Isopoda are typically dorso-ventrally compressed peracarid crustaceans without a carapace (because secondarily lost). Of all the crustaceans, isopods are the most diverse in body form and the body is by far not always flattened. Isopods can be distinguished from other similar crustaceans because they have only one pair of uropods and lack strong clawed first thoracic legs (or pereiopods).
The cephalon is fused with the first (rarely also with the second) thoracal segment. There are seven (rarely six) free thoracal segments (or pereion somites). Abdomen with six segments (pleon somites); some or all pleon somites may be fused with the terminal telson, forming a pleotelson. The eyes are composite, unstalked, or absent
Appendages: one pair of maxillipeds, seven pairs of uniramous pereiopods, or walking legs (except in the Family Gnathiidae, that have one pair of maxillipeds, one pair of pylopods and five pairs of pereiopods). The pereiopods are generally more or less similar "(iso-poda").
The pereiopods are without a chelae, but a subchela may be present; without epipodites. In females, some of the anterior pereiopods are provided with oöstegites: lamellae that enclose the embryos in a brood chamber.
The pleopods are biramous and leaf-like; the pleopods 1-5 are respiratory, the sixth pair is the uropod.
The cephalon bears a pair of antennules (a1) and a pair of usually larger antennae (a2), each consisting of a basal peduncle and a distal flagellum, followed by ventrally placed mandibles, maxilulles and maxillae. All the cephalic appendages are usually shielded under a pair of maxillipedes, the appendages of the first true pereion somite that is fused to the head.
The seven pereion somites each bear a pair of uniramous pereiopods, consisting of seven articles (or segments) : (1) coxa, (2) basis , (3) ischium, (4) merus, (5) carpus, (6) propodus, or propus, (7) dactylus (or dactyl).
Extending laterally from the coxae at the articulations between the body and the legs, are the coxal plates, which are often visible, from above, at the sides of the dorsal tergal plates; ventrally are the sternal plates.
The pleon somites bear biramous pleopods (but may be absent in some species or females of some parasitic isopods). The sixth pair of pleopods are the uropods and differ much in shape from the other pleopods, in fact they are of systematic importance.
The telson is an unpaired appendage of the sixth pleon somite (or abdominal segment), to which it is fused. The telson differs very much in shape and is therefore also an important systematic character.
Isopoda are ubiquists, occurring in the marine environment from the littoral to abyssal depths, or in estuaries and freshwaters; even a group of isopods became, as the only crustaceans, completely terrestrial in habit. Generally, isopods are benthic, some families are entirely parasitic. Isopods occur in the plankton occasionally; juveniles (or praniza) of the Family Gnathiidae are parasitic on fish, but can be found swimming around in the plankton also.
[Holthuis, 1956; Naylor, 1972]
The key to the major planktonic isopod species in the North Sea starts at Page 181: Isopoda.
The following isopod species are included: