Main Page | Isopoda

Suggested Common Name: Isopods
Number of subordinate taxa: over 11000 species in 12 suborders globally, over 1500 species in 8 suborders currently known from our area.
Etymology: ἴσος (ísos) = equal + πόδιον (pódion) = leg [both Ancient Greek], either referring to all the legs pointing in a roughly similar direction (in comparison to Amphipoda [amphi- = around or on both sides in New Latin], which refers to the legs being splayed out in multiple directions) or refering to the similarity of the legs to each other. Unfortunately for the latter interpretation, numerous groups of isopods specialize one or more sets of legs for various actions. Common name is a very common angelization of the scientific name, similar to those of other peracaridans (Amphipoda, Tanaids, etc.).
Taxonomic History: Somewhat convoluted earlier on, with the known members of Isopoda being shoved into various now-obsolete groups containing other crustaceans before finally being placed in their own order by Latreille in 1817. After this point, the name has remained remarkably stable outside of the placement of the former suborder Chelifera into its own order Tanaidacea.
Size Range: Among the broadest of any crustacean order, ranging from around half a millimeter in some intersitial forms to over 300mm in the titanic genus Bathynomus.
Description: Body generally dorsoventrally compressed (terete to laterally compressed in some subtaxa); sexual dimorphism variable, ranging from negligible to extremely disparate. Head carapace absent. Eyes sessile (sometimes lobed). Antenna 2 peduncle generally 3-segmented. Maxilla 1 palp absent. Maxilla 2 palp absent. Thorax with only 1 segment fused to head (legs forming maxilliped), rarely 2 segments fused to head. Thoracopod 1 (maxillipeds) epipod present, cheeklike or at least otherwise forming a flap lateral to the basis (absent in some aberrant groups), not respiratory. Thoracopods 2-8 generally unmodified, if modified then most commonly with thoracopod 2 (pereopod 1) subchelate, in some groups (especially in Asellota:Janiroidea) most or all thoracopods immensely modified; leg flexion point between the basis and ischium; epipods and pereopodal gills absent. Pleopods 1-5 predominantly respiratory (but often used for swimming in many groups), when at rest laying flat against the venter of the pleon and/or telson, never reduced (except for in some extremely aberrant groups), all generally subsimilar (if modified then pleopods 1-2 modified for sexual reproduction and pleopods 3-5 respiratory). Pleopod 6 (uropod) terete in basal groups (Phreatoicidea and Asellota) but in most groups flat and fan-like, rami always uniarticulate. Molt cycle biphasic. Juveniles earlier stages manciform.
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. Most of this diversity is unequally spread across the continent, with large areas (especially the interior west of the United States and most of the Canadian taiga) having no native isopods. Diversity hotspots in our area include the Pacific coast (marine), the US Southeast (freshwater) and central and southern Mexico (terrestrial). Terrestrial isopods in particular are oddly sparse in temperate regions of North America, in contrast to the extreme diversity found in Europe (even away from the hyperdiverse Mediterranean region) and the moderate to extreme diversity in the temperate regions of the Southern Hemisphere. Although the specific cause of this sparsity is unknown, its likely a complex set of circumstances led to this oddity, including the glaciation of half the continent and the relatively narrow connection between the temperate zone and the tropical core of the region. Traditionally, North America has been split into two distinct biological regions, the Nearctic, containing the temperate parts of North America, and the Neotropics, containing the tropical parts of North America (including southern Florida). However, in certain groups of isopods (especially Philoscomorph woodlice and freshwater Asellids), it might be more appropriate to split the region between a North American core region (down to around Nicaragua and the northern Lesser Antilles) and a transition zone to South America (containing Costa Rica and Panama and the majority of the Lesser Antilles), which has a dramatically different fauna than the tropical parts of North America. In this view, the tropical portions of Mexico and Northern Central America along with the Greater Antilles represent the tropical core of North America, with the diversity tapering off in the harsher temperate regions (with the exception of the US Southeast, which has a massive radiation of Asellids and smaller radiations of some Oniscidean groups).
Taxonomically, Isopoda belongs to Peracarida, the largest superorder of Malacrostracan Crustaceans. The superorder Peracarida are defined by the presense of oostegites, extensions of the base of the legs in ♀♀ that form a brooding chamber holding the eggs and very young larvae. This means that unlike the vast majority of crustaceans, which lay eggs that hatch into free-swimming naupulus larvae, the members of Peracarida give live birth to (in most cases) minuature versions of the adults. In Isopoda and a few other related groups (including Tanaidacea and Cumacea), the first juvenile stage is slightly modified, with the final pereon segment reduced and the set of legs on that segment not growing in until a later stage. This early stage is called a manca, once the last pereon segment and the last set of legs grow in the young are typically refered to as juveniles. The orders with mancae are frequently put into a seperate group within Peracaridea called Mancoidea. Rarely in some groups the reduced last segment and the lack of the last set of legs is retained through adulthood, this state (a case of neoteny) appears to be most common in Gnathiidea (where the entire suborder is neotenic) and Anthuridea (where a few genera are neotenic).
Isopods overall are a very morphologically varied group, ranging from familiar pillbugs and woodlice to long-legged waterwalkers to incomprehensible sacks of eggs living inside crustacean hosts. Practically all isopods can be distinguished from other orders of peracaridans, though, by the unique respiratory pleopods that fold upward to lie along the bottom side of the pleon and/or the telson. All other peracaridan orders either breath through a modified extension of the maxillipeds in the carapace (Tanaidacea and Cumacea), through extensions of the pereopods (Amphipoda and Mysidacea) or through other means. In additon, isopods lack a carapace (a flap extending from the head to cover or fuse with one or more pereonites), which is only shared with Amphipoda and some minor mancoid orders. One last feature that might be unique to Isopoda is the unique biphasic molting, where the front and back halves of the body molt at different times (usually the dividing line is around the joint seperating pereonites 4-5.

