Main Page | Isopoda

Suggested Common Name: Isopods
Number of subordinate taxa: almost 10800 species in 12 suborders globally, over 1500 species in 8 suborders currently known from our area. The 4 suborders not in our area are all tiny, with the largest being Phreatoicidea (south-temperate freshwater) with 113 currently-described species
Etymology: ἴσος (ísos) = equal + πόδιον (pódion) = leg [both Ancient Greek], likely referring to the similarity of the pereopods to each other. Unfortunately for this name, numerous groups of isopods specialize specific sets of legs for specific actions and the legs are rarely all similar to each other, although it's true when comparing them to the often highly segregated pereopods of Amphipods and Decapods. Common name is an angelization of the scientific name.
Taxonomic History: Tetracera (in part) [unkn. origin, pre-1817] [INCOMPLETE]
Size Range: ranging from less than a millimeter to a quarter of a meter.
Description: (partially modified from Poore, 2002) Body usually dorsolaterally flattened (tubular in many burrowing/intersitial taxa, laterally compressed in extralimital Phreatoicidea. Head lacking carapace. Eyes sessile, never articulating, sometimes set on raised bumps or pedicels. Antenna 1 typically uniramous. Antenna 2 uniramous or with a small exopod (scale). Thoracomere 1 fused to head with the appendages on this segment forming the maxillipeds. Maxilliped epipod set laterally (sometimes absent); endite set distally (sometimes absent). Thoracopods 2-8 (=pereopods 1-7) uniramous (except in Atlantasellus dominicanus), coxae ring-shaped (in Phreatoicidea, Asellota and Microcerberidea) or fused to tergites as dorsal plates (Scutocoxifera), often with ventral plates. ♀ gonopores on pereonite 5. ♂ penes separate, fused or reduced, situated on coxal plate 7, pereonite 7 or pleonite 1, connected to base of copulatory pleopods at least during copulation; insemination internal. ♂ appendix masculina on medial or distal margin of pleopod 2 endopod, frequently fused with endopod. Pleopods 1-5 usually biramous (sometimes uniramous, rarely triramous), respiratory. Pleopods 1-2 present or absent. ♂ pleopod 2 frequently modified into a coupalatory structure. Pleonite 6 fused with telson. Pleopod 6 modified into a uropod (except Anuropidae) or very rarely absent. Life cycle ovoviviparous, with eggs and younger juveniles held in a brood pouch formed from oostegites (medial extensions of the coxae), rarely from internal pouches; young released as mancae (with pereopod 7 absent and pereonite 7 reduced). Molting biphasic.
Type taxon: Oniscidea Latreille, 1802 [assigned due to the first genus of isopods described being Oniscus]
Notes: Over a tenth (about 14%) of the world's isopods occur in North America. The North American isopod fauna can be split into multiple regions based on whether they are terrestrial, freshwater or marine. Terrestrial isopods are mostly Neotropical in our area, with a strong element reaching as far north as southern Florida and portions of it (in the form of native Venezillo) reaching the US southwest. The Nearctic fauna is extremely depauperate but from the native species present an affinity with the Palearctic can be made, with many cave genera (most noticeably Amerigoniscus and Brackenridgia) being close to or nestled in Eurasian clades. Most native Nearctic terrestrial isopods are coastal, with representatives being from both cosmopolitan and subcosmopolitan clades (Littorophiloscia, Armadilloniscus and Alloniscus) with a few regional endemics (Scyphacella and Detonella). A vast majority of the terrestrial isopods present north of Mexico, however, are more recent introductions from Europe (rarely other regions) that occured in the past 400 years, with the oldest records of land isopods being  the original description of Porcellio spinicornis (Brickwork Woodlouse) in Philadelphia in the late 18th century.
Freshwater isopods in North America can be divided into three ecological groups: ancient relicts from at least as far back as the Triassic, more "recent" clades derived from oceanic inundations into what is now land during the Cretaceous and the recent coastal stragglers. The ancient group contains Aselloidea and Microcerberus, which have been present in what is now North America in their present forms as far back as the Triassic, prior to Pangaea splitting up. Many Nearctic Aselloidea genera are closer to Palearctic genera than to each other, such as the western troglobitic genera being closer to Asellus proper and Caecidotea appearing to be close to 2 western European genera. Nearctic Microcerberus on the other hand are congeneric with species in the Balkans, over 8000km away! The more "recent" freshwater group are members of marine taxa that became stranded in inland waterways as water levels dropped since the onset of the Cenozoic ice ages. The most recognizable members of this ecological group are members of Cirolanidae (prov. com. name Dungeondivers) that formed an immense radiation of endemic troglobitic genera and species around the margins of the Caribbean Sea, ranging as far north as Atrolana in the Virginias but becoming the most diverse in Texas and Yucatán. Some Microcerberids also seem to have had followed a similar path, with Texicerberus and Mexicerberus appearing to have arisen this way. Finally, there are the coastal stragglers, members of predominantly marine taxa that have a heightened tolerance to lower salinities and thus wander up into coastal rivers and lakes. Prominent members of this ecological group include Chiridotea arenicola and various members of Cyathura in the eastern Nearctic and Saduria in the western Nearctic. Also in this ecological group are parasities that wander upstream with their hosts, including Anilocra acuta (a marine species that specifically hosts on gar, a freshwater fish) and the Probopyrus pandalicola complex, the latter which reaches as far north as the Ohio River drainage on its Glass Shrimp hosts.
Marine Isopods in our area have a more complex biogeography. Nearshore isopods in our area can be split into roughly 6 regions: North Atlantic boreal (south to the Gulf of Maine), North Atlantic temperate (south to the Blake Plateau), Caribbean coasts and islands (including the entire Gulf of Mexico), East Pacific tropical (north to the Gulf of California), Temperate North Pacific (Cabo San Luis to the Aleutian Islands) and Boreal North Pacific (Bering Sea and nearby regions). There is also a distinct deepsea region and a distinct pelagic region that both seem more or less monotonous across the World Ocean. The pelagic region is fairly depauperate of isopods, with only Idotea metalica, Anuropidae and potentially Xenuraega being the only cosmopolitan taxa that occur in our area. The Sargasso Sea also contains a small handful of  endemics, most noticeably Carpias minutus and two species of Bopyrids.
Taxonomically, Isopoda is the largest member of Peracarida, a fairly large superorder that is defined by the presence of oostegites, extensions of the pereonal coxa that forms a brood pouch that holds eggs and young. The young emerge as miniature adults called mancae, which have the rearmost pereopod (the seventh in isopods) absent. The last pereopod usually develops as the young moult, but in many neotenic taxa (including a few genera in Isopoda) never develop it. The second most specious order in Peracarida, Amphipoda, is also widespread and abundant, and many aquatic species can be confused for isopods by casual naturalists, but can easily be distinguished by the presence of 3 uropods (pleopods 4-6 modified into uropods) on a distinct tagma of the pleon called the urosome that often has its segments still distinct. In isopods, only the last pair of pleopods (pleopod 6) has been modified into uropods (except in Anuropidae), while the sixth pleonite is fused with the telson in all species. Another order, the Tanaids, used to be considered isopods (as suborder Chelifera) but can be distinguished by the chelate pereopod 1 (only present in Katianiridae in Isopoda), the distinct carapace and the fairly simple pleon morphology. Biphasic molting also appears to be unique to isopods, with all other Peracarideans (and arthropods in general) usually moulting in one large piece.

