of the Asellota
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.
- *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.
- *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.
- *Either pleopods I, II, or III will form a distinct operculum over more
posterior branchial pleopods (Asellota never swim with their pleopods).
- 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.
- *Pleopod I is absent in adult females and female-like manca instars 1-3.
- 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.
- *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.
- *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.
- 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).
- 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,
- 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).