A new relictual and highly troglomorphic species of Tomoceridae (Collembola) from a deep Croatian cave

Abstract Tritomurus veles sp. n. (Tomoceridae) is described from a Croatian cave. It is characterized by troglomorphic features (absence of eyes, reduced pigmentation, slender claw, pointed tibiotarsal tenent hairs) that only compare, among Tomoceridae, to the microendemic species Tritomurus falcifer from the Pyrénées. Tritomurus veles also shares with Tritomurus falcifer the absence of macrochaetae on head, a presumably non-adaptive character that within Tomoceridae is unique to these two species. Both species have no known epigean relatives in their respective distribution areas and can be considered as relictual.


Introduction
Th e family Tomoceridae includes 149 species in 16 genera, grouped in two subfamilies, Tomocerinae with 131 species and Lepidophorellinae with 18 species (Bellinger et al. 2010). Tomocerinae are distributed across the whole Holarctic region, extend-
Derivatio nominis. Named after Veles-a Slavic god of earth, water and the underworld.
Description. Body length 3.4 to 3.7 mm. Habitus slender, color pale grey in alcohol with scattered black pigment and white patches (Figs 1, 2). Region around the base of Ant.I without pigment (Fig. 3). Narrow white median line from Th .II to Abd.II. Primary granulation of integument fi ne and regular, mostly composed of hexagonal meshes (Figs 6,25,32); some areas with fusion of primary granules resulting in quadrangular or irregular meshes (Fig. 15). Eyes absent, ocular spot weak (Fig.  3) or absent.
Ordinary chaetae numerous on body and appendages, slightly rugose at optical microscope magnifi cation and longitudinally rugose-striate at higher SEM magnification, distally tapering, with well-marked thin sub-basal ring, diff erentiated as medium mesochaetae and long to very long macrochaetae; macrochaetae thin, acuminate, curved, not basally swollen, with socket ring markedly protruding above integument level (Fig. 7); mesochaetae basally swollen in specimens on slide but not in those examined with SEM; socket rings of mesochaetae strongly protruding on Ant.III-IV and dens 31,32), not protruding on tergites . Dense clothing of mesochaetae, particularly laterally (Figs 4, 5); macrochaetae few, frequently detached in microscopic preparations (Fig. 5); minute microchaetae present on anal valves and empodium.
Antennae 1.5-1.8 times length of body (Figs 1, 2). Antennal segment ratio as I:II:III:IV = 1:1.79:15:4.58. Microchaetae at bases of Ant.I and II not diff erentiated. Ant.I and II with a dense clothing of mesochaetae, and S-chaetae of two main types: (i) dark, thin, straight, long (usually longer than surrounding mesochaetae), evenly distributed in large number among mesochaetae; (ii) hyaline, variously but moderatly thickened, shorter than surrounding mesochaetae, less evenly distributed (mostly ventrally and distally) (Fig. 10). Ant.III and IV annulated. Basal part of Ant.III ( Fig. 11) with irregularly arranged chaetae, progressively organized in whorls for most of its length, as well as on Ant.IV. Each whorl composed of a single row of 17-28 chaetae, including ordinary mesochaetae and a smaller number of S-like chaetae, mostly long and thin, some shorter and thicker (Figs 11,12). Serial pattern not evident in chaetal arrangement across successive whorls. Ant.III sense organ not clearly diff erentiated. Two protruding papillae and a spiniform process (pin chaeta) simple, without lateral process apically on Ant.IV (Fig. 13); subapical organite not seen.
Dorsal chaetotaxy dense and regular on head and tergites, consisting of small rounded scales, mesochaetae and a few macrochaetae, but no microchaeta (Fig. 5). Chaetal row along the posterior part of antennal basis fi eld made of mesochaetae identical to those of the head. A line of numerous (more than 30) short equal mesochaetae regularly and closely arranged at the posterior edge of head. Clothing of mesochaetae on tergites denser where scales are absent, i.e. laterally and behind posterior row of macrochaetae, especially on Abd.IV and Abd.V (Fig. 5); mesochaetae more variable in size behind posterior macrochaetae than on the remaining of these tergites.
Each femur with very long, and thin ventro-basal macrochaeta, the two-thirds length of femur, inserted perpendicular to integument (Fig. 22); rest of femoral clothing mesochaetae. Each trochanter with one long, thin ventro-basal macrochaeta similar to that of femur (Fig. 22). Trochanteral-femoral organ not diff erentiated.

