Pygolabis, Wilson, George D. F., 2003

Wilson, George D. F., 2003, A new genus of Tainisopidae fam. nov. (Crustacea: Isopoda) from the Pilbara, Western Australia, Zootaxa 245, pp. 1-20 : 7-12

Pygolabis gen. nov. ( Figs. 1­7 )

Type Species. Pygolabis humphreysi sp. nov., here designated.

Etymology. The genus name is derived from the Greek words "pyge" (tail) and "labis" (forceps or tongs, feminine).

Diagnosis. Pereonite 1 lateral length subequal or shorter than other pereonites. Pleotelson distal margin produced medially and concave distolaterally, region anterior to uropods elongate, uropods inserting ventrally approximately halfway along length of pleotelson; ventral surface anterior and medial to insertion of uropods flat, anus opening posteroventrally, in distal half of pleotelson just posteromedial to uropod insertions, preanal ridge present. Mandible incisor in medial view broader than molar process. Penes narrow and tapering distally, width much less than half length. Pleopod II appendix masculina biarticulate, shorter than exopod, expanding distally after constricted midpoint; ventral surface with deep groove lined with elongate cuticular hairs and combs; dorsal surface strongly expanded laterally with denticulate ridge (visible on ventral side), with large spines along ridge projecting anteromedially, distal spines distinctly longer than proximal spines; distal tip with rounded cup lined with dense cuticular combs. Pleopod II of female and III­V of both sexes endopods bilobed and tumescent, broader than long, with transverse folds, both lobes near same thickness. Pleopod II exopod uniarticulate in male, biarticulate in female. Pleopod V exopod not reaching anus. Uropods inserting approximately half way along pleotelson; protopod robust sensillate setae absent, with small tooth­like setae along medial margin; both rami without robust sensillate setae, long thin simple setae occurring along endopod lateral margin and all margins of exopod; endopod with row of small tooth­like setae, mostly placed proximally, distally pointed medially curved robust claw; exopod flattened and distally rounded.

Discussion. Despite being broadly similar to Tainisopus Wilson and Ponder, 1992 , Pygolabis gen. nov. differs markedly in having a powerful pair of grasping claw­like uropodal endopods (hence their name “tail­tongs”). Modifications of the pleotelson appear to service these tongs. The cuticle is thick and strong, and the enlarged anterior portion of the pleotelson has powerful extrinsic retractor muscles attached to the uropods. So strong is the association between the uropods and the pleotelson that several attempts to remove the uropods without damaging the pleotelson proved futile (hence the cracks seen in Figure 6E­G). The endopods are apparently used to grip the substrate and were found to be difficult to remove from nets in which they were captured (W.F. Humphreys, S. Anstee, pers. comm.). Similar structures are found on the tail appendages of other hypogean crustaceans: Hypsimetopidae (Wilson & Keable in preparation) or Bathynellacea (e.g., Schminke 1973). These unrelated animals having similar structures suggests that some unknown, possibly hydrological, characteristic of the hypogean environment may select for these modified grasping appendages. Some specimens, including the holotype and associated paratypes of P. humphreysi sp. nov. (field number BES4836), were captured in traps, suggesting that this species may be predatory or a scavenger. Whether the uropodal tongs of Pygolabis could be used to hold prey, similar to the forceps of Dermaptera (earwigs) or Japygidae , is unknown. Owing to the difficulty of observing this species in situ, live observations in an aquarium might assist our understanding of these unusual stygofaunal isopods.

The body shape also differs between the two genera: Tainisopus is broader and flatter (body length approximately 5 times width) compared to the distinctly thinner Pygolabis (body lengths 7­8 times width). The male pleopod II appendix masculina is also different: it is relatively undifferentiated in Tainisopus ( Wilson & Ponder 1992: fig. 8B­C), somewhat similar to that seen in other Flabellifera, whereas in Pygolabis , it bears a complex arrangement of grooves, denticles and spines ( Figs. 6 B­D, 7C, F). Similarly modified structures occur in the Asellota or Oniscidea (Wilson 1991), although not this particular form.

Figures

FIGURE 1. Pygolabis humphreysi gen. nov., sp. nov (holotype male WAM C 33562, light micrographs): A, C. Body, dorsal and lateral; B, D. Head, dorsal and lateral. (scale bars 1 mm)