Psathyropus

Psathyropus tenuipes, from here.

Belongs within: Sclerosomatidae.

Psathyropus is a genus of harvestmen from southern Asia, characterised (indistinctly) by the presence of two pseudoarticular nodules in femur II.

Tenuous legs and tenuous connections

In past postings on this site, I’ve expounded at length on the taxonomic nightmares associated with the harvestman family Sclerosomatidae. High diversity combined with systems based on features of questionable significance has led to an all-but-impassable morass. The genus Psathyropus stands as a good example of some of the challenges involved.

Psathyropus tenuipes (female?), copyright Ryosuke Kuwahara.

Psathyropus was first established as a genus by the German arachnologist Ludwig Carl Christian Koch (not to be confused with his father, Carl Ludwig Koch) in 1878 but was little used in subsequent years. It remained as something of a collective shrug for nearly a century until Suzuki (1973) reassessed its type species P. tenuipes. Suzuki transferred this species to Metagagrella, synonymising the genera, but seemingly ignored the fact that Psathyropus was the older of the two. As noted by Crawford (1992), it seems likely that Suzuki intended to treat the little-known Psathyropus as a nomen oblitum and thus preserve the more commonly used name. However, the case did not qualify for such treatment. Crawford (1992) therefore listed Metagagrella as a junior synonym of Psathyropus though formal combinations for most species involved would not appear in print until Kury et al. (2020).

However, this may be jumping the gun. As with other sclerosomatid genera, the reasons for synonymising Psathyropus and Metagagrella are questionable. When Suzuki (1973) revised P. tenuipes, he also synonymised species with it that had been assigned to the genera Gagrella and Melanopa. Historically, these genera have been distinguished by the length of the legs (Melanopa has shorter legs than the other two) and the number of pseudoarticulations in the leg femora (Gagrella has a single pseudoarticulation in the second femur, Metagagrella has two). However, it looks as though both features may vary within a single species. Suzuki pointed out that populations of P. tenuipes included individuals with either one or two pseudoarticulations in the second femur, making it potentially assignable to either Gagrella or Metagagrella (indeed, numbers of femoral pseudoarticulations may not even remain constant between the left and right legs of a single individual—Suzuki 1982). His decision to go with Metagagrella was essentially arbitrary, based on slightly more individuals having two than one. Which is all to say, there is little reason to assume that the species assigned to ‘Metagagrella’ form a single evolutionary cluster, and even less reason to assume that Psathyropus tenuipes forms part of that cluster. My own recommendation would be that the two genera remain separate until such a time as their relationships can be reliably reviewed. In which case, the only species that can be firmly attributed to Psathyropus is the type.

Psathyropus tenuipes (male?), copyright Ryosuke Kuwahara.

Psathyropus tenuipes is found in coastal regions of Japan and Far Eastern Russia (a long way from the bulk of ‘Metagagrella’ species in southern Asia, another reason to be circumspect about their association). It is mostly found on weathered cliffs alongside sandy beaches though it is found further inland in northern Japan (Tsurusaki & Shimada 2004). In parts of its range it is strictly maritime, requiring proximity to at least slightly saline water (Tsurusaki et al. 2020). This, is should be noted, is an unusual habitat to the normally desiccation-sensitive Opiliones. Populations in northern Japan have been recognized as a distinct subspecies P. tenuipes yezoensis based on their darker coloration, shorter dorsal spine and reduced denticulation, but Tsurusaki & Shimada (2004) found variation in these features to be more clinal and recommended against subspecific differentiation.

One feature of Psathyropus tenuipes that has attracted attention is the presence in their genome of B chromosomes (Tsurusaki & Shimada 2004). These are small, supernumerary chromosomes expressing little or no active genetic function. The number of B chromosomes varies between individuals within a population, even between cells within a single individual, with recorded ranges between none up to eight, though northerly individuals in Japan did tend to have more than their southerly counterparts. The function, if any, of B chromosomes remains unknown. A connection has been suggested with resistance to microbial parasites but, as noted by Tsurusaki & Shimada (2004), the conceptual basis for how this is supposed to work is weak. Perhaps more likely is that B chromosomes are just superfluous pieces of ‘selfish DNA’, their number moderated only by the individual’s capacity to function despite, not because of, their presence.