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

Simplified key to Isopoda suborders present in North America
1 a. Uropods terete (rounded in cross section), if flattened (pillbugs) then small not forming a tail fan with the telson or covering the uropods → 2
b. Uropods lamellar (strongly flattened in cross section), usually forming a tail fan with the telson OR folded under the telson and covering the uropods → 3

2 (1) a. Pleon with about 5 segments visible dorsally; terrestrial (uncommon in freshwater) → Oniscidea [Woodlice and allies]
b. Pleon with 0-2 segments visible dorsally; marine to freshwater → Asellota [Asellotes]

3 (1) a. Parasitic on crustaceans; life cycle divided into 4 distinct life stages, adults usually contorted past recognition → Epicaridea [Eldritch Isopods]
b. Free-living or parasitic on fishes; life cycle at most divided into two stages, adults usually recognizable as isopods → 4

4 (3) a. Pereopods (walking legs) 5; juveniles ectoparasitic on fishes, adults not feeding; marine → Gnathiidea [Fishgnats]
b. Pereopods 7 (6 in all mancae and rarely in adults); ecology various, but usually with adults feeding; marine to freshwater → 5

5 (4) a. Body elongate, tubular; uropod exopod folded over telson, the uropods and telson forming a flower-shaped digging mechanism; marine → Anthuridea [Wormpods]
b. Body broad to elongate, flattened to tubular; uropod exopod not folded over telson; freshwater and marine → 6

6 (5) a. Telson domed, forming a chamber for the pleopods underneath; pleon with 0-3 segments free from the telson → 7
b. Telson flat, not forming a chamber underneath. pleon usually with 4-5 (rarely 3) segments free from telson → 8

7 (6) a. Uropods folding like double-doors, covering pleopods at rest, rarely visible from a dorsal view; body somewhat long and tubular or broad but tapering to a sharp point posteriorly → Valvifera [Valvetails]
b. Uropods folding like a fan, forming a tailfan with telson, not covering pleopods at rest, visible dorsally; body very broad, either flattened or domed and conglobulating → Sphaeromatidea [Seapills and Trisobites]

8 (7) a. Uropods not forming a tail fan; very small wood- or plant-boring isopods usually with conspicuously ornamented telsons → Limnoriidea [Gribbles and allies]
b. Uropods forming a tail fan (except in Anuropidae, where they are pleopod-like); larger predatory or parasitic isopods, telson rarely ornamented → Cymothoida [Fishbiters]

