CRUSTACEA 255 CRUSTACEA species that can be classified as stygobites. Subterranean םThe taxonomic status of crustaceans has long been debated by 20 carcinologists (the scientists who study these arthropods). The cladocerans (particularly hyporheic species) are known from all group has been assigned to one of the phylum, subphylum, continents and most demonstrate only minor troglomorphic ad- or superclass levels, with five, six, and even ten classes being aptations. recognized. The purpose of this essay is not to enter into a taxonomic debate, but to recognize that crustaceans are one of Class Remipedia the oldest arthropod groups and certainly represent one of the Remipedia (Figure 1a) are among the most primitive of all extant largest, most diverse, and most successful groups of invertebrates crustaceans. Twelve species, placed in two families containing six on Earth, with some 40 000 species recognized (Hobbs, 2000). genera (Cryptocorynetes, Godzilliognomus, Godzillius, Lasionectes, Although crustaceans occupy a truly impressive number of Pleomothra, and Speleonectes), are restricted to anchialine caves aquatic environments (freshwater, brackish, marine), some spe- in the Bahamas, the Caicos Islands, Cuba, the Yucata´n Peninsula cies have become successful on land as well (e.g. isopods). Crusta- (Mexico), the Canary Islands, and the Cape Range Peninsula ceans have a chitinous exoskeleton impregnated with calcium (Western Australia). These small (up to 45 mm total length), carbonate, with three major body divisions: the cephalon, colourless, blind, elongate, centipede-like predators are found in 1מ Ͻ thorax, and abdomen, and they possess paired antennules, anten- their highest densities in oxygen-deficient waters ( 1mgl ), nae, mandibles, and maxillae. and are often found associated with other stygobitic crustaceans Not only have crustaceans been successful in epigean (above- such as caridean shrimps, cirolanid isopods, haziid amphipods, ground) habitats, but they have also moved into the hypogean mysids, ostracods, and thermosbaenaceans. (below-ground) realm, with six classes of crustaceans being rec- Class Copepoda ognized as inhabiting cave and other hypogean environments. (See also Crustacea: Copepoda.) Copepods are a large and di- Additionally, anchialine and interstitial habitats (although many verse group of crustaceans, and are the dominant forms of marine of the latter are not actually associated with karst landforms) plankton. Presently, more than 14 000 species are placed in harbour troglomorphic crustaceans. approximately 2300 genera in 210 families, yet the majority Like many other cavernicoles, many crustaceans are stygobites of subterranean species are placed in the following six orders: (aquatic obligate cavernicoles), while a few are troglobites (terres- Platycopioida and Misophrioida (occur in anchialine habitats), trial obligate cavernicoles). Both groups exhibit various morpho- Gelyelloida (found in caves in France and Switzerland), Cala- logical, physiological, and behavioural adaptations to living in noida, Cyclopoida, and Harpacticoida. Stygobitic species origi- a dark spelean environment where food is in short supply. Their nated from surface, marine, and freshwater ancestors and reached adaptations include reduction or loss of pigments and eyes, at- different groundwater habitats mostly through interstitial and tenuated appendages, and lowered metabolic rates. crevicular or karst routes. It is widely accepted that most obligate cavenicoles (including Calanoid copepods are mainly planktonic filter-feeders, rep- those other than crustaceans) evolved from epigean ancestors resented by some 2300 species, yet fewer than 20 are known with some degree of pre-adaptation to living in hypogean envi- from subterranean waters in a wide distribution ranging from ronments and are thought to have evolved by either natural the Americas to Madagascar and southeastern China. The cyclo- selection or by neutral mutation and genetic drift. poids (Figure 1b) are a poorly studied group, with approximately The six classes of crustaceans with subterranean representa- 150 species recognized from various caves, wells, anchialine habi- tives are Mystacocarida, Branchiopoda, Remipedia, Copepoda, tats, and groundwater interstices. These characteristically blind Ostracoda, and Malacostraca. The primitive Class Mystacocar- crustaceans lack pigments and are known from North and South ida is a meiobenthic group restricted to interstitial waters of America, Asia, Africa, Europe, and Madagascar. The harpacti- intertidal sand beaches and subtidal sand substrates. It is most coids (Figure 1c) are represented by nearly 3000 species assigned closely related to the Copepoda, and less than 20 species and to 40 families, at least 10 of which have stygobiont species. They subspecies are known from Africa, Europe, North and South are widely distributed in caves and various interstitial habitats, America, and Australia. Because mystacocarids are not known as well as in anchialine habitats. These vermiform crustaceans from karstic settings, there will be no further discussion here of can be observed scraping food from various substrates rather this group. Discussion of the remaining five classes of crustaceans than filtering water. follows. (The classes Copepoda and Ostracoda have their own entries in this Encyclopedia, as do three orders—Amphipoda, Class Ostracoda Isopoda, and Syncarida—of the Class Malacostraca.) (See also Crustacea: Ostracoda.) These bivalve crustaceans are fairly successful, being known from virtually all aquatic habitats, Class Branchiopoda but mostly from the marine environment. Some 5700 benthic Only the subclass Diplostraca includes species inhabiting subter- and planktonic species are recognized and more than 300 are ranean habitats (the hyporheos—or transition zone between endemic to subterranean waters, many assigned to the Order streams and groundwater, phreatic zone, caves). Although the Podocopida. Although some species are known only from caves Conchostraca (clam shrimps) and Cladocera (water fleas) are (e.g. Candona jeanneli in Marengo Cave, Indiana), most subter- included in this subclass, only the latter order has hypogean ranean forms have been observed in springs, interstitial waters, representatives, with fewer than 100 of the approximately 450 and anchialine habitats (Figure 1d). Stygobites are reported from recognized species occupying subterranean waters. These are Afghanistan, Africa, Bahamas, Bermuda, Canary Islands, Cen- small (0.2–2 mm), are laterally compressed, and there are only tral and Southern Europe, India, Jamaica, Japan, Java and West- 5603$$0C36 04-01-03 06:51:51 256 CRUSTACEA Crustacea: Figure 1. Line drawings of dorsal (d) and lateral (l) views of representative stygobiont crustaceans: a. Remipedia (d) Speleonectes lucayensis from the Bahamas (after Hobbs, 2000; total length 19.8 mm); b. cyclopoid copepod (d) Kieferiella delamarei from France (after Hobbs, 2000; length 740 ␮m); c. harpacticoid copepod (d) Novocrinia trifida from Belize (after Huys and Iliffe, 1998; length 505 ␮m); d. thaumatocyprid ostracod (l) Danielopolina wilkensi from the Canary Islands (after Kornicker and Iliffe, 1995; length 0.41 mm). ern Malaysia, Mexico, North, Central, and South America, and (or with greatly reduced eyes), stygobites that evolved from shal- South Korea. low-marine ancestors and moved into anchialine habitats, caves, springs, thermal springs, wells, and occupy freshwater and saline Class Malacostraca environments. The group is widespread (found in Australia, the Clearly this is the largest class of crustaceans, with approximately Bahamas, Cambodia, Costa Rica, Croatia, Cuba, France, 23 000 species placed in six superorders, four of which contain Greece, Haiti, Italy, Lanzarote, Mallorca, Mexico, Morocco, hypogean species. The Superorder Syncarida (see Crustacea: Puerto Rico, Sicily, Spain, United States, and the Virgin Islands) Syncarida) is a poorly studied, freshwater group which is divided and consists of at least 34 species assigned to six genera in two into three orders, all of which share the absence of a carapace families. or cephalic shield. The Order Anaspidacea (Figure 2a) is known The Superorder Peracarida consists of eight orders, six of only from Australia, including the island of Tasmania, and occu- which have known subterranean species. Common characters pies a variety of aquatic habitats, including springs and caves, of this group include a carapace that is never fused to all the with one family (Psammaspididae) that is exclusively stygobitic. thoracomeres, although the carapace in isopods and amphipods The Order Stygocaridacea (Figure 2b) occupies interstitial waters secondarily has been lost, the first pair of thoracopods (thoracic in epigean and hypogean environments in New Zealand and somites with a pair of appendages) is modified as a pair of maxil- South America. Fewer than 10 species are assigned to four gen- lipeds, the females have a brood pouch, and the pleopods lack era. The Order Bathynellacea (Figure 2c) has the greatest species an appendix interna (a slender appendage of the endopod). The richness of the three orders, with approximately 150 species rec- Order Mysidacea is a diverse group of crustaceans with more ognized from interstitial epigean, well, and cave habitats. This than 1000 species widely distributed in marine, freshwater, and ancient, relict group is known from Africa, Asia, Australia, Eu- groundwater habitats. Stygobitic mysids (Figure 3a) occur