Definition of the Asellota

Author: George D.F. (Buz) Wilson

The Asellota are perhaps the easiest isopods to define owing to their specialised copulatory apparatus. To make it clear what is meant by Asellota, this list of features is offered as a definition. An asterisk is placed on the features that are easiest to identify on specimens in a dish. See the glossary for explanations of the terms.

1. *Pleonites 4-5, and often pleonite 3, are fused to pleotelson, making an enlarged and often wide terminal segment. Although fusion of pleonites is an evolutionary trend in many isopods, this feature defines all Asellota, and is an apomorphy of the entire clade.
2. *Pleonites 1-2, and rarely 3, if distinct from pleotelson, are only small rings or cuticular bars visible ventrally. Sometimes the pleonite articulation will be absent and the pleotelson is a single large segment.
3. *Either pleopods I, II, or III will form a distinct operculum over more posterior branchial pleopods (Asellota never swim with their pleopods).
4. Male pleopods II with specialised copulatory apparatus consisting of an enlarged protopod, a geniculate (knee-like) endopod, and typically a well-muscled exopod. This is a defining synapomorphy of the Asellota.
5. *Pleopod I is absent in adult females and female-like manca instars 1-3.
6. Antenna with a distinct basal (precoxal) segment; articulation is not expressed in some advanced groups). This feature is plesiomorphic but absent in many isopod taxa. As a result, the antenna will have 3 basal podomeres, and three robust distal podomeres, followed by a flagellum of varying lengths.
7. *Pereopodal coxae are ring-like (plesiomorphic) and do not form broad plates characteristic of the higher isopods (see Brusca and Wilson, 1991; referred to as "Scuticoxifera" in Dryer and Wägele, 2002). The coxae may have lateral spines or other projections.
8. *Uropods with narrow rami, also a plesiomorphy (Brusca and Wilson, 1991). The uropods vary enormously among the Asellota, but they are never broad and fan like as in the flabelliferan isopods.
9. As far as is known, Asellota have a specialised sperm conduit in the female alternatively named either the cuticular organ or the spermathecal duct. The opening to this duct will either be adjacent to the oopore on pereonite 5 or on the anterolateral or anterodorsal surface of pereonite 5 (Wilson, 1991).
10. Females with medial oostegites on the medial margins of coxae of pereopods I-IV (thoracopods II-V, and sometimes extending posteriorly from the coxa of the maxilliped (thoracopod I). This feature is shared with the Phreatoicidea. Other isopods can have oostegites on more posterior coxae (Brusca and Wilson, 1991).
11. Although the precise definition has not been clarified, all Asellota are prognathous, i.e., the mandibles are rotated anteriorly so that the mandibular incisors meet at the anterior margin of the head, instead of below the head as in most isopods (e.g., Phreatoicidea). As a result, the clypeus and/or labrum are often visible in dorsal view. Some taxa (e.g., Sugoniscus or Thaumastosoma) are even more prognathous so that the mandibles extend anteriorly and are visible in dorsal view

Many other features are basally plesiomorphic in the asellotans, such as the form of the antennulae and antennae, but are modied considerably among the Janiroidea. This definition excludes the families Atlantasellidae (living in saltwater caves on islands) and Microcerberidae (living interstitially on beaches). Both families lack the distinctive male copulatory apparatus found in the Asellota (on the male pleopod II: a geniculate endopod and a strong exopod with a copulatory hook). In addition, the pleonites 1-2 are larger and much more distinct in these two families than they are in most Asellota (Stenasellidae can also have large segments). The Atlantasellidae and Microcerberidae should be classified in the suborder Microcerberidea (see Brusca and Wilson, 1991).