Microthalestris variabilis, sp. nov.

Microthalestris variabilis sp. nov.

urn:lsid:zoobank.org:act: 5CD772EB-A539-4FF2-B83E-531ED6767E80

Parastenhelia? costata Pallares, 1982 sensu Mielke (1990)

Based solely on P1 morphology P. gracilis appears morphologically close (and potentially conspecific) with P. costata, another Argentine species originally described from Bahía Thetis on the east coast of Tierra de Fuego (Pallares 1982: 9–11; Plates I–II) and which occurs sympatrically with M. minuta comb. nov. (Pallares 1982: 12–14; Plate III). Bick & Arlt (2013) recorded both species in considerable numbers from a soft-bottom area in Maxwell Bay (King George Island, South Shetlands) but it is not clear how they distinguished them. Mielke (1990) reported a second population on Isla Grande Tierra del Fuego from Bahía Lapataia near Ushuaia which he tentatively attributed to P. costata. An exceptionally high degree of variability in the armature of P2–P4 was observed in a relatively small sample (13 ♀♀, 6 ♂♂), causing Mielke (1990) to suggest that both Tierra del Fuego species, P. costata and P. minuta represented populations of the same species. Accepting this view would unnecessarily blur the species boundaries in Microthaletris even further and, considering the recent progress that has been made (e.g. Gee 2006), be a retrograde step in the revision of the genus. Mielke’s (1990) specimens differ from the P. costata type population in the smaller body size [580–780 μm (♀), 420–560 μm (♂) vs 789–960 μm (♀), 600–706 μm (♂)], the shape of caudal ramus seta IV (with bulbous swelling near the base), the presence of only one or two inner setae (instead of three) on P3–P4 exp-3, the length of the inner seta on the male P3 enp-2, and the weakly developed endopodal lobe of the male P5, reaching barely beyond the proximal exopod segment (vs reaching well beyond the middle exopod segment). The female P5 differs in the shape of the exopod which is rounder in the type population and shows a wider spacing between the proximal and middle outer setae, and in the length of the proximal inner seta of the endopodal lobe which is distinctly shorter in Mielke’s (1990) females. Based on this suite of differences we elect to attribute separate specific status to the Bahía Lapataia material rather than treating it as a population of P. costata.

Original description. Mielke (1990): 158–164, Figs 1–5.

Type material. The dissected female specimen illustrated by Mielke (1990: Figs 1–3, 4A, 5) is here designated as the holotype of P. variabilis sp. nov. (ICZN Arts 16.4 and 72.5.6). The species can be differentiated by the characters mentioned in the diagnosis below and those illustrated by Mielke (1990) (ICZN Art. 13.1).

Type locality. Argentina, Province of Tierra del Fuego, Antarctica and South Atlantic Islands; Isla Grande Tierra del Fuego, Bahía Lapataia near Ushuaia.

Differential diagnosis. Microthalestris. Body length 580–780 μm in ♀, 420–560 μm in ♂. Antenna with 2- segmented exopod bearing two setae on exp-1 and two lateral and three apical elements on exp-2; endopod without penicillate spines. P1 exopod about 70% length of endopod; exp-2 elongate, about 1.8 times as long as exp-1, and about 40% length of enp-1; insertion point of inner seta of enp-1 at 40% of inner margin length; exp-3 with two unipinnate spines and two geniculate setae; enp-2 with one minute seta, one geniculate and one non-geniculate claw. Armature pattern of ♀ P2–P4:

P2 endopod ♂ 2-segmented (enp-2 and -3 forming compound segment), armature pattern [1.221]. P3 endopod ♂ 3-segmented, with apophysis on enp-3, armature pattern [1.1.02 + apo]. P 5 ♀ with ovate exopod (about 1.9 times as long as maximum width), with six elements, distal outer one longest, outer apical one short and naked; endopodal lobe with five elements, innermost one well developed. P 5 ♂ exopod 3-segmented, with seven elements, outer seta of exp-1 long and naked; endopodal lobe with two short spiniform elements.Armature of P 6 ♂ unconfirmed. Caudal ramus seta V normally developed, seta IV with bulbous swelling near base.

Etymology. The species name alludes to the high level of variability observed in the swimming leg armature.