Macrothrix lourdesae, Sousa & Freitas & Perbiche-Neves & Bertini, 2021

Sousa, Francisco Diogo R., Freitas, Daniele Fátima Bastos De, Perbiche-Neves, Gilmar & Bertini, Giovana, 2021, A new species of Macrothrix (Crustacea, Branchiopoda, Macrothricidae) from the Neotropics with description of the marthae-group, Zootaxa 4926 (1), pp. 93-104 : 96-102

Macrothrix lourdesae sp. nov.

( Figs. 2–5 )

Etymology. Named in honor of Dr. Lourdes Maria Abdu Elmoor-Loureiro, for her important contribution to ecology, biogeography and taxonomy of Cladocera in Brazil and South America.

Type locality. Valo Grande Channel , Ribeira de Iguape River Basin, Iguape, S „o Paulo State, Brazil (24°42’07”S, 47°33’58”W) .

Holotype. Undissected adult parthenogenetic female in a tube with 92% ethanol deposited at the Museum of Zoology of the University of S„o Paulo under access number MZUSP41420 .

Paratypes. Thirteen adult parthenogenetic females from the Valo Grande Channel , Ribeira de Iguape River Basin , S „o Paulo , Brazil (24°42’07”S, 47°33’58”W), material collected from 07/2014–05/2015 by Daniele Fátima Bastos de Freitas and Giovana Bertini ( FDRS0694 ) .

Diagnosis. Female. Body ovoid in lateral view. Carapace with dorsal and ventral margins serrated; ventral margin with two rows of spines or denticles separated by a thick edge; several serrated leaf-like setae articulated to ventral margin. Ventral margin of head with a large and rounded protuberance near antennule base, concave before labrum. Labrum triangular-shaped, separated from head by a slight incision. Antennule rod-like, about 7 times longer than wide; near to articulation with rostrum there is a slightly dilated part; apex not dilated. Antenna II typical for the genus with antennal formula: spines 0101/001, setae 0013/113. Five pairs of thoracic limbs. Inner Distal Lobe armed with three apical pectinate and strongly chitinized setae; one ejector hook present on the corm of first limb. Second limb armed with eight scrapers of which six bear robust spines or denticles. Exopodite of third limb elongated and armed with two distal and one lateral setae; seta 1 of distal endite longer and heavily pectinate. Fourth limb without exopodite; inner surface of endites without setae. Fifth limb with epipodite exceptionally large; exopodite with two setae, one of them wide, with pectinate apex. Postabdomen not bilobed and not laterally compressed; ventral and postanal margins markedly short. Postabdominal claws short and curved, about 0.3 of anal margin length.

Description. Adult parthenogenetic female. General habitus ( Figs. 2A , 5D ). Body ovoid in lateral view; postero-dorsal angle as a short triangular-shaped prominence; dorsal margin regularly arched from the tip of rostrum to posterior-most point interrupted by head pore; posterior part of ventral margin with a wide rounded prominence; in dorsal and ventral view body laterally compressed, without lateral projection.

Carapace ( Figs. 2 A–G, 5D–F) with well-visible sculpture; dorsal and ventral margins serrated; most of ventral margin supplied by of two rows of spines or denticles separated by a thick edge (more evident towards posterior part of carapace); several serrated leaf-like setae articulated on the base of spines of internal row, anterior and median longer than posterior setae; anterior-most and posterior-most part of ventral margin smooth.

Head ( Figs. 2 H–J, 5E). Dorsal margin with slight supraoccular dome above compound eye; ventral margin with a large and rounded protuberance near antennule base, concave before labrum; ocellus markedly smaller than compound eye; dorsal head pore large, rounded, with a fine ring, positioned near to posterior part of head shield.

Rostrum ( Fig. 2I ). With moderate lateral compression.

Labrum ( Fig. 2J ). Triangular-shaped, separated from marginal line of head by a slight incision; ventral margin naked, anterior part prominent in lateral view; distal part setulated.

Maxilla ( Fig. 2N ). Well developed, with three setulated setae.

Antennule ( Fig. 2 L–M). Rod-like, about 7 times longer than wide; near to articulation with rostrum there is a part slightly dilated, apex not dilated; ventrolateral basal sensory seta slender, about 0.25 of antennular body length; antennular body armed with up to six transverse rows of spinules and up to three groups of 2–3 spines, distal part with seven spines surrounding insertion of aesthetascs; nine aesthetascs, of which one of them is markedly longer than the rest, each aesthetasc has a forked apex.

