Leipanthura casuarina , new genus and species of anthurid isopod from Australian coral reefs without a “ five-petalled ” tail ( Isopoda , Cymothoida , Anthuroidea )

A new minute anthurid isopod, 2.7 mm long, is described. It is notable in having a uropod with almost cylindrical terminal rami, lacking the typical anthuroid uropodal structure in which the fl attened exopod is attached to the peduncle dorsolaterally and more proximal to the terminal endopod. Th e species has all the other features of Anthuroidea (cylindrical body, mandibular spine row absent or evident as lamina dentata, maxilla 2 fused with the lower lip as a hypopharynx) and many features of the family Anthuridae (paired statocysts, fused pleonites, compact mandible, pereopods 2 and 3 propodus with single palmar robust seta) and is placed in this family as a new genus and species, Leipanthura casuarina, close to Anthura, Exallanthura and Ptilanthura. Leipanthura casuarina is also unusual in lacking pereopod 7 in the adult.


Introduction
Th e Anthuroidea Leach, 1814 are a distinctive group of mostly marine isopods.All have a characteristic uropodal structure in which the fl at exopod is attached to the peduncle dorsolaterally and proximal to the terminal endopod such that Leach likened the "tail" (pleotelson and two uropods) of the type species to a fi ve-petalled fl ower.In addition, all species have a cylindrical body without dorsal coxal plates, the mandibular spine row absent or evident as lamina dentata, and maxilla 2 fused with the lower lip as a hypopharynx.While most literature deals with them as members of the suborder Anthuridea Monod, 1922, a recent revision places them as a superfamily within Cymothoida Wägele, 1989 (Brandt and Poore 2003).Th e superfamily was revised (as Anthuridea) by Poore (2001).In that work, more than 500 species were listed in 57 genera in six families following a cladistic analysis of family and generic relationships.Schotte et al. (2008 onwards) also listed the species.
Th e discovery of three tiny individuals of a new species with anthuroid features but lacking the characteristic uropodal structure, necessitates a reappraisal of superfamily and family defi nitions.While it is tempting to treat the new species as a new family (or superfamily!), it has so many characteristics of the family Anthuridae Leach, 1814 that only a new genus can be justifi ed.
Th e family diagnosis, diff erentiating Anthuridae from the other fi ve anthuroid families, repeats that of Poore (2001) but adds characters dealing with pereopod 7 and the uropod.Th e generic diagnosis follows the format used by Poore (2001) for anthurid genera.Pereopods, antennae and mouthparts were drawn in situ after ensuring the limbs sat in one plane.Mouthparts were confi rmed from a dissection of the paratype.Material is deposited in the Museum of Tropical Queensland, Queensland Museum, Townsville (QM) and Western Australian Museum, Perth (WAM).
Remarks.Th is family diagnosis is slightly modifi ed from that of Poore (2001) who adjusted earlier concepts, e.g., of Wägele (1981), by removing some genera to Expanathuridae Poore, 2001.Th e signifi cant change in this new diagnosis is to accommodate the absence of pereopod 7.
Th e new genus, Leipanthura, could be identifi ed in a fi rst couplet of a key to genera of Anthuridae as follows: Etymology.Anthura, from Greek anthos, a fl ower, and oura, a tail, describes the telson and uropods of Anthura gracilis "… which, when alive, much resemble a fi vepetaled fl ower …" (Leach 1814).Greek leipo, meaning to be without, refl ects the absence of the anthuroid tail in this monotypic genus.
