margin absent.

Antennule ( Fig. 7View FIGURE 7 G) short, length 1.6 times the width, not reaching tip of rostrum; two rows of setulae on antennular body of distinct size. Antennular sensory seta slender, about same length of antennular body, inserted at the second-quarter of antennule. Nine aesthetascs of similar length and longer than antennular body.

Antenna ( Fig. 7View FIGURE 7 H). Coxal setae not studied. Basal segment thick, armed with thin spinulae and a long and thick spine. Branches/basal segment ratio about 2. Spine on first endopodite segment long, exceeding middle-length of second endopodite segment, naked. Apical spines long, naked. Antennal formula: spines 001/101 (exo/endo); setae 013/003 (exo/endo). All segments with thin spinulae.

Abdomen ( Fig. 7View FIGURE 7 A) as long as thorax, armed with two rows of abdominal setae.

Postabdomen ( Fig. 7View FIGURE 7 I) short, height/length ratio about 0.38; ventral margin straight, with two rows of spinulae. Preanal margin longer than postanal margin and slightly shorter than anal margin. Anal margin large, concave, with two distinct angles; it has four groups of strong spines and seven lateral fascicles. Postanal margin short, constricted, relatively straight, armed with tree thick spines; groups of lateral spines and fascicles present. Postabdominal seta ( Fig. 7View FIGURE 7 I) not studied. Postabdominal claw ( Fig. 7View FIGURE 7 I) relatively slender, longer than preanal and anal margin; apex sharp and slightly curved; pecten arranged in two groups, proximal group armed with three thick spines increasing in size towards claw apex; distal group armed with thin spinulae. Distalmost basal spine relatively slender, about 1/3 the length of the postabdominal claw, and about two times as long as width of claw at its base; proximal spinulae modified in a spine relatively long, about middle-length of distalmost basal spine.

Six pairs of limbs.

First limb ( Fig. 8View FIGURE 8 A –C, N). Epipodite ovoid, with a long finger-like projection. ODL with a thin seta longer than IDL setae, serrulations undistinguished; accessory seta not studied. IDL (en 4) with two groups of strong setulae; three setae present; first seta short, naked; setae 2–3 bisegmented, long, subequal in length, armed with fine and short setulae. Endite 3 with four setae (1, a –c); seta (c) armed with thin setulae; seta 1 robust, laterally armed with short spines; a long element closer seta 1; posterior setae a –b setulated, shorter than the other two. Endite 2 with a row of thick spinulae; three setae present (d –f); seta (f) longer than seta (d), with thick spinulae and setulae on lateral face; seta (e) thick, long, armed with thick spinulae and setulae on lateral face; seta (d) relatively short, setulated from median portion. Endite 1 with three posterior setae (g –i); seta (i) half-length of seta (g); setae (g –h) of subequal length, bisegmented and slightly setulated. Ejector hooks, gnathobase and ventral face not totally studied.

Second limb ( Fig. 8View FIGURE 8 D). Exopodite subrectangular, with distal row of thin setulae. Seta on exopodite longer than exopodite itself. Inner limb portion armed with eight scrapers; anterior seta soft, inserted near base of the first scraper; scrapers 1–2 of similar length, armed with fine denticulation; scraper 3 of same length of scrapers 1–2, armed with thick denticles; scapers 7–6 of similar length and armed with strong denticles; scraper 8 short and armed with thick denticles. Proximal portion of gnathobase short and wide, gnathobasic brush extended, armed with thin setulae. Distal portion of gnathobase with sensillum and three setae; apex of first seta not studied; second seta with distal portion geniculated and armed with short denticles; third seta elongated, with sharp apex. Filter comb with seven setae, first seta short and densely setulated; other setae long.

Third limb ( Fig. 8View FIGURE 8 E –G, N). Epipodite oval, a finger-like projection relatively short. Exopodite quadrangular, with five distal and two lateral setae; seventh and sixth setae not totally studied; fourth seta shorter than first seta; third and fourth setae of similar length, shorter than half length of second seta; first seta longer than half length of the second seta; setae 3–5 plumose; second seta armed only in its distal portion, with short setulae on a side and thick spinulae on the other; first seta armed bilaterally with thick spinulae. Distal endite with three setae (1–3), two scraper-like setae with thin denticles (1–2), third seta slightly curved and armed with many setulae implanted bilaterally (3); four plumose posterior setae increasing in length toward gnathobase (a –d). Basal endite with four soft anterior setae (4–7); a large sensilum near to base of the fourth soft seta. Gnathobase armed with four elements; first element a cylindrical sensillum, second element a long geniculate setulated seta, third and fourth elements clearly separated, with naked acute tip. Filter comb with seven setae.