Systematics of Psathyropus

Characters (from Roewer 1954 and Suzuki 1985, as Metagagrella): Femora I–IV with 0.2.0.0 nodules; scutum with median spine(s) on area 2 only or one each on areas 1 and 2; eyemound low, upper section granular, without larger spines; femora I and III cylindrical, longer than body.

<==Psathyropus Koch 1878 [incl. Metagagrella Roewer 1910]C92
    |--*P. tenuipes Koch 1878C92 (see below for synonymy)
    |--P. aurolucens (Roewer 1954) [=Metagagrella aurolucens]KM20
    |--P. bengalensis (Roewer 1954) [=Metagagrella bengalensis]KM20
    |--P. bilineatus (Roewer 1954) [=Metagagrella bilineata]KM20
    |--P. bimaculatus (Roewer 1954) [=Metagagrella bimaculata]KM20
    |--P. biseriatus (Roewer 1912) [=Metagagrella biseriata]KM20
    |--P. cingulatus (Roewer 1954) [=Metagagrella cingulata]KM20
    |--P. conjugatus (Roewer 1954) [=Metagagrella conjugata]KM20
    |--P. conspicuus (Roewer 1954) [=Metagagrella conspicua]KM20
    |--‘Metagagrella’ crassa Suzuki 1985S85
    |--P. cuprilucidus (Roewer 1954) [=Metagagrella cuprilucida]KM20
    |--‘Metagagrella’ curvispina Suzuki 1985S85
    |--P. distinctus (Sato & Suzuki 1938) [=Metagagrella distincta]KM20
    |--P. formosa (Roewer 1911)CS12 (see below for synonymy)
    |--‘Metagagrella’ fusca Suzuki 1985S85
    |--P. granulatus (Roewer 1954) [=Metagagrella granulata]KM20
    |--P. granulosus (Suzuki 1977) [=Metagagrella granulosa]KM20
    |--P. guttatus (Roewer 1954) [=Metagagrella guttata]KM20
    |--P. hainanensis (Wang 1941) [=Metagagrella hainanensis]KM20
    |--P. hirtus (Roewer 1915) [=Metagagrella hirta]KM20
    |--P. hsuehshanensis (Suzuki 1977) [=Metagagrella hsuehshanensis]KM20
    |--P. koyamai (Suzuki 1979) [=Metagagrella koyamai]KM20
    |--P. luteomaculatus (Suzuki 1970) [=Metagagrella luteomaculata]KM20
    |--P. mandalayius (Roewer 1954) [=Metagagrella mandalayia]KM20
    |--P. micans (Roewer 1926) [=Metagagrella micans]KM20
    |--P. minax (Thorell 1889)KM20 [=Gagrella minaxKM20, Metagagrella minaxR23]
    |    |--P. m. minaxKM20
    |    `--P. m. trispinosus (Thorell 1889)KM20 [=Gagrella minax trispinosaKM20, Metagagrella minax trispinosaR23]
    |--P. mysoreanus (Roewer 1954) [=Metagagrella mysoreana]KM20
    |--P. niger (Roewer 1912) [=Metagagrella nigra]KM20
    |--P. octomaculatus (Roewer 1954) [=Metagagrella octomaculata]KM20
    |--P. perakanus (Roewer 1954) [=Metagagrella perakana]KM20
    |--‘Metagagrella’ punctata Suzuki 1985S85
    |--P. pustulatus (Roewer 1910) [=Metagagrella pustulata]KM20
    |--P. roeweri (Suzuki 1974) [=Metagagrella roeweri]KM20
    |--P. rufoscutum (Suzuki 1982) [=Metagagrella rufoscuta]KM20
    |--P. rufus (Roewer 1954) [=Metagagrella rufa]KM20
    |--P. satarensis (Roewer 1954) [=Metagagrella satarensis]KM20
    |--P. silvestrii (Roewer 1927) [=Metagagrella silvestrii]KM20
    |--‘Metagagrella’ similis Suzki 1985S85
    |--P. sinensis (Schenkel 1953) [=Metagagrella sinensis]KM20
    |--P. sordidatus (Thorell 1889) [=Gagrella sordidata, *Metagagrella sordidata]KM20
    |--P. sulcatus (Roewer 1954) [=Metagagrella sulcata]KM20
    |--P. tenuis (Roewer 1954) [=Metagagrella tenuis]KM20
    |--P. tongkingensis (Roewer 1954) [=Metagagrella tongkingensis]KM20
    `--P. versicolor (Suzuki 1964) [=Metagagrella versicolor]KM20

