Epicarideans are ectoparasites of other crustaceans (malacostracans, ostracods, copepods, and cirripeds). Females are usually greatly distorted, being little more than an egg sac in some species. Males are symmetrical, but minute and live on the body of the female. Eyes are usually present in males, but reduced or absent in females. The antennules (first antennae) are very reduced, usually of only two or three articles; a 3-articulate peduncle is generally apparent only in juvenile stages. The antennae (second antennae) are vestigial in adults. The mouthparts are reduced, forming a suctorial cone with a pair of piercing stylets formed from the mandibles; a mandibular palp is absent. The maxillules and maxillae are reduced or absent.
There are no good references for the Epicaridea as a whole, although Strömberg (1971) reviews the embryology (including several California species), and Jay (1989) cites several other papers containing general information. The California fauna is poorly known, both taxonomically and biologically. About 700 species of epicarideans have been described world-wide, in eleven families. Three of these families are represented in California waters by sixteen species, six of which occur in the intertidal region and are included in the key.
Species in the family Bopyridae retain complete, or nearly complete body segmentation, and usually have six or seven pereopods on one side but far fewer on the other side. The sides of the pleonites are often produced as large lateral plates (epimeres) that resemble pleopods. Adult bopyrids are parasites either on the abdomen or in the branchial chamber of decapod crustaceans. In branchial parasites, the female attaches ventrally to the host's branchiostegite, inducing a bulge in the host's carapace. Males are much smaller and usually found on the ventral side of the pleon of the female isopod. Females brood many small eggs in an oöstegial brood pouch, that hatch as a free-swimming epicaridium stage. The epicaridium attaches to an intermediate host, a calanoid copepod. Once on the copepod, the isopod molts into a microniscus stage, and then into the cryptoniscus stage. The cryptoniscus detaches from the copepod, is free-swimming, and eventually attaches to the definitive host. All species are probably sequential hermaphrodites. About 500 species have been described world-wide.
Species in the family Entoniscidae are internal parasites of crabs and shrimps. Females are usually modified beyond recognition, with the marsupium grossly inflated and in some cases extending dorsally over the head. Males and mancas, however, are less distorted with a flattened body, complete segmentation, and pereopods. Mature females are surrounded by a host response sheath, with an external communication to the environment via a small hole or furrow in the carapace of the hosts. Most are parasitic castrators, and in some cases entoniscids can feminize male hosts. Good references on the biology of this family include: Giard (1887), Giard and Bonnier (1887), Veillet (1945), and Reinhard (1956).
The following key is based mainly on adult females.(figures 31-34, 35)
Key to Intertidal Species of Epicaridea
|1.|| Female without segmentation, simply an egg sac; antennae and mouthparts absent. |
(one California species, Hemioniscus balani, parasitic in barnacles of the genus Balanus and Chthamalus)
|-||Female with distinct or weak segmentation; not simply an egg sac; antennae and mouthparts present, although may be greatly reduced.||2|
|2.|| Body of female without indication of rigid exoskeleton, seemingly undifferentiated, but body divisions and segmentation present; pereonites expanded laterally into thin plates; maxillipeds are the only recognizable mouthparts; pereopods stubby or absent; endoparasites in body cavity of decapod crustaceans. |
(one California species, Portunion conformis, in body cavity of the crab Hemigrapsus spp.)
|-||Female distinctly segmented; pereonites not expanded laterally into thin plates; mouthparts rudimentary; pereopods prehensile, seven present on one side, but all except first may be absent on the other side; parasites of branchial cavity or on pleopods of decapod crustaceans.||Bopyridae 3|
|3.||Pleon with lateral plates (epimeres or pleural lamellae) elongate, those of female fringed with long, branched processes, those of male without such digitations; in branchial cavity of ghost shrimps of the genus Callianassa.||Ione cornuta|
|-||Pleon in both sexes with pleural lamellae rudimentary or absent (caution: do not confuse lateral biramous pleopods with pleural lamellae).||4|
|4.||Female pleopods not prominent, relatively short, not noticeable in dorsal view; in branchial chamber of the snapping shrimp Synalpheus lockingtoni and Alpheopsis equidactylus.||Bopyriscus calmani|
|-||Female pleopods prominent, long, visible in dorsal view.||5|
|5.||Pleopods biramous, with narrow branches arising from a peduncle or stem, extending laterally from narrow pleon; among pleopods of the mud shrimp Upogebia pugettensis.||Phyllodurus abdominalis|
|-||Pleopods biramous, lanceolate, not arising from a peduncle, extending posteriorly from pleon; in branchial chamber of the pelagic galatheid crab Pleuroncodes planipes.||Munidion pleuroncodis|