Macrothrix spinosa King 1853

Macrothrix cf. spinosa King, 1853 (Figs 9 –14)

Material examined: Six females from Mahahual, three females from Galeana, two females from Popalillo (Mexico), and three females from a pond near Raxruja (Guatemala).

Additional material: Five specimens from Santa Fe, Argentina.

Body 0.28–0.31 mm (n = 6), more or less rounded, surface of shell strongly squamose (Fig 9). Head shield with a sole “pore” (window) bordered by a cuticle thickening (Figs 10 and 13). First antenna dilated distally, with 5–7 transverse rows of fine spinules, on its distal part several series of longer spinules are inserted on the internal side (Fig 11). When females from Mexico and Guatemala were compared with females from Santa Fe (Argentina), we observed that the antennules in the latter presented a heavier armature (see Fig. 14, and compare with 11), represented by three rows of larger spinules. Second antenna stout, all segments also covered by numerous rows of spinules (Fig. 12), antennal formula with spines 0–1–0–1/0–0–1, setae 0–0–1–3/1–1–3. Numeration of setae in Figs 15­20 is the same as in the redescription of M. elegans by Kotov et al. (2004).

Limb I (Fig. 15, 16), ODL bearing a long apical seta, with strong setules in the second half, and a small unilaterally setulated lateral seta (Fig. 16). IDL with three setae of different length, unilaterally setulated along their second half (Fig. 16). Endite 3 with four setae. Seta “a” strong, bisegmented, with long setules in one side of distal part; setae “b” and “c” similar, stout, with a row of short setules on one side (Fig. 15). Seta 1, the smallest, naked. Endite 2 with three bisegmented setae (“d”, “e” and “f”), with dense setulation at their distal segment and short setules basally, with stronger appearance. Endite 1 with two long setae (“g” and “h”), with fine, long, unilateral setulation, seta “g” with short setules basally. Forks on endites 1 and 2 broad at base. Fork on endite 1 (marked with number 3), left part acute, right part with three teeth. Fork on Endite 2 (marked with number 2), left part acute, right part with two teeth (Fig. 15).

Limb II (Fig. 17). Exopodite elongated with one seta bilaterally armed with long setules and a series of spinules along distal margin. Endopodite with eight scrapers. Scraper 1 and 2 long, with broad basis, bisegmented, with distal segments finely setulated. Remaining scrapers shorter, with robuster denticles. Scrapers 3–5 with a single denticle more developed than the others. Gnathobase distally with four setae. Filter comb with four long setae, bisegmented, and bilaterally setulated in distal part.

Limb III (Fig. 18), exopodite elongated, with four setae (1–4), three of them located distally, distalmost bilaterally setulated, while the two others unilaterally setulated. Seta 4 shorter, located more proximally than the others, bilaterally setulated. Distal endite anteriorly with two bisegmented setae, feathered in distal part, and one unisegemented seta. Posteriorly, seta “a” with bifurcated tip, followed by six (only three illustrated) bisegmented setae. Three modified setae on basal endite (sensorial?). Gnathobase as a lobe with rows of small spinules and a small globular projection.

Limb IV (Fig. 19), exopodite small, with distal group of three bilaterally feathered setae, one of them more densely setulated that the other two. At inner margin a row of four elements (1–4), seta 1 longest, setulated distally in one side, each setae 2–4 with an inflated basal and elongated distal part, densely setulated. Posteriorly, a row of five erect setae, similar in size. On gnathobase, a single seta continues this posterior row of setae, according to Kotov et al. (2004), it is a remainder of filter plate IV. Distal armature of gnathobase, with three elements, a large lobe with a bunch of hairs in the tip, a setae fully setulated in the second half, and a small seta with dilated distal part and setules in both sides.

Limb V (Fig. 20), exopodite with two setae, endopodite with a single seta, gnathobase as a setulated lobe. Although this species was previously discussed by Garfias­Espejo & E lías­Gutiérrez (2004), here we include some important remarks about it.

Probably M. spinosa s. str. is not present on the American continent, which makes sense since it was described from Australia. It has recently been found that the majority of the species of Macrothrix seem to be restricted in their distribution (Ciros­Pérez et al. 1996; Ciros­Pérez & Elías­Gutiérrez 1997a; Frey 1988; Elías­Gutiérrez & Smirnov 2000; Silva­Briano et al., 1999). Indeed, the species from Mexico is a close relative of M. spinosa. At present, it is difficult to establish the limits of spinosa, especially because the type material no longer exists (Smirnov, 1992). As indicated, the material identified by us as spinosa only showed differences in the antennule, while it in all other features is similar.