Technical key to global Isopoda suborders

1 a. Telson deeper than wide (subequal in Hypsimetopidae), uropods usually ventral to ventrolateral, terete; body laterally compressed (less obvious in Hypsimetopidae), with conspicuous vertical pleonite epimera covering most of pleopod protopods from lateral view (absent in Hypsimetopidae); at least some pleopod protopods with epipods; Gondwanan (South Africa, India, Australia and New Zealand) freshwater → Phreatoicidea [extralimital]

b. Telson generally wider than deep, uropods various; body dorsoventrally compressed to terete, pleonite epimera not conspicuously vertical, not obscuring pleopod protopods from lateral view; pleopod protopods without epipods; distribution various (including range described above) → 2

2 (1) a. Coxae generally ring-like (some or all platelike in most Microcerberidae, frequently with acute projections in many Janiroidea); pleopods 1-2 with extreme sexual dimorphism, in ♂♂ with a uniramous pleopod 1 (absent in Microcebreroidea) and a complex pleopod 2 with a geniculate endopod (colinear in Microcerberoidea) indistinguishably fused with appendix masculina, in ♀♀ with pleopod 1 absent and pleopod 2 uniramous; pleon extremely short, at most 2(-3)-segmented, much shorter than telson → Asellota

b. Coxae forming coxal plates (frequently fused to adjacent tergites); pleopods 1-2 with very little sexual dimorphism, at most with endopods more elongate and acute in males (Oniscidea and Calabozoidea), appendix masculina free from endopod (fused to endopod in Oniscidea and Calabozoidea, appearing fused through the appendix masculina migrating to the tip of the endopod in Tainisopidea and some Sphaeromatidea), ♀♀ pleopods 1-2 present (both absent in Calabozoidea:Brasileirinidae); pleon usually not as short as above, 0-5-segmented, frequently with segments variously fused to each other or to telson → 3

3 (2) a. Maxilliped reduced to a 2-segmented appendage (commonly with setae) or vestigial to absent, other mouthparts stylet-like, lacking any sort of processes (at most mandible toothed apically), held in an oral cone defined by 4 plates, or absent; life cycle highly derived, with 4 clear stages: a free-swimming epicaridium stage, an ectoparasitic microniscus stage (usually on copepods and other planktonic crustaceans), a free-swimming cryptoniscus stage and a highly derived adult stage; adult ♂♂ somewhat isopodiform (“normal”-looking) but highly simplified, very fleshy-appearing (Bopyroidea) or neotenic, closely resembling the cryptoniscus larvae (Cryptoniscoidea), adult ♀♀ highly derived, ranging from isopodiform but highly distorted and extremely fleshy to an incomprehensible sack of eggs pinned down to host with an anchor; ecto- and endoparasitic on crustaceans → Epicaridea

b. Maxilliped not as above, if other mouthparts stylet-like (Gnathiidea and some Cymothoida) then maxillipeds different in appearance (rarely reduced) and parasitic on vertebrates; life cycle not as above, juveniles effectively smaller versions of the adults (some vertebrate-parasitic isopods with a distinct stage before adulthood but these still mostly resemble the adults in some form or fashion); adults normal, isopodiform, generally not extremely modified; ecology various, including vertebrate parasites (crustacean ectoparasites in Tachaea) → 4

4 (3) a. Pereopods reduced to 5 pairs, with pereopod 1 modified into a second maxilliped (pylopod) and pereopod 7 absent due to neoteny; ♂ mandibles conspicously protruding foward, usually lacking recognizable processes (outside of knobs and teeth); pereon boxy, highly inflatable in juveniles (pranzias), inflated in ♀♀ and boxy with the consistency of hard cheese when cut in ♂♂; pranzias vertebrate-ectoparasitic in a manner similar to biting midges on land, adults non-feeding → Gnathiidea
b. Pereopods in 7 pairs (6 in mancae and adults in some neotenic taxa), pereopod 1 ambulatory or subchelate but not modified into a second maxilliped; ♂ mandibles not projecting foward (or if projecting foward in sucking species then with the entire mouth rotated foward), incisor, lacinia, spine row and/or molar usually present (absent in some derived species); pereon various, not inflatable nor the consistency of hard cheese; ecology various, including vertebrate parasites, but adults in general feeding (except for some ovigerous Sphaeromatidea:Sphaeromatidae) → 5

5 (4) a. Uropod exopod folded over telson, the uropods and telson together forming a “flower-shaped” digging apparatus; body extremely slender, cylindrical, animals burrowing into sediment and crevices → Anthuridea

b. Uropod exopod not folded over telson, usually plane with endopod, the uropods and telson not forming a digging apparatus; body generally broad or slender, ecology various (including burrowing) → 6

