Steneotarsonemus (Steneotarsonemoides), Mitrofanov & Sharonov, 1988

Ganguly, Moumi, Mondal, Priyankar & Karmakar, Krishna, 2021, Taxonomic notes on subgenus Steneotarsonemoides (Acari: Tarsonemidae) with description of a new species of Steneotarsonemus from Tiger grass in the northern hill zone of West Bengal, India, Zootaxa 5023 (3), pp. 405-420 : 406-407

Subgenus Steneotarsonemoides Mitrofanov & Sharonov, 1988

The subgenus Steneotarsonemoides was originally established by Mitrofanov and Sharnov (1988) and reconsidered by Seeman et al. (2016) based on the characters such as trachea with a conspicuous swelling posterior to stigmata i.e., a post-stigmatal atrial chamber, anterior margin of prodorsal shield with a lobe like extension hanging part over gnathosoma, sejugal apodeme visible only as small remnants of the distal part on either side and variable development of poststernal apodeme. So far, six species of Steneotarsonemoides are described viz. S. panshini Wainstein and Beglarov, 1968 ; S. azureus Mitrofanov and Sharanov,1988 ; S. cerinus Mitrofanov & Sharnov, 1988 ; S. porrectus Mitrofanov & Sharnov, 1988 ; S. hippodromus Seeman, Loch, Knihinicki and McMaugh, 2016 ; S. indianensis Karmakar and Mondal, 2021 . Though none of the authors was able to establish the autapomorphic character(s) of this subgenus, emphasis has been given towards the shape of flange hanging anteriorly from prodorsal plate as an autapomorphy. However, upon checking the illustration and description of the mentioned species we found this character as highly variable among the species of Steneotarsonemoides such as, triangular in S. cerinus ; rounded lobe like in S. porrectus and S. azureus , trapezoidal in S. hippodromus and flat ribbon like in S. indianensis which exhibits the poor stability of the character “tectum-like flange” ( Seeman et al. 2016, in key to the subgenera of Steneotarsonemus ) and the failure to establish it as an autapomorphic subgeneric character. It is also probable that many species of Steneotarsonemus described earlier possess such narrow band of flange at anterior margin of prodorsal shield but were overlooked due to inadequate microscopic techniques as these structures are often hyaline and extremely difficult to recognize. Upon investigating the paratypes of Steneotarsonemus indianensis, Karmakar & Mondal , we encountered a great degree of variation regarding the width of this flange and it was also infolded in some of the specimens which also reflects its membranous property. In addition to this, other characters such as presence of atrial chamber, patterns of sejugal apodeme and poststernal apodeme are also very much variable not only in the subgeneus Steneotarsonemoides but among all the species of the genus Steneotarsonemus which suggests towards considering them as synapomorphies rather than autapomorphies. Erection of the subgenus Steneotarsonemoides based on such unstable characters not only complicate the assignment of species but also fail to preserve its phylogenetic integrity.

In addition to subgenus Steneotarsonemus, Lindquist (1986) proposed three more subgenera viz. Mahunkacarus , Neosteneotarsonemus and Parasteneotarsonemus with unique defining characters within the genus Steneotarsonemus . The subgenus Mahunkacarus with five described species is clearly distinguishable from other species of Steneotarsonemus by vestigial bothridial setae sc1 as an autapomorphic character. Neosteneotarsonemus with six described species share apomorphies such as prodorsal bothridial seta sc1 slender or narrowly lanceolate in adult female and unguinal seta u’’ modified on leg II and III of adult and larva with complete bifurcation or expanded or hyaline. Parasteneotarsonemus with a single described species exhibits unique characters such as 3 pairs of prodorsal setae and knob like tarsal claw on leg IV of males and femur II with 2 setae in larvae and adults which might be considered as autapomorphies but requires the support by a greater number of species. The subgenus Carextarsonemus originally erected by Mitrofanov and Sharonov (1988) and also considered by Seeman et al. (2016) share a unique autapomorphy, apodemes 3 with anterior projection near medial extremity. In this context, the defining characters of subgenus Steneotarsonemoides are very unstable and often overlapping with the characters of subgenus Steneotarsonemus as discussed earlier. We have retained the subgeneric status of Steneotarsonemoides in this paper due to our inability (also unavailability) to check the type specimens and included the new species under the same subgenus as all the defining characters presented in earlier publications regarding Steneotarsonemoides are matching with this species but readers are advised not to consider the subgenus as sensu stricto. Additionally, we propose for a thorough revision and phylogenetic analysis of the genus Steneotarsonemus (with molecular approaches, if required) with utmost urgency to solve such dilemma and paradoxical taxonomic status.