Macrothrix agsensis Dumont, Silva

Garfias-Espejo, Tania, Elías-Gutiérrez, Manuel & Silva-Briano, Marcelo, 2007, On Macrothrix agsensis Dumont, Silva-Briano & Babu, 2002 (Cladocera: Anomopoda: Macrothricidae), with description of the male and ephippial females, and comments on the distribution of the genus in Mexico, Zootaxa 1632, pp. 49-60 : 50-56

Macrothrix agsensis Dumont, Silva –Briano & Babu, 2002

Macrothrix agsensis Dumont et al. 2002: 11 –12, figs 140–171.

Macrothrix smirnovi Ciros-Pérez & Elías, 1997 in Dumont et al. 2002: 13, figs 185–188 (in part, male only). Not Macrothrix smirnovi Ciros-Pérez & Elías-Gutiérrez, 1997: 115 –123, figs 1–35.

Amended diagnosis. Parthenogenetic female

In dorsal view body regularly ovoid, dorsal margin not serrated. Body compressed laterally, whole surface head smooth, valves slightly reticulated. Dorsal margin of the valves not elevated above dorsal margin of head, with dorsal keel. Postero–dorsal angle with a slightly pointed triangle. Dorsal head pore large and oval. Labrum bi–segmented with transversal lines at the edge. Postabdomen subovoid in lateral view, preanal zone with small heel, and with sculpture as longitudinal lines on sides of postabdomen. Postabdominal seta with short distal segment carrying some setae at its tip, no setules on basal segment. Postabdominal claw small with lateral teeth. Antenna I rod–like, dorsal margin with four rows of thin spines, sensory seta located ventrolaterally. Nine terminal aesthetascs of different size. Antenna II large, coxal region with long apical spine on outer surface. Swimming setae 0-0-1-3/1-1-3, spines 0-1-1-1-1/0-0-1. Lateral seta on first endopod segment remarkably larger than the rest, armed with fine spinules from basis to tip.

Outer Distal Lobe (ODL) of limb I supplied with a long apical seta and small lateral seta, Inner Distal Lobe (IDL) with three bisegmented setae of different size, unilaterally setulated in distal part. Setae e–f of endite 2 with five spines along their inner apical rim. One ejector hook. Epipodite elongated. Limb II, exopodite oval with plumose apical seta, endopodite with 8 scrapers, none of which have enlarged subapical teeth. Filter plate with four setae, smallest seta armed similarly with other setae. Exopodite of limb III with four plumose setae, seta 3 sligthly longer than seta 2. A row of six long setae on back endopodite. Exopodite of limb IV with two bilaterally feathered setae of similar size. Limb V with three lobed pre–epipodite, epipodite medium sized. Gnathobase composed of three setae of different length.

Ephippial female

Slightly higher than parthenogenetic female, body compressed laterally, with thick dorsal keel. Valves supplied with ornamentation in the form of hexagonal–rounded cells, central part of each cell slightly inflated. Ephippia transparent, with slight brown pigmentation, generally with one or two eggs.

Adult male

Body oval and acute, dorsal margin curved from tip of rostrum to posterior dorsal corner. Head about half of body length. Head shield not reticulated, head pore big and oval. Postabdomen similar to that of female, no long setules in region of anus, gonopores open near claw basis. Postabdominal setae with short distal segment. Antenna I long, slightly curved, with two sensory setae located ventro-laterally, close each other. Groups of strong spines border the ventral angulation. Thoracic limb I with copulatory hook, slightly bent, with 2–3 ridges running from the middle to tip. ODL and IDL as in female.

As a detailed description of the parthenogenetic females is given by Dumont, et al. (2002), we concentrate just in the details added to these descriptions after our review of more extensive material:

Adult Female

Body length 760–800 µm /440–510 µm height (n = 10). Antenna I ( Figs. 3–6 ) with dorsal margin armed with five rows of long, thin spines. Nine terminal aesthetascs of different length. Two of them larger than the others, all with minute claws at apex.

Limb I ( Fig. 14 ) with outer distal lobe (ODL) bearing a long apical seta, with its distal segment unilaterally armed with short lateral setules, and a small bilaterally setulated seta; inner distal lobe (IDL) with three bisegmented setae of different size, each unilaterally setulated distally, the two smaller hook-shaped ones.

Seta e–f (3–4 in Dumont, et al. 2002) with five spines along their inner apical rim. A single ejector hook setulated at its tip. Epipodite elongated.

