Genus Grimaldina Richard, 1892

Richard 1892: p. 214–218; Sars 1901: p. 28–31; Silva-Briano 1998: p. 149–151; Garfias-Espejo & Elías-Gutiérrez 2003: p. 108; Hollwedel et al. 2003: p. 78; Elías-Gutiérrez et al. 2006: p. 12–13.

Comments. Richard (1892) did not diagnose the genus (see also Smirnov 1976; 1992). And the following authors gave only a brief diagnosis (Smirnov 1976; 1992) or a formal description of the species (Sars 1901; Silva-Briano 1998; Garfias-Espejo & Elías-Gutiérrez 2003; Hollwedel et al. 2003; Elías-Gutiérrez et al. 2006) based on which it is difficult to separate Grimaldina from other macrothricid genera. The first attempt to estimate the position of Grimaldina within the Macrothricidae was made by Dumont and Silva-Briano (1998). In their proposed scheme, Grimaldina and a few other genera were transferred to the "Non-Macrothricinae" subfamily based on thoracic limb structure (Dumont & Silva-Briano 1998), although some errors and inaccuracies were made in their morphological description. This classification has not been generally accepted, because not all macrothricid taxa are equally well studied.

Emended genus diagnosis. Large macrothricids, about 0.7 mm. Living animals usually have reddish-brown color. Body compressed laterally, in lateral view subrectangular, length slightly greater than height. Dorsal margin regularly arched in anterior portion and almost straight in its posterior part; a depression between head and valves fully absent. At posterior margin a row of small stiff setulae on the inner side of valve; posteroventral portion with short setulated setae, subequal in size. Ventral margin of valves with setulated, relatively long setae different in size. Outer surface of valves with a sculpture consisting of elongated scales. Head relatively large, not keeled. Ventral surface of head with a protuberance near bases of antenna I. A single frontal head pore located in anterior part of head between bases of antennae I. Major and lateral dorsal head pores absent. Labrum large, subquadrangular, with setulated apex. Postabdomen large for macrothricids, semicircular, strongly compressed laterally. Preanal margin long, curved, subdivided into two lobes. Preanal margin with minute teeth, forming small groups distally; armature of anal margin as groups of fine setulae, plus a single setulated seta located proximally, also a bunch of 5–6 setulated setae laterally. Postanal margin with a row of 6–7 setulated setae, some specimens from the Paraná River in Argentina has 8 setae (J. C. Paggi, personal communication), among them the distalmost setae shorter than others, and fine groups of setules above them. Postabdominal setae longer than postabdomen. Postabdominal claw relatively robust, long. At its base, a relatively long basal spine and a small denticle. Antenna I long, thin, “rod like”, with a row of small denticles, decreasing in size distally. Antennular sensory seta slender, arising at 1/4 of its length from base. Antenna II with elongated branches. Antennal formula: setae 0-0-1-3/1-1-3, spines 0-1-0-1/0-0-1. The largest seta (on proximal segment of antenna II endopod) asymmetrically armed by short setulae along one side and more by robust spinules (located relatively sparsely or densely) along other side. Spines of both apical antennal segments thin, exopod apical spine slightly longer than endopod spine. Five pairs of thoracic limbs. Limb I without accessory seta, maxillar process with two setae. Limb II with eight scrapers, covered by relatively fine (for macrothricids) denticles, and additional elements: a fork-like projection near scraper 3, a long sensillum and soft seta near scraper 4, and soft seta near scraper 8. Filter plate with six setae. Exopodites of limb III–V with 6, 4 and 1 setae, respectively. Armature of posterior setae on endites of limb III–IV represented by fine setules, unlike other macrothricids with well-developed denticles and stiff swiping setulae. Gnathobase filter plate of limb III–IV with seven and six setae, respectively; limb V without differentiated filter plate.

Type species. Grimaldina brazzai Richard, 1892. Type was fixed by monotypy (ICZN 2000, case 68.3).