Isotomurus festus Potapov & Porco & Deharveng 2018, sp. nov.

Isotomurus festus sp. nov.

Figs 1–26.

Type material: Holotype, Russian Far East, the Kuril Islands, Kunashir Isl., Kurilsky Reserve, vicinity of Alekhinskaya outpost, behind Sernaya Bay, bank of spring, in moss sprinkled with water, 1.vi.2011. A. Matalin leg. Many paratypes in the same locality.

The holotype and 15 paratypes are deposited in the collection of the Department of Zoology & Ecology, Moscow State Pedagogical University. 17 paratypes including 12 vouchers of barcoded specimens are deposited in the collection of the Museum national d'Histoire naturelle (MNHN) in Paris.

Description. Body size 1.9–2.3 mm. Body shape as common for the genus, with rather prognathous head (Fig. 1). Brightly orange when alive (probably true only for colour forms with light background, see below). In alcohol background pale yellowish. Eye areas black, connected by broad band on anterior part of head, bases of antennae paler. Posterior part of head with large black medial spot. Antennae and foots diffusely dark, furca paler. Other characteristics of colour pattern of body varies (see below).

8+8 ommatidia of subequal size in typical position. PAO broad elliptical, as long as nearest ommatidium. Maxillary outer lobe with 4 sublobal hairs and bifurcate palp (Fig. 11). Labrum with 4 prelabral setae, labral edge as common for the genus, with four sharp ridges and ventro-apical ciliation. Frontoclypeal area with many setae. Labial palp with all usual papillae and complete set (16) of guards, ~ 9 proximal, 5–6 basomedian and 5 basolateral setae present (Fig. 12). Ventral side of head with 5–9+5–9 postlabial setae (Fig. 12). Maxillary head with lamella 1 slender, not expanded, with one row of stronger cilia along edge, other lamellae with fields of thin denticulation. (Figs 9–10). Antennae about 1.4–1.7 as long as head diagonal. With rather few s-setae on three first antennal segments. Ant. 1 with 5–9 s-setae: 5–3 short and thick ("spine-like") and 2–4 long (Fig. 7), Ant. 3 with few s-setae, “AO area” with two thick inner s-setae, group of slender s-setae in more lateral position, ms-seta and one thick lateral s-seta, with several erect s-setae around (Fig. 6). Ant. 4 with minute secondary branch present on subapical pin-seta (bifurcate pin-seta).

Body with cover of finely ciliated setae unequal in length (Fig. 23), ciliation often visible only under highest magnification. Neither trichobothria nor true macrosetae differentiated. Ratio longest setae to length of Abd. V as 0.3–0.4. The s-setae short (Fig. 22), longer on posterior part of trunk (Fig. 23). Number of s-setae in full grown specimens as 4,4/4,5–7,>14,>15,>14 (Fig. 21), first instar juveniles with somewhat fewer s-setae on posterior tergites (Fig. 24). The ms-setae 1,1/1,1,1 in number. Accp-s-setae in front of p-row in grown specimens, Abd. III– V with asymmetrical arrangement and number of s-setae with some in medial position (unpaired). On Abd. IV and V considerable part of dorsal area covered with as-s-setae (Fig. 21).

Legs with slender and long claws, ratio tibiotarsus: outer edge of claw as 0.8–1.0 on Leg 1 and 1.5–2.0 on Leg 3 (Figs 2–4). Claw with a pair of small lateral tooth, an outer one, and rudimentary inner tooth (often invisible) (Fig. 5). Empodial appendage without inner tooth. Tibiotarsi in juvenile specimens and first instar with 8 distal setae. Retinaculum with 4+4 teeth and 10–15 setae. Ventral tube with 3+3 laterodistal, 0–5+0–5 anterior and 16–20 posterior setae. Manubrium densely covered by setae. Manubrial thickening simple, without teeth in medial part. Dens rather slender, continuously narrowed, with rather few short setae on posterior side which cover only its basal part (ca 1/5 of its length). One of posterior setae in basal part slightly longer than others. Posterior side of dens crenulated, wrinkles regular in distal half and more disordered in basal half (Fig. 8). Mucro with four teeth (Fig. 13–16) and a seta on outer side. "Apical" tooth large (as large as subapical). Subapical tooth with two lamellae running to mucro base. Middle tooth scoop-like with two lamellae running back, outer lamella wider. Outer proximal tooth of common shape. Ratio manubrium: dens: mucro as 6.8–9.7: 10.5–16.5: 1. Ratio mucro: outer edge of claw III as 0.2 5–0.34. First instar juveniles without ventroapical manubrial setae and seta on mucro. Males are sporadic and thus the population consists mostly of females. Fully adult males have not been seen by us and so presence of modified setae on Abd. III–IV remains to be studied.

Colour forms. Type population consists of several colour forms mixed together. Pale variant is almost without pigmentation on corpus apart from weak diffuse spots on lateral parts of body and bases of legs, dark spot is also often seen on front part of mesothorax (Fig. 18). Most individuals have a broad mid-dorsal band interrupted in intersegmental areas and running from thorax to last abdominal segment. The size of spots forming the band become narrow towards posterior, the spots are much weaker or absent at the end of abdomen (Fig 17). Lateral parts of Th. II–Abd. II have dark irregular spots. In some individuals the main colour pattern is masked by darker background and therefore medial band is hardly visible (Fig. 20). Some specimens are, in fact, with homogeneous diffuse and rather pale colouration (Fig. 19). Colour photos of the described forms and their variability (apart from diffusely dark one) are shown in Figs 25–26. Association of colour pattern with size of the individuals or sex has not been detected.

In the subsequent molecular analysis, the variants with clear medial band were named “banded” (Figs 17, 20), while all others, without such a band, were called “pale” (Figs 18–19).

Name derivation. The name reflects bright (festal) colour of alive specimens.

Distribution and ecology. Known only from the type locality. It was found in mass population forming orange layer in wet mosses sprinkled with running water of mountain spring (Fig. 27).

Discussion. Three species, i.e. I. festus sp. nov., I. chaos Potapov & Babenko, 2011 and I. antennalis Bagnall, 1940, combine in a compact group which probably deserves the rank of supra-specific taxon. So far we named it as the ‘antennalis’ group (Potapov 2001) which is defined by the lack of trichobothria and macrosetae on the abdominal segments. S-chaetotaxy is disproportionate in all mentioned species: tergites of thorax and anterior half of abdomen have few s-setae while posterior half of abdomen possesses many such setae. Thus both I. festus sp. nov. (Fig. 17) and I. chaos (Fig. 1 in Potapov & Babenko 2011) have only 3 accp-s-setae on Th. II, III, and Abd. I and many irregularly scattered s-setae on the last abdominal segments. Isotomurus antennalis is also characterized by a similar s-pattern on body (Fjellberg, pers. comm.). The new species differs from the other two members of the group by a combination of very slender claws, large apical tooth of mucro and weak setal covering of posterior side of the dens. All three characters are uncommon within the genus Isotomurus: slender claws are shared with I. alticolus (Carl, 1899) (Europe) and I. takahashii (Yosii, 1940) (Japan), large apical tooth with I. annectens Yosii, 1963 (Japan). Isotomurus takahashii was described as a member of Isotoma, the genus having no trichobothria. Afterwards Yosii (1963) wrote that " setae sensuales [=trichobothria, authors' note] were previously overlooked, but they are rather short, filiform and ciliated on all sides" and moved accordingly I. takahashii to the genus Isotomurus. The species belongs to group other than ‘antennalis’. Isotomurus annectens differs from I. festus sp. nov. by normal shape of claw and presence of trichobothria.