Limb II ( Fig. 15 ) with exopodite as an oval lobe, with a bilaterally setulated seta. Endopodite with eight scrapers, increasing in size distally, none with any enlarged teeth. Scrapers 1–2 with a more delicate feathering, 3–8 with robust denticles. A modified seta with blunt tip near scraper 4. Filter comb composed of four long setae; seta 4 much thicker than others.

Limb III ( Fig. 16 ) with exopodite bearing four setae, seta 1 with unilaterally setulated distal segment, setae 2–4 bilaterally setulated from base to tip; seta 3 slightly longer than seta 2. Endopodite with posterior row of six setae, on distal endite: seta “a” stiff, with fine setules basally and robust denticles distally; seta “b” bisegmented, with distal part unilaterally armed with long fine setules; seta “c” small, bisegmented, bilaterally feathered distally. All of them with same length. Basal endite with three bisegmented soft seta (d–f). Anterior row with six setae and a small receptor. Seta 1 hooked, bilaterally setulated at its distal tip, seta 2 unilaterally setulated at its distal part, seta 3 hooked, long unilaterally setulated at its distal part. A small receptor between seta 3 and 4. Seta 4 small, bottle shaped, bilaterally setulated at the tip, seta 5 and 6 long, bottle shaped, with setules at their distal portion. Gnathobase blunt with rows of long and thick setae. Near its base, there is a mamillated receptor. Two setose setae at posterior end. No filter plate.

Limb IV ( Fig. 17 ) with exopodite bearing two short setae, subequal in length, bilaterally setulated. Endopodite with posterior row of five setae (named a–e), all with same length and bilaterally setulated. Anterior row with four setae and one receptor. Seta 1 broad at its basis, with long setules on it, naked in the rest. Seta 2-4 similar in shape and length, with cluster of dense setules at their tip, finally a small seta, naked. Gnathobase with an erect setose seta. Lateral seta absent.

Limb V ( Fig. 18 ) with pre–epipodite three–lobed, exopodite with a single seta, endopodite as a rounded lobe, epipodite elongated, and medium sized. Gnathobase V composed of three setae of different length. Juvenile female (Instar II)

Body length 420–470 µm /310–360 µm height (n= 10). In contrast to adult female, body is more elongate and high, ovoid, with a weaker border between head and valves, and almost straight postero–ventral angle. Antenna I thinner, relatively larger than in adult female, with nine aesthetascs of similar size. Rows of setules on antenna I and II weakly developed. Postabdominal claw weakly developed. IDL of limb I with shorter setules, and with three setae of different size, but the differences in their size are less than in adult female.

Ephippial female

Body length 750–810 µm /460–520 µm height (n=10), rounded in lateral view, proportions similar to a parthenogenetic female of the same size ( Fig. 19 ). In anterior view body broadly round, and a thick dorsal keel. Most of the valves are modified into the ephippium, supplied with a special ornamentation in the form of hexagonal cells, central part of each cell slightly inflated. Ephippium strongly chitinized and translucent, with most of its region yellowish brown in coloration, with two eggs. The reticulation by the hexagonal cells is hardly visible in the majority of females when observed in the light microscope.

Adult Male

Body length 400–480 µm /240–340 µm height (n=10), elongated, dorsum slightly curved, postero–dorsal angle sharp, almost straight ( Figs. 21–22 ). No reticulation on postabdomen sides. Gonopores open near ventral midline at a notch, located at a distance about two basal postabdominal claw lengths from tip. Postabdominal claw reduced in size, with serrated dorsal margin. Antenna I large and extending near halfway of the valve length, dilated in the middle region, where two groups of strong spinules are present and two long sensory setae. A third seta present, shorter. Limb I, ODL with one long seta, with 22–25 bean–like projections along its margin. Copulatory hook slightly bent, with 2–3 ridges running from the middle to tip. A single ejector hook on this limb.

Description (including developmental stages). Shape and shell: oval in lateral view ( Figs. 21–22 , 33 ), dorsal margin curved from tip of rostrum to the postero–dorsal corner, with maximum height at the middle (Instar II dorsal margin slightly curved, see Fig. 20 ). Postero–dorsal corner with a sharp angle, almost straight. No serrations along the dorsal margin. Ventral margin of valves arched, distinctly serrated. Valves without well expressed reticulation. The two immature instars have a general shape similar to females.

Head: laterally without depression in rostral region. It is arched in lateral view, head shield not reticulate, dorsal margin not serrated ( Fig. 33 ). Ventral margin almost straight, and smooth. Dorsal headpore relatively big. Compound eye located close to the dorsal margin. Ocellus smaller than the compound eye located near tip of rostrum. Labrum bisegmented, with transversal lines along the edge, with series of small hairy tubercles at the apex.