6 (5) a. Pleonites 1-2 embedded into pleonite 7, not reaching lateral margins; uropods subterete (flattened in some conglobulating Oniscidea); ♂ pleopod 2 endopod fused with appendix masculina; mandible palp absent; terrestrial and freshwater → 7

b. Pleonites 1-2 free, reaching lateral margins; uropods lamellar, forming a tail fan with telson (folding ventrally to cover pleopods in Valvifera, smaller and not creating a tail fan in Tainisopidea and some Limnoriidea); ♂ pleopod 2 endopod free from appendix masculina (appendix masculina apical on endopod in Tainisopidea and some Sphaeromatidae, creating a single bisegmented structure); mandible palp present (absent in Valvifera and Plakarthrium, reduced to absent in some Limnoriidea and Cymothoidea); marine → 8

7 (6) a. Antenna 1 fairly large, prominent, set dorsally of antenna 2; maxilla 2 triramous; uropods minute, lacking rami; restricted to freshwater in South America → Calabozoidea [extralimital]

b. Antenna 1 nearly microscopic, reduced to a 3-segmented stub set medially of antenna 2 (making the animal appear to only have 1 set of antennae); maxilla 2 uniramous; uropods usually larger, with rami (may be reduced to 1 ramus in some taxa); cosmopolitan in terrestrial environments (occasionally occurring in freshwater and marine environments) → Oniscidea

8 (6) a. Telson domed, ventral surface creating a branchial chamber housing the pleopods outlined by ventromarginal ridges; pleopod 3 endopod usually subtriangular, the distomedial corner at least sharper than the distolateral corner → 9

b. Telson flat, ventral surface not creating a branchial chamber, lacking ventromarginal ridges; pleopod 3 endopod ovate, blunt-tipped → 10

9 (8) a. Uropods set ventrolaterally, articulating laterally, forming a tail fan with the telson; generally broad ovate animals, either strongly ventrolaterally compressed or domed but conglobulating; mandible palp present (absent in Plakarthrium) → Sphaeromatidea

b. Uropods set ventrally, articulating like double-doors, forming an operculum over the branchial chamber; generally elongate animals (broad in Chaetiliidae but then distinctively tapering to an acute posterior point), usually somewhat ventrolaterally compressed or terete; mandible palp absent (present in Holognathus stewarti) → Valvifera

10(8) a. Pleopods 3-5 (and 2 in ♀♀) endopod with 2-3 large lamellae, making the pleopods appear tri- or quadriramous; antenna 1 peduncle segment 3 with scale; ♂ pleopod 2 appendix masculina apical on endopod, the combined structure appearing as a single biarticulate ramus; freshwater, restricted to western Australia → Tainisopidea [extralimital]

b. Pleopods 3-5 endopod without large lamellae; antenna 1 peduncle segment 3 without scale (with scale in Bathynomus and most of Limnoriidea); ♂ pleopod 2 appendix masculina usually basiomedial on endopod (uncommonly set more distally); primarily marine (some members of Cymothoida freshwater), cosmopolitan → 11

11(10) a. Pereopods 3-4 segments extremely broad, dactyl immensely reduced; pereonite 7 coxal plate much smaller than other coxal plates; only known from 2 collections from off of southern Australia → Phoratopidea [extralimital]

b. Pereopods 3-4 segments not as broad (somewhat broad in some burrowing Cirolanidae), dactyl not reduced; pereonite 7 coxal plate similar in size to other coxal plates (reduced in Hadromasticidae) → 12

12(11) a. Mandible molar present; maxilliped endite very long, reaching past palp segment 3; antenna 1 peduncle segment 3 with scale (except in Lynseia himantopoda, Keuphyliidae and potentially Hadromasticidae); uropods ventral, not quite lamellar, not forming a tailfan (ventrolateral, lamellar and forming a tailfan in Hadromasticidae); uncommon marine detritivores, woodborers and plant miners → Limnoriidea

b. Mandible molar absent; maxilliped endite reduced, not reaching past palp segment 2; antenna 1 peduncle segment 3 without scale (except in Bathynomus); uropods ventrolateral, lamellar, forming a tailfan (reduced, pleopod-like in Anuropidae); common to abundant marine carnivores, scavengers and vertebrate parasites (uncommonly in freshwater, rarely wood-boring) → Cymothoida

Sources

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