Guide to California Isopods

Introduction

The order Isopoda is one of nine orders in the crustacean superorder Peracarida. Peracarids are the "marsupial crustaceans," distinguished from the three other eumalacostracan superorders (Hoplocarida, Syncarida, Eucarida) by the following combination of characters: embryos brooded by females in a marsupium constructed by specialized thoracic coxal endites called oöstegites (except in Thermosbaenacea, which carry broods beneath the carapace); absence of caudal rami on the telson; maxilliped basis produced into an anteriorly directed bladelike endite; mandible with an articulated accessory process in adults, between the molar and incisor processes (the lacinia mobilis); carapace, when present, not fused with posterior pereonites and usually reduced in size; and, in most, young hatching as mancas, a pre-juvenile stage lacking the last pair of thoracopods. All peracarids undergo direct development with brooding, hence true larval forms do not occur in this superorder. About 25,000 species of peracarids have been described.

Isopods can be distinguished from other peracarids, and crustaceans in general, by the following combination of characteristics: (1) body usually flattened (except in Anthuridea and Phreatoicidea); (2) head (cephalon) compact, with unstalked compound eyes, two pairs of antennae (first pair minute in Oniscidea), and mouthparts comprising a pair of mandibles, two pairs of maxillae (maxillules and maxillae), and one pair of maxillipeds; (3) a long thorax of eight thoracomeres, the first (and also the second in Gnathiidea) fused with the head and bearing the maxillipeds, the remaining seven (called pereonites) being free and collectively comprising a body division called the pereon; (4) seven pairs of uniramous legs, or pereopods, all more or less alike (hence, "iso-pod"), except gnathiids which have only 5 pairs of walking legs; (5) appendages never chelate (i.e., the subterminal article, or propodus, is not modified into "hand" that works with the terminal article, or dactyl, as a true claw); (6) a relatively short abdomen (pleon) composed of six somites, or pleonites, at least one of which is always fused to the terminal anal plate (telson) to form a pleotelson; (7) six pairs of biramous pleonal appendages, including five pairs of platelike respiratory/natatory pleopods and a single pair of fanlike or sticklike, uniarticulate (unjointed) uropods; (8) heart located primarily in the pleon, and (9) biphasic molting (i.e., posterior half of body molts before anterior half). For illustrations of general isopod morphology see figures 1 - 8.

All isopods possess one of two fundamental morphologies, being "short-tailed" or "long-tailed" (Brusca and Wilson, 1991). In the more primitive, short-tailed isopods the telsonic region is very small, positioning the anus and uropods terminally or subterminally on the pleotelson (Phreatoicidea, Asellota, Microcerberidea, Oniscidea, Calabozoidea). The more highly derived, long-tailed isopods have the telsonic region greatly elongated, thus shifting the anus and uropods to a subterminal position on the pleotelson (Flabellifera, Anthuridea, Gnathiidea, Epicaridea, Valvifera).

Isopods can be sexed in several ways. If oöstegites, or a marsupium, are present, one is obviously examining a female. The openings of the oviducts in females (near the base of the legs on the fifth pereonite) are difficult to observe. If oöstegites are absent, males can be distinguished by the presence of paired penes on the sternum of pereonite 7 (or pleonite 1) or appendices masculinae (sing. appendix masculina) on the endopods of the second pleopods. Absence of penes, appendices masculinae, and oöstegites indicates the specimen is either a nongravid female or a juvenile that has not yet developed secondary sexual features.

Isopods are a large, diverse order with ten named suborders, all but two (Phreatoicidea and Calabozoidea) of which occur in California. They are found in all seas and at all depths, in fresh and brackish waters, and on land (the Oniscidea). The approximately 10,000 species are more-or-less equally split between marine and terrestrial/freshwater environments. Several general guides to marine isopods of Pacific North and Middle America have been published. These include: Richardson (1905) (still a valuable reference, although obviously out-of-date), Schultz (1969), Brusca (1980) (keys to common Gulf of California species), Brusca and Iverson (1985) (the only summary treatment available for the tropical eastern Pacific region), Wetzer et al. (1997), and Light's Manual. Kensley and Schotte (1989) is also a very useful reference. Key citations to the original literature for the Pacific coast of California are provided in this chapter, and the history of Pacific isopodology (with a complete bibliography) can be found in Wetzer et al. (1997).

The California marine isopod fauna numbers 190 named species (eight of which are questionable species, nomen dubia or species inquirenda), representing thirty-six families in eight suborders. Although we have enumerated all species known from California waters (see list below), the keys treat only species occurring in the intertidal and supralittoral zones, plus the commonly encountered fish parasites of the family Cymothoidae. However, the coverage of the key includes the entire coastline of California and Oregon. At least fourteen species are thought to have been introduced to California’s coastal zone: one cirolanid, four sphaeromatids, one asellotan, one epicaridean and seven oniscids.

In the sea, isopods compare in ecological importance to the related Amphipoda and Tanaidacea, notably as intermediate links in food chains. They typically predominate (numerically), along with tanaids, bivalves, and polychaetes, in soft bottom sediment samples from continental shelves. On some tropical coasts, isopods may constitute the majority of prey items consumed by rocky shore fishes. In the Arctic region, they are one of the primary food items of gray whales. Intertidal isopods are predominantly benthic and cryptic, living under rocks, in crevices, empty shells and worm tubes, and among sessile and sedentary organisms, such as algae, sponges, hydroids, ectoprocts, mussels, urchins, barnacles, and ascidians. Some burrow in natural substrates including mud, sand, soft rocks, and driftwood, and some burrowers, such as the Limnoria (the gribbles) and Sphaeroma, do extensive damage to pilings and wooden boats. In the tropics, some species of Sphaeroma burrow into mangroves, weakening the prop roots and causing them to break more easily, which typically stimulates the growth of multiple new rootlets, leading to the classic stairstep structure of red mangrove prop roots (Perry and Brusca, 1989). Several species are important scavengers on shore wrack or dead animals (e.g. Ligia, Tylos). Cirolanids, corallanids and tridentellids are voracious carnivores, functioning both as predators and scavengers. Epicarideans are all parasites on other crustaceans, cymothoids are all parasites on fishes, and aegids are "temporary parasites" on fishes. Some invertebrate parasites, notably acanthocephalans, use isopods as intermediate hosts.

Identification of isopods often requires dissection and microscopic examination of appendages and other structures using fine-pointed "jewelers" forceps under a binocular dissecting microscope. Dissected parts may be mounted on microscope slides in glycerin or a more permanent medium for observation under a compound microscope.