Postabdomen: roughly similar in shape to that of female. Ovoid in lateral view, preanal zone with a small heel, dorsal margin not bilobed ( Fig. 28 ). Anus bordered with 7–9 regularly distributed groups of spines. Gonopore open at a notch, near ventral midline ( Fig. 36 ). No reticulation on postabdomen. Postabdominal claw small, with serrated distal margin and lateral teeth ( Fig. 36 ). Postabdominal seta with long proximal segment, distal segment relatively short, with some setules at its tip. Instar II shows a small end–claw and poorly developed spinules on the postabdomen.

Antenna I large ( Figs. 24 and 34 ), its length nearly half length of the valve, S–shaped, not dilating distally, but there is an expanded area in the middle. With three sensory setae, two of them located ventro–laterally, and one laterally. In the instar II they are not well expressed. One of the ventro–lateral seta inserted more proximally, shorter than the other. Two notorious groups of strong spinules in the mid–expanded area. They are evident from instar II, although less developed. Distal end with nine terminal aesthetascs, each with bifurcated tip, as in female; two of them two times larger and located separately from the rest.

Antenna II as in female. Swimming setae 0–0–1–3/1–1–3; spines 0–1(1)–1–1/0–0–1. Largest seta with fine spines from basis to tip ( Fig. 23 ).

Limb I, with a large, strong, bent hook with 2–3 ridges form the middle to the tip; with a small and hyaline teeth in the middle ( Fig. 26, 27 ). ODL with one bisegmented seta, with 22–23 bean–shaped projections on the distal segment. IDL as in females. Endopodite with setules of seta e–f more delicate than those of females. One ejector hook (EH in Fig. 35 ). Limbs II–V as in female.

Instar II (pre-reproductive) male

Body length 270–400 µm /230– 420 µm height (n= 5). There are some appreciable ontogenetic changes over the two instars. Maybe the most notable are the changes in body shape ( Fig. 20 ) and the secondary sexual characters, for example the presence of a small copulatory hook, without ridges, only with a small groove at the top ( Fig. 25 ). Also, the bean–like projections on seta of ODL are not visible in the juvenile instars.

Figures

FIGURES 1 – 9. Macrothrix agsensis parthenogenetic female, from Km 217 Chihuahua state, Mexico; 1) Body in lateral view; 2) Head, labrum and antenna I; 3 – 6) Antenna I and its distal portion in inner and outer view; 7) Setulae at middle portion of ventral margin; 8) Antenna II; 9) Largest endopod seta on anntena II, lateral view. Scale bars: a) Fig. 1; b) Fig. 2; c) Figs. 3 – 9.

FIGURES 10 – 18. parthenogenetic female, from Km 217 Chihuahua state, Mexico; 10 – 11) Postabdomen; 10) Lateral view; 11) Detail of distal part; 12 – 13) Postabdominal claws in outer and inner view; 14) Limb I, with one ejector hook; 15) Limb II and distal armature of its gnathobase; 16) Limb III, with distal armature of gnathobase; 17) Limb IV, exopodite with two setae; 18) Limb V. E 1 – E 3 Endite 1 – 3; EH Ejector hook; EP Epipodite; EX Exopodite; FC Filter comb; GT Gnathobase; IDL Internal distal lobe; ODL Outer distal lobe; PEP Pre-epipodite.

FIGURE 19 – 28. 19. Macrothrix agsensis ephippial female, body in lateral view from Km 217, Chihuahua state, Mexico. 20 – 28. Macrothrix agsensis male, from Km 217 Chihuahua state, Mexico; 20) Immature male, body in lateral view; 21 – 22) Mature male, in lateral view; 23) Antenna II; 24) Antenna I; 25) Limb I of immature male, hook not developed; 26) Limb I of mature specimen; 27) Hook of mature male; 28) Postabdomen. EP Epipodite; IDL Internal distal lobe; ODL Outer distal lobe. Scale bars: a) Fig. 19; b) Figs. 20 – 22; c) Figs. 23 – 28.

FIGURES 29 – 36. 29 – 32. Macrothrix agsensis, female from Km 217 Chihuahua state, Mexico; Scanning Electron Microscope photographs; 29) Lateral view; 30) Postero-dorsal portion of valves; 31) Head, antenna I, labrum, and antenna II; 32) Postabdomen; 33 – 36. Macrothrix agsensis male, from Km 217 Chihuahua state, Mexico; 33) Lateral view; 34) Closer view of antenna I; 35) Limb I, ejector hook (arrow), and hook below; 36) Postabdominal claws and gonopore (arrow). EH Ejector hook.