Platyaxius Sakai, 1994

Poore, Gary C. B. & Collins, David J., 2009, Australian Axiidae (Crustacea: Decapoda: Axiidea), Memoirs of Museum Victoria 66 (2), pp. 221-287 : 274

Platyaxius Sakai, 1994

Platyaxius Sakai, 1994: 180–181 .

Diagnosis. Carapace smooth, with scattered long setae; cervical groove visible laterally over half or more of distance to anterolateral margin. Rostrum triangular, broad, laterally weakly dentate, as long as or slightly longer than eyestalks, slightly depressed below level of carapace, continuous with definite lateral carinae; supraocular spines absent; lateral carina weakly dentate or unarmed; submedian carina dentate; median carina, dentate; postcervical carina absent. Abdominal somite 1 pleuron triangular; pleuron 2 posteriorly rectangular; pleura 3–5 posteriorly rounded. Eyestalk cylindrical, articulating; cornea pigmented. Antenna, scaphocerite short. Maxilliped 3 exopod not clearly bent at base of flagellum. Pleurobranchs absent above pereopods 2–4; podobranchs and arthrobranchs well developed; epipods present on maxilliped 2 to pereopod 4. Pereopods 1 asymmetrical, with propodus cylindrical; carpus-dactylus upper and lower margins smooth, propodus with obsolete distal tooth on upper margin. Pereopods 3–5 propodi with transverse rows of robust setae; dactyli 3 and 4 triangular, with scattered robust setae; dactylus 5 spatulate, with row of robust setae along oblique margin. Pleopods 3–5, appendix interna present. Pleopod 1 of male absent. Pleopod 2 of male with appendix interna and appendix masculina. Uropodal exopod without transverse suture. Telson with lateral teeth, with posterolateral robust setae; apex deeply rounded and continuous with lateral margins.

Remarks. The type species, Platyaxius brevirostris Sakai, 1994 from the Australian North West Shelf, is recognised by the absence of a suture on the uropodal exopod, absence of the male pleopod 1 and lateral teeth on the rostrum. Sakai compared his new genus to species of Eiconaxius Bate, 1888 (see Sakai, 1992 for descriptions of five species) and the two genera share similar uropods, pleopods, swollen chelipeds and a rounded telson, differing largely in the dentition of the rostrum. The dactyli of pereopods 3–5 of species of Eiconaxius are spatulate, non-tapering and with a row of robust setae on the oblique distal margin. See, for example, E. farreae Ortmann, 1891 figured by Sakai and Ohta (2005) and E. mallacoota sp. nov. (fig. 24). The same unusual form is seen only on pereopod 5 of the new species described here; dactyli of pereopods 3 and 4 are tapering with several facial robust setae. Sakai (1994) illustrated only pereopod 3 of P. brevirostris and did not mention pleurobranchs in his diagnosis. The new species described below differs from all species of Eiconaxius in lacking pleurobranchs above pereopods 2–4.

Axius odontorhynchus De Man, 1905 was included in Spongiaxius by Sakai and de Saint Laurent (1989) but is better placed in Platyaxius . It too has denticulate rostrum margins, swollen chelipeds and non-tapering dactylus only on pereopod 5 ( De Man, 1925b: pl. 1 figs. 1l, m).

Sakai and Ohta (2005) removed Eiconaxius from Axiidae and placed it in its own family, Eiconaxiidae , on the basis of ‘P3–5 dactyli … rounded in shape … and their ventral margins spinulate …’ and other features that are shared with one or more other axiid genera. Eiconaxius and Platyaxius are clearly related but whether or not they belong in a clade separate from another containing all other axiids remains to be investigated.