Asellota Latreille, 1802

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Suggested Common Name: Asellotes
Number of subordinate taxa: about 2300 species in 4 superfamilies and 1 incertae sedis family worldwide, 487 species in all four superfamilies occurring in our area, making it the second most diverse superorder in our area (just short of Cymothoida's 489 species)
Etymology: after Asellus Geoffroy, 1762, see that account for details. Common name is an anglicized version of the scientific name
Taxonomic History: Asellota Latreille, 1802.
Description: Body generally broad (elongate in some derived species), dorsoventrally compressed to terete; post-manca juveniles not modified, adult sexual dimorphism minute to extremely conspicuous, pleopods 1-2 always with a high amount of sexual dimorphism, other features variably sexually dimorphic depending on the taxon. Antenna 1 unmodified, usually small but prominent, set dorsally of antenna 2; scale absent. Antenna 2 scale present (frequently reduced or lost). Mandible lacinia mobilis variable, present on both sides or reduced on one or both sides; spine row consisting of denticulate spines (frequently reduced to absent); molar present to absent, extremely variable, generally cylindrical with a distal grinding surface but can be flat and blade-like, distal margin with or without teeth, spines (when present) distally dentate, articulation not described; palp present, 3-segmented. Maxilla 1 present, biramous; exopod with 9 or more robust setae; endopod subsimilar in length to exopod with 3-4 brush-like setae to much shorter than exopod with 1-2 brush-like setae. Maxilla 2 triramous (reduced and vaguely biramous in Microcerberidae); endites generally slender, longer than wide, with numerous apical setae (very broad with 1 seta each in Microcerberidae). Maxilliped epipod present (absent in some derived groups), elongate to somewhat ovate; endite long, truncate with distal setae, usually at least reaching base of palp segment 3, usually longer than wide, plumose robust setae present (reduced in some derived taxa); palp 5-segmented,  segments 4-5 roughly conlinear, without terminal hooks. Pereonite 1 usually articulated to head. Pereonite 7 subsimilar to other pereonites. Pereopod 1 variable, ranging from ambulatory to subchelate to chelate. Pereopods 2-7 coxae all ringlike (ringlike on 2-4, platelike on 5-7 on most Microcerberoids, all modified into coxal plates in Robustura, plates in either case articulated). Pereopod 7 present in non-mancae. Penes lateral, at pereopod 7 bases. Pleonites 1-5 variously fused, usually into 1-2(-3) segments anterior to telson but can also be entirely fused to telson, combined length much shorter than telson. Pleonite 1 (when present) subequal in width to pleonite 2. Pleopods 1-5 epipods absent. Pleopod 1 sexual dimorphism extremely prominent, ♂♂ uniramous (absent in Microcerberoidea), commonly with endite and protopod fused or entirely fused into one piece, covering pleopod 2 appendages, ♀♀ absent. Pleopod 2 sexual dimorphism extremely prominent, ♂♂ biramous, exopod often ovate, often assisting in “thrusting”, endopod geniculate (colinear in Microcerberoidea), indistinguishably fused with appendix masculina, ♀♀ usually vastly simplified (absent in Microcerberoidea), uniramous, either very small and simple or larger and operculate (frequently as a single fused piece). Pleopod 3 endopod ovate, tip blunt. Telson wider than deep; ventral surface flat, without ventrolateral shelf, uropod notches present to absent. Uropod generally terminal or subterminal but can vary; rami terete.
Type taxon: Aselloidea Latreille, 1802
Notes: This superorder is the second largest suborder in Isopoda, trailing Oniscidea by over 1500 species but with over 1000 species more than the next specious suborder Cymothoida. Similarly to Oniscidea, the taxonomy within Asellota is in constant flux, with the largest superfamily in the suborder, Janiroidea, very likely being paraphyletic. The current superfamilial classification is based off of details of the reproductive pleopods, but within Janiroidea the supposed diagnostic traits are highly variable among subordinate members, with some families even approaching some of the other superfamilies in terms of pleopod details. Thus, to facilitate identification, global and region keys to families (and unplaced genera) are provided below alongside the superfamilial key.
Asellota contains two very large radiations into two very disparate environements. Members of the families Asellidae and Stenasellidae, which make up the monophyletic superfamily Aselloidea, are solely found in freshwater environments, where they probably have occured since the middle Proterozic. Although mostly restricted to a fairly spotty range around the Holarctic (some genera of Stenasellidae occur in the Afrotropics), the Aselloids are extremely diverse in areas they occur in, especially in wetter unglaciated regions such as the southeastern US and southern Europe. Members of Stenasellidae are entirely subterranean, while members of Asellidae are primarily epigean with massive radiations into frequently isolated subterranean environments. Some members of Asellidae have been introduced and become invasive in areas outside their native ranges (although interestingly enough only in the native range of the superfamily), in our area that includes multiple species of Conasellus native to the eastern US but introduced to the western US and Asellus hilgendorfi, which has been introduced to the San Joaquin delta in California.
The other massive radiation in Asellota occurs in bathyal and abyssal marine waters, where members of the paraphyletic superfamily Janiroidea are among the most diverse groups of crustaceans. Unlike the Aselloidea, which have remained morphologically consistent across the entire radiation, members of Janiroidea dramatically diverge from each other morphologically. Some of the more extreme deriverations include splitting the pereon into two distinct tagma (especially prominent in Munnopsidae), 

Subordinate taxa: Aselloidea, Gnathostenetroidioidea, Janiroidea, Microcerberoidea, Stenetrioidea

Key to superfamilies
1 a. Pleopod 3 operculate, with shorter pleopod(s) visible in front of it; marine or freshwater → 2
b. Pleopod 3 not operculate, either pleopod 1 (♂), pleopod 2 or a combination of the two acting as an operculum, the resulting operculum with no shorter pleopods in front of it; marine (some Janiroidea freshwater along the immediate coast) → 3

2 (1) a. ♂ pleopod 1 broad, opercular, with protopod present, endites free; ♂ pleopod 2 small and hidden behind pleopod 1; ♀  pleopod 2 with protopod present; tropical shallow water and N Caribbean anchialine → Gnathostenetroidoidea
b. ♂ pleopod 1 narrow, protopod fused to endites; ♂ pleopod 2 large, forming operculum with pleopod 1; ♀ pleopod 2 lacking protopod; widespread habitats from polar to tropical, especially abundant in the deep sea → Janiroidea

3 (1) a. ♂ pleopod 2 endopod straight, set distally on and colinear with protopod; ♀ pleopod 2 completely absent; pereopods 5-7 coxae formed into coxal plates (ring-like in extralimital Afrocerberus and Protocerberus); body elongate tubular (Microcerberidae) or robust and conglobulating (Atlantasellidae); occurring solely in subterranean and intersitial environments --> Microcerberoidea
b. ♂ pleopod 2 endopod geniculate, often 2-semgented, set distomedially and not colinear with protopod; ♀ pleopod 2 present, small or moderately large; pereopods 5-7 coxae ring-like; body shape and ecology various, although especially common in freshwater or deepsea environments (also occurring, sometimes quite abundantly, in subterranean and intersitial environments) --> 4

4 (3) a. ♂ pleopod 1 protopods separated; ♂ pleopod 2 exopods large, circular, often fringed with setae; ♀ pleopod 2 separated; Nearctic freshwater south to Guatemala (sometimes swept to sea after storms) → Aselloidea
b. ♂ pleopod 1 protopods fused; ♂  pleopod 2 exopods small; ♀  pleopod 2 fused; tropical-marine → Stenetrioidea

Sources

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Published: Jan 1, 2023
Updated: Aug 15, 2025