Ecology
Tritomurus veles sp. n. was collected from -170 to -430 meters in the deep pit named Amfora jama. All specimens were found far away from entrance, in total darkness on or adjacent to the thin water-fi lm fl owing on vertical walls (hygropetric habitat). Th ey walked on the water fi lm with legs widely spread; if the water current or water drops detached them from the wall, they fl oated downstream and obtained purchase in another place. While walking on the wall with thin water-fi lm, only the legs were immersed in water while body was held above the water surface. Th is species was not observed entering or on the surface of the pools. Water temperature at a depth of -350 meters was 4.7°C and air temperatures were 5.1-5.4°C.
Th e morphology and environment of the new species are similar to that of Tritomurus falcifer, which lives in the hygropetric habitat of caves on the Arbas massif of the French Pyrénées. Both species have the ventro-distal labral brush particularly well developed, apparently more than other epigean representatives of the family. Th is mouthpart modifi cation recalls similar fi ltering structures observed in other species of the cave hygropetric (Moldovan et al. 2004;Sket 2004), and suggests special feeding habits. Th e guts of the collected specimens were fi lled with very fi ne clay-like material, without the mycelium or spores typical of epigean Tomocerinae. Th e new species probably ingests clay as do many other troglobitic Collembola (unpublished observations).
Large parasitic worms (larva of Nematomorpha), were visible inside the body of several specimens (Fig. 35).

Relationships
Tritomurus veles sp. n. is strikingly similar at fi rst sight to the rare cave species T. falcifer Cassagnau from Pyrénées by obvious troglomorphic traits: slender habitus, pigment reduction, anophthalmy, claw elongation, and reduction of tenent hairs to short, pointed chaetae. Such characters are assumed to be adaptive to cave life, and occur in most obligate subterranean species of Collembola (Th ibaud and Deharveng 1994). But the two species also share several non-troglomorphic characters, including those listed as defi ning the genus Tritomurus by Bellinger et al. (2010): presence of scales on the body, absence of post-antennal organ and of eyes, trochanteral and femoral organs not diff erentiated, mucro elongate and setose with two basal teeth, the outer one devoid of toothlet, dens without large basal outer macrochaetae and without inner basal scale-like spine. Two other characters not considered as adaptive in the literature, the absence of macrochaetae on head and the absence of internal lobulations on claw, are also unique to T. falcifer and T. veles sp. n. among Tomocerinae.
According to the most recent generic key for Tomoceridae (Bellinger et al. 2010), T. veles species is a member of Tritomurus. Th is genus is, however, poorly defi ned, as indicated by Cassagnau (1958), and the assignment of T. falcifer and T. veles sp. n.
to Tritomurus is unsatisfying. Tritomurus scutellatus Frauenfeld, 1854 from Slovenian caves, the type species of the genus, diff ers from falcifer and veles sp. n. in signifi cant characters, including the presence of cephalic macrochaetae and a much diff erent claw structure (claw not elongate and with very large basal wings in T. scutellatus). Th e features that separate T. falcifer and T. veles sp. n. from T. scutellatus and from all other Tomocerinae may well justify placing them in a separate genus. Th e placement of these species in Tritomurus is therefore provisional, pending redescriptions of T. scutellatus and T. falcifer based on fresh material.

Troglomorphy
All species of Tomocerinae living outside caves have 5+5 or 6+6 eyes. Th ose living in caves may have the complete 6+6 eye set for the family (e.g., Tomocerus problematicus Cassagnau, 1964 in the Pyrénées and Plutomurus unidentatus (Börner, 1901) in central and northern Europe), or a reduced number of eyes, but all species with reduced eye number are cave-restricted. Among reduced-eyed species, several also exhibit partial or total loss of pigment and have pointed tenent hairs, two characters often observed in subterranean Collembola. Claw elongation, considered another correlate to cave life in most troglomorphic Collembola (Christiansen 1961), is not observed in cave Tomoceridae, except Tritomurus falcifer and T. veles sp. n.

Distribution
Th e species is only known from the type locality. A number of caves were explored during the fi ve-year project 2002-2006 "Inventory and Mapping of the Subterranean and Spring Fauna of Biokovo Nature Park," but T. veles sp. n. was not collected in any other cave, perhaps because hygropetric zones in most of these caves were practically inaccessible. Tomoceridae with reduced numbers of eyes (less than 4+4) belong to four genera: Lethemurus Yosii with 2 blind species from Japan and North America; Plutomurus Yosii with 9 species and 3 subspecies with 3+3 or fewer eyes, from Eastern Asia and North America; Tomolonus Mills with one 3+3-eyed species from North America; and Tritomurus with 3 blind species, including T. velves sp. n. European cave species of Plutomurus, the most diverse of these genera, have the full complement of 6+6 eyes except P. sorosi Kniss & Th ibaud, 1999 from Georgia with 4-5 eyes per side. Th us, Tritomurus is the only genus of Tomoceridae with blind and fully troglomorphic species in the Western Palaearctic region. Both T. falcifer and T. veles sp. n. have extremely narrow distribution and no close relatives is known in their respective regional fauna which are relatively well sampled, as well as among Tomocerinae, suggesting a relictual status.