Fourth limb ( Fig. 8View FIGURE 8 H –J). Pre-epipodite not studied. Epipodite oval, with a long projection. Exopodite round, with six setae; setae 5–6 subequal in length, relatively short; fourth seta of similar length to second seta; third seta markedly short; second seta armed with short spinulae and lateral setulae; first seta longer than half length of second seta, bilaterally armed with thick and short spinulae. Basal endite with one scraper-like (1) and three flaming-torch setae decreasing in size towards the base (2–4); first flaming-torch seta longer and thicker than others (2), armed with long and thick setulae; second flaming-torch seta (3) half-length of first flaming-torch seta; one element observed. Basal endite with three soft setae (a –c), increasing in length towards the base. Gnathobase armed with a sensillum and setulated seta implanted on robust base. Filter comb with five slender setae.

Fifth limb ( Fig. 8View FIGURE 8 K –L). Pre-epipodite oval and densely setulated. Epipodite oval, with a long finger-like projection. Exopodite rectangular, not divided into lobes, about two times as long as wide; four plumose setae; setae 3-2 of similar length; first seta with thick base, longer than second seta. Internal lobe elongated, subrectangular and with setulae; two setulated setae of different length on inner face of the lobe, no elements between these setae. Gnathobase armed with one element and two setae on filter comb.

Sixth limb ( Fig. 8View FIGURE 8 M) wide and oval, armed laterally with long and fine setulae.

Ephippial female, ephippium and male unknown.

Differential diagnosis. Monospilus  sp. can be differentiated from other species of the genus by the following combination of characters: carapace without lateral projections, antennules not reaching the tip of rostrum, labral keel with an obtuse apex and armed with a single row of setulae; long seta (e) of the endite 2 of the first limb, and markedly robust first and second flaming torch setae on the fourth limb. Besides, Monospilus  sp. differs from M. daedalus  because the former exhibits an element on the endite 3 of the first limb, and the seventh scraper is armed with strong denticles. Regarding M. dispar  , Monospilus  sp. differs in the postabdomen armature, in the presence of an element on the endite 3 of the first limb and in the armature of the antenna (for comparison see Alonso, 1996 and Hudec, 2010). On the exopodite of the fifth limb, the morphology of the first seta differs from M. macroerosus  sp. nov. and M. brachyspinus  sp. nov. Other differences among Monospilus  species are shown in Table 1. Unfortunately, the limited material available did not allow the formal deposition of some type specimens at a museum collection. Nonetheless, Monospilus  sp. clearly is an undescribed species new for science.

Distribution and biology. Monospilus  sp. is known from the Salto Grande Reservoir only, Uruguay ( Fig. 9View FIGURE 9 A). Naliato (2012) found this specimen in a plankton sample. We believed that its occurrence within the zooplankton is accidental. The setae on the inner portion of the second limb point to a scraper habit. However, new efforts are necessary to improve the knowledge about the ecology and biology of Monospilus  sp., as well as to provide a full formal description of this species.

TABLE]. Main morphological đifferences between South American species of Monospilus  . Source: M. dispar  from Alonso (1996) anđ Huđec (2010), M. daedalus  from Kotov anđ Sinev (2011) Kotov et al. (2012). Abbreviations : see Material anđ Methođs. * structure not observeđ. 

M. dispar  M. daedalus  M. macroerosus  sp. nov. M. brachyspinus  sp. nov. Monospilus  sp. Length (mm) up 0.47 0.32+0.37 0.37+0.39 up 0.35 up 0.32 Carapace—lateral projection - + - - -

Rostrum—projection đownwarđ đownwarđ upwarđ đownwarđ đownwarđ Labral Keel—setulae one row * absent three rows one row —specializeđ setulae on first segment base absent absent long short absent —branches/ basal segment ratio about 2 about 2.5 about 1.5 about 2 about 2 —en3, seta (1) armature spines đecreasing đistally nakeđ short đenticles spines đecreasing đistally short đenticles —en3, seta (c) crown-shapeđ - - + - -

—en2, seta (e) crown-shapeđ - - + - -

—en2, seta (f), length similar similar longer than seta (e) shorter than seta (e) shorter than seta (e) —scarper 3 anđ 7 armature thick spinulae thick spinulae heavy đenticles thick spinulae thick đenticles —ex, setae 6+7, length 7> 6 similar 7 <6 7> 6 *

—ex, setae 1+2, length seta 1 up half-length of 2 seta 1 longer than seta 1 up half-length of 2 seta 1 longer than half- seta 1 longer than half-

half-length of 2 length of 2 length of 2 —ex, seta 1 length same length others shorter than others shorter than others same length others long than others —fc, setae * 2 3 3 2

PA—n° marginal đenticles 4+6 3+6 4+5 5 3

PA—Postabđominal claw apex not sharp sharp not sharp not sharp sharp

PA—pecten spines + thick spinulae spines + thin spinulae saw-shapeđ spines + thick spinulae spines + thin spinulae PA—proximalmost basal spine length about 0.35 of the about 0.37+0.4 of the longer than half of the about 0.2 of the about 0.6 of the

đistalmost spine đistalmost spine đistalmost spine đistalmost basal spine đistalmost spine