Subordinate taxa: Asellota, Cymothoida, Epicaridea, Limnoriidea, Microcerberidea, Oniscidea, Sphaeromatidea, Valvifera

Key to suborders of Isopoda
1 a. Parasitic on crustaceans, often with a highly derived shape due to parasitic lifestyle; marine (Probopyrus frequently freshwater) --> Epicaridea
b. Freeliving or parasitic on vertebrates or jelly animals, body shape typical of isopods; marine, freshwater or terrestrial --> 2

2 (1) a. Uropods Asellotiform (=set distally on telson, with terete protopods and rami); pleon with 0-2(-3) free pleonites; all or at least some pereopodal coxae free, ring-like (entirely modified into coxal plates in Coxicerberus); ♀ pleopod 1 absent; ♂ pleopod 2 appendix masculina fused with endopod --> 3
b. Uropods Flabelliform (=set laterally or ventrally on telson, with flattened protopods and rami) or with lateral but terete uropods (Onsicidea); pleon with 0-5 free pleonites; all pereopodal coxae modified into coxal plates, often fused with tergites; ♀ pleopod 1 usually present; ♂ pleopod 2 appendix masculina free (fused with endopod in Oniscidea) (Scutocoxifera) --> 4

3 (2) a.  ♂ pleopod 2 endopod straight, set distally and colinear with protopod; ♀ pleopod 2 absent; pereopods 5-7 coxae formed into coxal plates (ring-like in extralimital Afrocerberus and Protocerberus); body elongate-tubular (Microcerberidae) or conglobulating (Atlantasellidae), occuring in subterranean and intersitial environments --> Microcerberidea
b. ♂ pleopod 2 endopod geniculate, often 2-semgneted, set distomedially and not colinear with protopod; ♀ pleopod 2 present; pereopods 5-7 coxae ring-like; body shape and ecology various, although especially common in freshwater or deepsea environments --> Asellota

4 (3) a. Antenna 1 extremely reduced, practically microscopic (making it appear as if theres only 1 set of antennae at first glance), 1-3-segmented; ♂ pleopod 2 appendix masculina fused with endopod; underside of body with advanced water-conducting system formed from cuticular scales; terrestrial and secondarily semiaquatic (especially in caves) --> Oniscidea
b. Antenna 1 not as above, usually visible at first glance; ♂ pleopod 2 appendix masculina free; underside of body without water-conducting system; marine and freshwater --> 5

5 (4) a. Underside of telson domed with marginal ridges, forming a branchial chamber --> 6
b. Underside of telson flat, lacking marginal ridges --> 7

6 (5) a. Uropods set under the telson, forming a "double-doors" operculum over the pleopods; body usually somewhat elongate (compact in Chaetiliidae) --> Valvifera
b. Uropods set next to the telson, forming a tail fan; animals rotund or flat (generally compact) with many common species being able to conglobate --> Sphaeromatidea

7 (6) a. Mandible lacking a molar; maxilliped endite long and slender, reaching at least to palp segment 4; tiny social marine wood- or seagrass-borers usually with conspicous ridges on the telson --> Limnoriidea
b. Mandible with molar blade-like or conical (sometimes absent); maxilliped endite short (not reaching past palp segment 1) or absent; ecology varied, usually highly carnivorous --> Cymothoida

Sources

Brandt, A., & Poore, G. C. (2003). Higher classification of the flabelliferan and related Isopoda based on a reappraisal of relationships. Invertebrate Systematics, 17(6):893-923.

Poore, G. C. (2002). Isopoda. Zoological Catalogue of Australia, 19:26-31.

Wägele, J. W. (1983). On the origin of the Microcerberidae (Crustacea: Isopoda). Journal of Zoological Systematics and Evolutionary Research, 21(4):249-262.

Published: Jan 1, 2023
Updated: Nov 1, 2023