Remarks.Several features place this enigmatic new genus well within the family Anthuridae.Th e overall narrow body form, arrangement of pereonites, short antennal fl agella, compact mouthparts, simple pereopods with few robust setae, fused pleonites, operculiform fi rst pleopodal exopods, and pleotelson with paired statocysts are typically anthurid.Th e species is notable within Anthuridae for the absence of pereopod 7 in an adult female.Th e observation that the holotype bears oostegites confi rms that this is a neotenous characteristic (all isopods hatch without pereopods 7, this stage being called the manca).Th e condition is seen in one other anthurid, Exallanthura Kensley, 1980, four genera of Paranthuridae Menzies & Glynn, 1968(Poore 19842001) and Curassanthura Kensley, 1981in Leptanthuridae Poore, 2001(Wägele 1982).Exallanthura also shares with Leipanthura a completely fused maxillipedal palp but diff ers in having a well developed anterodorsal uropodal exopod, more swollen pereopod 1 propodus and a mandibular palp of one article.Th e only species, Exallanthura sexpes Kensley, 1980, is known from only two individuals that may be mancas; as is commonly the case with specimens of anthurids they lack features that would identify them as fully developed males (multiarticulate antenna 1) or females (oostegites).Ptilanthura Harger, 1878 has a similar uropod, mandibular palp, maxilliped and pereopod 1 to Exallanthura but examples possess pereopod 7 (Kensley 1996).
Th e fl attened uropodal peduncle and its almost cylindrical rami that characterise the new genus are unique within Anthuroidea and must be regarded as a reversal to the form seen in Gnathiidae, the probable sister taxon of Anthuroidea (Brandt and Poore 2003;Cohen and Poore 1994).Placing the genus outside Anthuroidea demands numerous convergences in many other characters.Paratype.Collected with holotype, QM W31120 (juvenile, 2.6 mm, plus 1 microslide).
Etymology.Casuarina, from the type locality and continuing the convention initiated by Poore and Lew Ton (1985) of naming Australian anthuroids after Australian plant genera (noun in apposition).
Description of holotype.Total length, 2.5 mm.Body with well-spaced patches of brown pigment all over (see fi g. 4); 14 times as long as wide.Head longer than wide, smooth, with short, broad rostral projection; eyes lateral, of about a dozen ommatidia.Pereonites smooth, of equal width, pereonites 2-5 of similar lengths, pereonite 6, 0.8 length of pereonite 5, pereonite 7, 0.4 length of pereonite 6. Fused pleonites 1-5 smooth, pleonite 1 indicated ventrolaterally as a slight notch on deep pleural fl ange, others with minute lateral seta, pleonite 5 posterolaterally lobed around base of pleotelson and uropods; pleotelson half as long as pleonites 1-5, as long as width at base, evenly convex dorsally, tapering to broadly semicircular apex with pair of distal setae, with pair of large statocysts clearly visible.Upper lip domed anteriorly.Mandible with incisor barely toothed, lamina dentata with 3 teeth (molar process not seen); palp of 3 articles, article 2 longer than 1, article 3 half as long as 2, with 3 short distal setae.Maxilla 1 outer lobe with 2 or 3 subdistal teeth (inner lobe not seen).Maxillipedal of fused articles with 1 mesial seta, 1 lateral seta, 2 subterminal facial setae, and 5 setae on lateral apical lobe (fused article 5); epipod rounded.
Remarks.Th e illustration of the female (fi g. 1a) shows pleopod 1 in its operculate position; that of the juvenile (fi g. 1b) with the pleopods open.Apart from the absence of oostegites, the juvenile paratype seems indistinguishable from the female.
Th e specimen from Ningaloo Reef, WA, on the opposite side of Australia from the type locality, was not dissected (fi g. 4) but diff ered in ways that could be interpreted as being of taxonomic value only after looking at more than the three specimens available now.General proportions and shapes were similar.Nevertheless, the distal palmar setae on pereopods 2−6 were noticeably trifi d (fl agellated and simple in type specimens) and the numbers of setae diff ered.Th e WA specimen possessed four setae on the mandibular palp article 3 (not three), three apical setae on the uropodal endopod (not two), and three setae ventrolaterally on the uropodal exopod (not two).
Th e species, at 2.7 mm long, is one of the smallest anthuroids known, considerably narrower than some expanathurids of similar lengths (Poore and Lew Ton 2002).