Suborder ASELLOTA Latreille, 1802

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Suborder ASELLOTA Latreille, 1802

Compiler and date details

April 2011 - Kelley Merrin, Museum Victoria, Melbourne, Victoria

Gary C. B. Poore & Helen M. Lew Ton

Introduction

The Asellota are a large group of isopods in which the first two or three pleopods are variously modified to form an operculum over the remaining pleopods; pleopod 1 is always absent in the female. The group is noted for the extreme variety of body shapes, sometimes being extremely elongate and slender, sometimes broad and flat. Many species are highly ornamented, and others are among the smallest isopods known. The group reaches its greatest diversity in the deep sea (Menzies 1962; Wolff 1962) and in Antarctic and Subantarctic waters (Kussakin 1967).

The higher systematic arrangement of the families is at present uncertain. Several superfamilies have been erected from time to time. The Aselloidea Latreille, 1802 (comprising three families) are not known from Australia, but are represented by numerous genera and species from freshwater environments of the Northern Hemisphere. The absence of the Aselloidea from Australia is noteworthy; extensive freshwater invertebrate collections have been made and the absence is probably real.

Most Australian Asellota belong in the families usually assigned to the Janiroidea Sars, 1897 and one genus, Stenetrium, to the Stenetrioidea Hansen, 1905.

The 'Janiroidea' may be loosely defined as follows: Pereonites without sternal keels or spines in midline. Pleon variously shaped, smooth or spinose, but posterolateral margin without single spines anterior to uropodal insertions. Pereopod 1 simple, subchelate or chelate; carpus elongate, not small or triangular, partially or wholly participating in opposition to propodus and dactylus. Female spermathecal duct ('cuticular organ') either adjacent to oopore or separate and situated anterodorsally on pereonite 5, without coupling apparatus around external opening. Male penes on posterior margin of sternum of pereonite 7, forming cone or tapering and adjacent. Male pleopods 1 and 2 forming operculum, or rarely pleopod 3 forming operculum with pleopods 1 and 2 being smaller. Female pleopods 1 absent; pleopods 2 fused and forming operculum. Male pleopods 1 fused to single elongate unit with medial sperm channel along line of fusion, with distal tip having posterior opening of sperm channel and stylet guides; anterolateral section of male pleopod 1 with locking ridges engaging pleopods 2. Male pleopod 2 endopod proximal article with exopod coupling ridge; distal article of endopod forming elongate sharp stylet with posteromedial opening of sperm tube; sperm tube extending to stylet tip; exopod fused into single article with blunt, anteriorly directed hook on dorsomedial part of distal tip. Pleopod 3 biramous, both rami lamellar, rarely sexually dimorphic.

In terms of the same characters 'Stenetrioidea' are defined as follows: Pereonites with sternal keels or spines in midline; pereonite 7 with large, posteriorly directed spine. Pleon broad, with convex lateral fields; posterolateral margin with single spines anterior to uropodal insertions. Pereopod 1 propodo-subchelate; carpus trapezoidal. Female pereopod 1 propodus with numerous setae on lower margin. Cuticular spermathecal duct present; cuticular part of duct short with wide atrium on anteroventral surface of pereonite 5, in common groove with ventral oopore; spermatheca thin-walled, attaching to ventral cuticle and spermathecal duct. Male penes positioned between coxae 7 and midline, not on coxae. Male pleopods 1 and 2 not opercular, subequal, rami less than half pleotelson length. Female pleopods 1 absent; pleopods 2 fused and small, not operculiform. Male pleopod 1 peduncle fused medially; rami without distal stylet guides. Male pleopod 2 mostly covered by pleopod 1, basal article enlarged, distally tapering; endopod position on distal half of peduncle; appendix masculina with groove on posterior surface; exopod inserting distomesially. Pleopod 3 biramous; exopod opercular, with oblique suture, not sexually dimorphic.

Wilson (1987) questioned the superfamily arrangement at the same time as proposing another, Pseudojaniroidea, for the family Pseudojaniridae. The relationship between the Stenetriidae and Pseudojaniridae was reviewed by Serov & Wilson (1995, 1999) who placed the two in Stenetrioidea which they diagnosed in detail (1999). Serov & Wilson (1999) provided a key to the two families and 12 genera.

None of these superfamilies will accommodate the Vermectiadidae, a family with many primitive stenetrioid and pseudojaniroid features (Just & Poore 1992). Another small superfamily, Gnathostenetroidoidea Kussakin, 1967, originally proposed for just one genus, Gnathostenetroides, is represented in Australia only by undescribed species from deep water off south-eastern states.

In a paper describing an Australian pseudojanirid species (Poore & Just 1990) and in another describing Vermectias nelladanae from Macquarie Island (Just & Poore 1992) the two authors doubted the validity of the superfamily arrangement then in use. This view was reinforced by Wilson (1994) in a study of Janiridae. His unpublished phylogenetic analyses suggest that none of the superfamilies is likely to be monophyletic (G.D.F. Wilson pers. comm.). Given the uncertainty no superfamilies are used in this Catalogue.

Wolff (1962) provided a useful practical key to families and many genera.

Although the Australian marine asellote fauna is exceptionally diverse (249 species in 25 families having been recorded from the continental slope of south-eastern Australia: Poore et al. 1994), few species have been described from Australia. The families were not listed in this work but included among the many undescribed species are most of the families described in the literature and some possibly new, many more than appear in this Catalogue.

The Asellota are highly variable and difficult to diagnose. The following diagnosis is drawn from the synapomorphies of Wägele (1989) and Brusca & Wilson (1991).

Diagnosis

Isopoda with at least pleonites 4 and 5 fused with telson, usually pleonite 3 and sometimes pleonites 1 and 2 fused into pleotelson. Pleopod 1 of female absent. Pleopod 1 of male without endopod, sometimes peduncles and sometimes rami of pair fused. Pleopod 2 of male with endopod modified as a folded gonopod.