Psathyropus formosa (Roewer 1911)CS12 [=Metagagrella formosaS77; incl. M. formosa bispinosa Roewer 1923S77, M. duriuscula Roewer 1915S77]

*Psathyropus tenuipes Koch 1878C92 [=Metagagrella tenuipesTS04; incl. Met. abei Sato & Suzuki 1938S73, Melanopa biseriata Sato & Suzuki 1938S73, Gagrella japonica Roewer 1910TT05, Melanopa japonica Roewer 1910S73, Metagagrella suzukii Roewer 1954S54, Gagrella japonica yezoensis Suzuki 1949TS04, Met. tenuipes yezoensisTT05, Psathyropus tenuipes yezoensisTS04]

*Type species of generic name indicated

References

[CS18] Chen, S.-L., & H.-T. Shih. 2018. A review of the harvestman genus Metadentobunus (Opiliones: Sclerosomatidae: Gagrellinae) with a description of a new species from Taiwan. Zoological Studies 57: 57.

[C92] Crawford, R. L. 1992. Catalogue of the genera and type species of the harvestman superfamily Phalangioidea (Arachnida). Burke Museum Contributions in Anthropology and Natural History 8: 1–60.

[KM20] Kury, A. B., A. C. Mendes, L. Cardoso, M. S. Kury & A. A. Granado. 2020. WCO-Lite: Online world catalogue of harvestmen (Arachnida, Opiliones). Version 1.0—Checklist of all valid nomina in Opiliones with authors and dates of publication up to 2018. Published by the author: Rio de Janeiro.

[R23] Roewer, C.-F. 1923. Die Weberknechte der Erde: Systematische Bearbeitung der bisher bekannnten Opiliones. Gustav Fischer: Jena.

Roewer, C. F. 1954. Indoaustralische Gagrellinae (Opiliones, Arachnidae). (Weitere Weberknechte XVIII). 2. Teil. Senckenbergiana Biologica 35 (5–6): 237–292.

[S73] Suzuki, S. 1973. Opiliones from the South-west Islands, Japan. Journal of Science of the Hiroshima University, Series B, Division 1 (Zoology) 24: 205–279.

[S77] Suzuki, S. 1977. Opiliones from Taiwan (Arachnida). Journal of Science of the Hiroshima University, Series B, Division 1 (Zoology) 27 (1): 121–157.

Suzuki, S. 1982. Four new harvestmen from Thailand (Arachnida, Opiliones, Gagrellidae). Annotationes Zoologicae Japonenses 55 (3): 167–174.

[S85] Suzuki, S. 1985. A synopsis of the Opiliones of Thailand (Arachnida) II. Palpatores. Steenstrupia 11 (7): 209–257.

[TS04] Tsurusaki, N., & T. Shimada. 2004. Geographic and seasonal variations of the number of B chromosomes and external morphology in Psathyropus tenuipes (Arachnida: Opiliones). Cytogenetic and Genome Research 106: 365–375.

Tsurusaki, N., M. Kawaguchi, Y. Funakura, T. Matsumoto & Y. Obae. 2020. Distributions of a halophilous and a riparian species of harvestmen along Sendai River, Tottori City, with the first records of harvestmen in Tottori Sand Dunes. Acta Arachnologica 69 (2): 95–103.

[TT05] Tsurusaki, N., M. Takanashi, N. Nagase & T. Shimada. 2005. Fauna and biogeography of harvestmen (Arachnida: Opiliones) of the Oki Islands, Japan. Acta Arachnologica 54 (1): 51–63.

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