Broadening the definition of the genus Thalassaphorura Bagnall, 1949 (Collembola, Onychiuridae) with a new aberrant species from China

Abstract A new species belonging to the tribe Thalassaphorurini, Thalassaphorura problematica sp. n., is described from Northeast China. The new species is closest to the large genus Thalassaphorura by its simple vesicles in PAO and its furcal rudiment, but it does not fit the definition of the genus by the absence of chaeta d0 on head, the number of chaetae in the distal whorl of tibiotarsi and the labium type. We discuss the relative weakness of these last characters at generic level, which lead us to assign the new species to Thalassaphorura instead of erecting a new genus. The diagnosis of Thalassaphorura is broadened accordingly.


introduction
The tribe Thalassaphorurini was established by Pomorski (1998), characterized by a furcal rudiment in a form of a finely granulated area with four small chaetae in two rows posteriorly. During recent investigations on Collembola in Northeast China, we recorded seven species belonging to three genera of the tribe Thalassaphorurini: Allonychiurus (A. songi Sun & Wu, 2012), Sensillonychiurus (S. changchunensis Sun & Wu, 2012, S. pseudoreducta Sun & Wu, 2012, S. reducta Sun & Wu, 2012, and Thalassaphorura (T. encarpata (Denis, 1931), T. lifouensis (Thibaud & Weiner, 1997), T. macrospinata Sun & Wu, 2012). Meanwhile, we also found a new species of the tribe Thalassaphorurini, with a combination of morphological characters that did not fit any of the known genera of the tribe. In the present paper, we assign the new species to the genus Thalassaphorura according to its simple vesicles in PAO and its furcal rudiment, rather than erecting a new genus. We broaden accordingly the diagnosis of Thalassaphorura and discuss the inconsistent characters of the new species. An updated key to the genera of the tribe Thalassaphorurini is provided.

Material and methods
Specimens were collected by Berlese extraction of forest soil and humus, cleared in lactic acid and then mounted in Marc André II solution. They were studied using a Nikon Eclipse 80i microscope. The material is deposited in the Key Laboratory of Wetland Ecology and Environment, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun.
Derivatio nominis. Named for its unusual characters among Thalassaphorura. Discussion. The new species is closest to the genus Thalassaphorura by its simple vesicles in PAO and the furcal rudiment. However, it does not match the definition of this genus proposed by Sun et al. (2010), nor those given previously by Weiner (1996), Fjellberg (1999) or Pomorski (1998) for three characters: absence of chaeta d0 on head, 6 chaetae in the distal whorl of tibiotarsi of all legs, and labium type AB. In order not to erect a new genus in a tribe in need of revision (Sun et al. 2011) and for a species otherwise very similar to existing Thalassaphorura, we placed our new species in the genus Thalassaphorura and broadened its diagnosis.
The new species belongs to the species-group of Thalassaphorura which has modified ventral chaetae in the adult male ("male ventral organ"), including the species Assigning the new species to this genus led us to re-examine three important taxonomic characters that separate the new species from most other Thalassaphorura.
The distal tibiotarsal chaetae have been recently checked in the genera Allonychiurus, Onychiurus and Thalassaphorura (Sun et al. 2010;Sun et al. 2011;Sun and Zhang 2012), showing that this character has a limited taxonomical value to discriminate these genera. In addition, paratypes of T. petaloides (Rusek, 1981) from Iraq and specimens of the same species from southern China were found to actually have 15 (1, 7, 7), 14 (1, 7, 6) and 14 (1, 7, 6)) chaetae on tibiotarsi I, II and III. Together with reduced tibiotarsal chaetotaxy of the new species described here, this leads us to extend the diagnosis of Thalassaphorura to species with 6, 7 or 9 chaetae in the distal row of tibiotarsus. Chaeta d0 on head is considered as a stable character at the generic level. It is present in all species of Thalassaphorura (Sun et al. 2011) except T. jailolonis (Yoshii & Suhardjono, 1992) from Malukku (Indonesia) and the new species T. problematica sp. n. The species jailolonis was described in Jailolaphorura Yoshii & Suhardjono, 1992 (a subgenus of Onychiurus , upgraded to genus level by Weiner in 1996), but was subsequently transferred to Thalassaphorura by Bellinger et al. (1996Bellinger et al. ( -2013 according to a personal communication of Pomorski in 2002. This assignation is however uncertain because the chaetotaxy of the furcal rest is unknown in T. jailolonis. At this point, we consider that the diagnosis of the genus Thalassaphorura should provisionally state that d0 is present or absent on head, waiting for a re-examination of T. jailolonis on fresh material. The third character, labium type, is not stable in several genera of Thalassaphorurini, being AC or ABC in Allonychiurus and Sensillonychiurus (Babenko et al. 2011), and even A, AC or ABC in Thalassaphorura (Sun et al. 2010). In our new species, labium is still of another type -AB. Moreover, labial type is undescribed in many species. This high intra-generic variability implies that this character should not be considered diagnostic at a generic level among Thalassaphorurini.
An amended diagnosis of the genus Thalassaphorura and an updated key of the genera of Thalassaphorurini integrating these changes are given below.
Thalassaphorura Bagnall, 1949 http://species-id.net/wiki/Thalassaphorura Type species: Onychiurus thalassophilus Bagnall, 1937 Diagnosis. Postantennal organ oval, with numerous simple vesicles perpendicular to the long axis; antennal basis more or less indicated; clubs of AIIIO smooth, ribbed or granulated; Ant. IV with S-chaetae differentiated or not, ms close to the second row of chaetae, and no bulb on Ant. IV; labral chaetae formula 4/1,4,2; no multiplication of dorsal pseudocelli, 3 (rarely 4 or 2) pseudocelli in the antenno-basal group, 3-4 (rarely 2 or 5) pseudocelli per half-tergum on Abd. IV, 3 (rarely 4 or 2) pseudocelli per halftergum on Abd. V (1-3 in a postero-internal group, one in a postero-lateral group); chaeta d0 on head present, rarely absent; Th. I usually with pseudocelli; Abd. VI with one or two axial chaetae (a0 or m0, or both); anal spines present or absent; distal whorl of tibiotarsal chaetae as 6, 7 or 9, no clavate tenent hairs; furcal rudiment as a finely granulated area with 4 small dental chaetae in two rows posteriorly, one manubrial row of chaetae present posteriorly to dental chaetae.  Babenko et al. (2011), but is still assigned to the tribe Thalassaphorurini by Bellinger et al. (1996Bellinger et al. ( -2013. Here we prefer to accept the synonym. The genus Dungeraphorura, of uncertain tribal position, has closer relation to the genera of the tribe Thalassaphorurini by three key characters, simple vesicles in postantennal organ, presence of d0 chaeta on head and 9 distal chaetae on tibiotarsi (Gulgenova and Potapov 2012). In the present key, we don't include this genus because of its furcal rudiment, reduced to a cuticular pocket while it is reduced to a finely granulated area in the tribe.