Antenna II ( Fig. 3 A–H). Coxal region folded with sensory seta similar in length to first endopodite segment; basal segment robust, covered by transverse rows of fine spines and spinules, with an apical spine and an apical bisegmented sensory seta; apical spine of similar length to spine of second segment of exopodite, sensory setae not reaching the distal part of second segment of exopodite ( Fig. 3B ). Endopodite and exopodite subequal in size with cylindrical segments covered by rows of spinules ( Fig. 3A ). Exopodite with four segments; first segment of exopodite shorter than rest, armed with robust spines laterally ( Fig. 3B ); second segment of exopodite elongated, spine about middle length than the segment itself ( Fig. 3B ); third segment of exopodite elongated armed with a bisegmented seta, proximal part with lateral short spines and one relatively long spine positioned apically, plumose distal part ( Fig. 3H ); fourth segment of exopodite armed with a spine and three apical bisegmented setae (two of them not represented), spine about 0.25 the length of the segment itself, two setae armed with lateral spines on the proximal part and plumose in distal part ( Fig. 3H ), one seta plumose ( Fig. 3I ). Endopodite with three segments; first segment of endopodite with a very long sclerotized and bisegmented seta which bears, on the proximal part, thin lateral spines, distal part armed with 9-10 robust spines reducing in size distally, between distalmost spines there are several spinules ( Fig. 3 E–F); second segment of endopodite armed with a long (shorter than seta on the first segment) and bisegmented seta which bears, on the proximal part, short spines and one relatively long spine positioned apically, distal part with short and fine spines ( Fig. 3G ); third segment of endopodite with a spine and three apical bisegmented setae, spine about 0.3 of length than the segment itself, one seta armed with lateral spines on the proximal part and plumose in distal part ( Fig. 3H ), two setae plumose ( Fig. 3I ). Antennal formula: spines 0101/001, setae 0013/113.

Thoracic limbs ( Figs. 4 A–K, 5G): five pairs.

Limb I ( Fig. 4 A–C). Epipodite quadrangular, with a long finger-like projection. ODL with a short accessory seta (as) and a bisegmented seta armed with thin spines, of which the proximal-most are longer than the distal one. IDL (en4) with three rows of chitinized setae on the posterior surface, distal row bears five setae markedly longer than others. Three apical pectinate and strongly chitinized setae present namely: third seta slightly longer than second seta, pectinate part inserted from the mid-length of its own seta; second seta markedly longer than first seta, pectinate part inserted in the last third of setae length; first seta with pectinate part inserted from the mid-length of its own seta. Endite 3 armed with an anterior seta (1) and three posterior setae (a–c); anterior seta (1) markedly short and plumose; posterior seta (a) plumose, bisegmented and geniculated, about 2.8 times longer than seta (c), ratio width base/length about 0.19; posterior seta (b) plumose, about 1.6 times longer than seta (c), ratio width base/length about 0.27; posterior seta (c) short and plumose with a wide base, ratio width base/length about 0.5. Endite 2 with three posterior setae (d–f) of similar length, bisegmented, armed with spinulae inserted unilaterally, on the inner face of endite there is an anterior fork-like seta (2) and two short elements. Endite 1 with two posterior setae (g–h) of similar length, armed with short setulae; on the inner face of endite there is an anterior fork-like seta (3). A single ejector hook (ejh) armed with short spinulae. Gnathobase with two setae.

Limb II ( Fig. 4 D–F). Epipodite rectangular, with a long finger-like projection. Exopodite naked, armed with a seta longer than exopodite’s own length, plumose in distal part. Inner limb portion with eight robust and bisegmented scrapers; base of scraper 1 bears a row of long setulae; scrapers 1–2 longer than other scrapers; scraper 1 slightly shorter than scraper 2, armed with chitinized setulae, of which proximal ones are longer; scraper 2 with similar morphology to scraper 1; scrapers 3–5 of similar length; scrapers 3–4 armed with short and wide denticles; scraper 5 armed with short spines; scraper 6 markedly shorter than scrapers 1–5 and 7–8; scrapers 6–8 armed with relatively long denticles; near to scraper 8 base there is a setulated soft seta (ss) slightly shorter than scraper 8 length. Gnathobase with proximal portion setulated, distal portion armed with four elements (1’– 4’); filter comb with four setulated setae.

Limb III ( Figs. 4 G–H, 5G). Exopodite rectangular, about 1.5 times higher than wide, with two distal setae and a single lateral seta; first seta about 0.53 of second seta length; third seta slightly plumose, about 0.15 of second seta length. Distal endite with three anterior setae (1–3); seta 1 robust, markedly longer and heavily pectinate; seta 2 about 0.65 of seta 1 length, armed with thin denticles; seta 3 about 0.85 of seta 2 length, armed with thin spines. Basal endite with an element and three setae (4–6). Six long and plumose posterior setae decreasing in length towards gnathobase (a–f). Gnathobase armed with a bottle shaped sensillum (s) on the proximal part; distal part with one densely setulated element and two naked elements (1’–2’). Filter comb absent.

Limb IV ( Fig. 4 I–J). Without exopodite; epipodite circular positioned on the inner surface of the limb. The armature is composed only by the naked seta 1 on the distal endite and three (2–4) bisegmented and plumose setae. Basal endite probably reduced. Gnathobase armed with a short sensillum on the proximal part; distal part armed with a plumose seta and two naked elements (1’–2’). Filter comb absent.

Limb V ( Figs. 4K , 5G ). Epipodite exceptionally large. Two distal setae have unknown homology; first seta thin and plumose; second seta heavily modified, wide, with apex pectinate. Inner lobe narrow and setulated, without setae.

Postabdomen ( Fig. 5 A–C). Rectangular in lateral view, not bilobed; wide in dorsal view, not laterally compressed; dorsal margin armed with about 25 transverse rows of short spinulae, of which four rows bordering anal margin. Ventral margin convex, naked, about 5 times shorter than the postabdomen length. Preanal margin about 3.7 times longer than anal margin. Postanal margin markedly short, including only the terminal claws. Postabdominal claws ( Fig. 5 A–B) short and curved, about 0.3 of anal margin length. Postabdominal seta not studied as were absent in studied specimens.

Size. In adult females length 0.52–0.63 mm; maximum height at middle of body, height/length ratio about 0.6.

Ephippial females and males. Not studied.

Remarks. Several morphological traits might be used to distinguish Macrothrix lourdesae sp. nov. from species of the hirsuticornis -group, laticornis -group, rosea-triserialis -group and paulensis -group, namely: (i) on the ventral margin of head there is a protuberance near antennule base; (ii) IDL (en 4) with three chitinized setae; (iii) seta 1 of distal endite of third limb markedly long and heavily pectinate; (iv) exopodite of fourth limb absent; (v) fifth limb extremely modified with exopodite bearing a seta with apex pectinate; and (vi) postanal margin of postabdomen very short. Macrothrix lourdesae sp. nov. and M. marthae clearly form a separate species group; however, they differ one from another in the length and morphology of setae 1 and (c) of first limb. Also, M. lourdesae sp. nov. has three setae on the exopodite of third limb while M. marthae bears only two setae. The postabdomen of M. lourdesae sp. nov. has a shorter ventral margin when compared to M. marthae .

Distribution and biology. Macrothrix lourdesae sp. nov. is found only in the type locality, so far. The Valo Grande Channel presented sampling values of temperature of 24.8±2.6ºC, salinity of 0.03± 0.01g /L- 1, electric conductivity of 76.6±13.8µs. cm-1, dissolved oxygen of 5.6± 0.5mg /L, water transparency (Secchi disk) of 0.4± 0.1m and pH of 7.4±1.4. The presence of M. lourdesae sp. nov. can be associated mainly with the macrophyte species that occurs in the Valo Grande Channel, such as Pistia stratiotes L., Salvinia L. sp. and Eichhornia crassipes (Mart.) Solms. Regarding other Macrothricidae species, M. lourdesae sp. nov. co-occurs with M. elegans Sars, 1901 and Grimaldina freyi Neretina & Kotov, 2017 ( Neretina & Kotov, 2017b). Macrothrix squamosa Sars, 1901 was also observed in Valo Grande Channel, but not in co-occurrence with M. lourdesae sp. nov.

Figures

FIGURE 2. Macrothrix lourdesae sp. nov. A, habitus of adult parthenogenetic female, lateral view. B–C, ventral margin of carapace with ventral setae. D, posterior part of carapace. E, leaf-like setae articulated on the base of spines of internal row of ventral margin. F, ventral view of adult parthenogenetic female. G, dorsal view of adult parthenogenetic female. H, head, lateral view, arrow indicating the protuberance. I, head, ventral view. J. ventral margin of head and labrum. K, dorsal organ, L, antennule. M, antennular sensory seta.

FIGURE 3. Macrothrix lourdesae sp. nov. A–H, antenna. A, general feature; two of three apical setae of exopodite were removed from illustration. B, sensory seta and first and second segments of exopodite. C, apical segment of exopodite. D, apical segment of endopodite. E–F, spines of chitinized seta on the first segment of endopodite. G, morphology of seta on the second segment of endopodite. H–I, morphology of apical setae present on the apical segment of endopodite and exopodite.

FIGURE 4. Macrothrix lourdesae sp. nov. A–K, thoracic limbs. A, first limb. B, first limb fork elements on the endites, arrows indicating elements. C, first limb Inner Distal Lobe and Outer Distal Lobe. D, second limb, arrow indicating the setulated soft seta. E, second limb, apical view of scraper 8 and setulated soft setae. F, second limb, morphology of setulated soft setae. G, third limb. H, third limb, basal endite, arrow indicating the element. I, fourth limb. J, fourth limb, apical view. K, fifth limb.

FIGURE 5. Macrothrix lourdesae sp. nov. A, lateral view of postabdomen and fifth limb. B–C, dorsal view of postabdomen. D, habitus of adult parthenogenetic female. E, head, lateral view, arrow indicating the protuberance. F, image showing setae on the valves and postabdomen; arrows indicating ornamentation on carapace. G, image showing ODL, IDL, and pectinate setae on endite of third limb and fifth limb.