rustaceansare one of the most popular invertebrate groups, even amongnonbiologists, for they include someof the world's most delec- table gourmet fare,such as lobsters,crabs, and shrimps (Figure 21.1). There are an estimated 70,000described living speciesof Crustacea,and probably five or ten times that number waiting to be discovered and named. They exhibit an incredible diversity of form, habit, and size. The smallest known crustaceans are less than 100 pm in length and live on the antennules of cope- pods. The largest are Japanesespider crabs (Macrocheiraknempferi),withleg spans of 4m, and giant Tasmanian crabs (Pseudocarcinusgigas) with carapace widths of 46 cm. The heaviest crustaceans are probablv American lobsters (Homarus americanus),which, before the present era of overfishing, attained weights in excessof 20 kilograms. The world's largest land arthropod by weight (and possibly the largest land invertebrate) is the coconut crab (Birgus latro), Classificationof The Animal weighing in at up to 4 kg. Crustaceans are found Kingdom(Metazoa) at all depths in every marine, brackislu and fresh- water envfuonment on Earth, including in pools at Non-Bilateria* Lophophorata 6,000m elevation (fairy shrimp and cladocerans in (a.k.a. the diploblasts) PHYLUM PHORONIDA northem Chile). Afew have become PHYLUM PORIFERA PHYLUM BRYOZOA successful on PHYLUM PLACOZOA PHYLUM BRACHIOPODA. land, the mostnotablebeing sowbugs and pillbugs I PHYLUM CNIDARIA EcpYsozoA (the terrestrial isopods). PHYLUM CTENOPHORA Nematoida Crustaceans are commonly the dominant or- PHYLUM NEMATODA Bilateria ganisms in aquatic subterranean ecosystems, and PHYLUM NEMATOMORPHA (a.k.a.the triploblasts) new species of these stygobionts continue to be Scalidophora PHYLUM XENACOELOMORPHA discovered as new caves are explored. They also PHYLUM KINORHYNCHA Protostomia PHYLUM PRIAPULA dominate ephemeral pool habitats, where many PHYLUM CHAETOGNATHA PHYLUM LORICIFERA undescribed species are known to occur.2 And SprRmn Panarthropoda crustaceans are the mostwidespread, diverse, and PHYLUM PLATYHELMINTHES PHYLUM TARDIGRADA PHYLUM GASTROTRICHA PHYLUM ONYCHOPHORA This chapter has been revised by Richard C. Bruscaand PHYLUM RHOMBOZOA Joel PHYLUM ARTHROPODA W. Martin PHYLUM OBTHONECTIDA SUBPHYLUMcRusrAcEA" lWhen most people hear the word "shrimp" PHYLUM NEMERTEA they think of SUBPHYLUM HEXAPODA edible shrimps, two crustaceangroups nestedwithin the PHYLUM MOLLUSCA SUBPHYLUM MYRIAPODA order Decar:oda(in the subordersDendrobranchiata and PHYLUM ANNELIDA SUBPHYLUM CHELICEFATA Pleocyemati).However, the term "shrimp" is applied to a PHYLUM ENTOPROCTA Deuterostomia number of long-tailed crustaceans,many not closely related PHYLUM CYCLIOPHORA PHYLUM ECHINODERMATA at all to decapods.So, in this generalsense, there are fairy tadpole mantis Gnathifera PHYLUM HEMICHORDATA shrimps, shrimps, shrimps, etc. In much of PHYLUM GNATHOSTOMULIDA the English-speakingworld, the word "prawn" is used for PHYLUM CHORDATA PHYLUM MICROGNATHOZOA the edible shrimps, thus eliminating some of the confusion. *Paraphyletic 2One PHYLUM ROTIFERA group study of ephemeralpools in northern California dis- covered30 nrobable undescribedand unnamed crustacean species(King et al.7996). 762 ChapterTwenty-One (A) PHYLUMARTHROPODA Crustacea: Crabs, Shrimps, and TheirKin 763 (o) Figure 21 .1 Diversity among the Crustacea. (A-D) Classes Remipedia, Cephalocarida, and Branchiopoda. (A) Spe/eonectes ondinae; note the remarkably homono- vivesi (Decapoda: Axiidea). (M) The Hawaiian regal lobster, mous body of this swimming crustacean (Remipedia). Enoplometopus (Decapoda: Achelata). (N,O) Two unusual (B) A cephalocarid. (C) A tadpole shrimp (Branchiopoda; amphipods (Peracarida: Amphipoda). (N) Cysfisoma, a Notostraca) from an ephemeral pool. (D) A clam shrimp huge (some exceed 10 cm), transparent, pelagic hyperiid (Branchiopoda: Diplostraca), carrying eggs. (E-Q) Class amphipod. (O) Cyamus scammoni, a parasitic caprellid Malacostraca. (E) The fiddler crab, Uca princeps amphipod that lives on the skin of gray whales. (P) A male (Decapoda: Brachyura). (D The giant Caribbean hermit and female cumacean; note the eggs in the marsupium crab, Petrochirus diogenes (Decapoda: Anomura). (G) A of the female (Peracarida: Cumacea). (Q) Ligia pacifica hermit crab (Paragiopagurus fasciatus) removed from its (Peracarida: lsopoda), the rock louse; Lrgra are inhabitants snail shell (Decapoda: Anomura). (H) The coconut crab, of the high spray zone on rocky shores worldwide. (R) The Birgus latro (Decapoda: Anomura), climbing a tree. mysid Slriel/a sp. (Peracarida: Mysida) with a parasitic epi- (l) Euceramus praelongus, the New England olive pit caridean isopod (Peracarida: lsopoda: Dajidae) attached to porcelain crab (Decapoda: Anomura). (J) The pelagic lob- its carapace (Western Australia). (S,T) Class Thecostraca. sterette, Pleuroncodes planipes (Decapoda: Anomura). (S) Acorn barnacles, Semibalanus balanoides (Cirripedia: (K) The cleaner shrimp, Lysmata californica (Decapoda: Thoracica). (l-) Lepas anatifera (Cirripedia: Thoracica), a Caridea). (L) The unusual rock-boring lobster shrimp, Axrus pelagic barnacle hanging from a floating timber. (Continued on next page) 764 ChapterTwenty-One Figure 21 .1 (continued) Diversity among the Crustacea. (U) Class Copepoda; the calanoid copepod Gaussra, a common planktonic genus. (V) Class Tantulocarida, Deoterthron, parasitic on other crustaceans. abundant animals inhabiting the world's oceans.The biomassof one species,the Antarctic krill (Euphausia superba),has been estimated at 500million tons at any given time, probably surpassingthe biomassof any other group of marine animals (and rivaling that of the BOX21A Characteristicsof the world's ants).The range of morphological diversity among crustaceansfar exceedsthat of even the insects. SubphylumCrustacea Many speciesof Crustaceaare threatenedby environ- 1. Body composed of a 6-segmented head, or cepha- mental degradation,over 500are listed on the IUCN lon (plus the acron), and a long postcephalic trunk; Red List, and about two dozen are protected by the trunk divided into two more or less distinct tagmata U.S.Environmental ProtectionAgency. Crustaceans (e,9., thorax and abdomen) in all but the remipedes are also the most common invasive invertebrates/and and ostracods (Figure21 .2) well over 100species have establishedthemselves in (anterior 2. Cephalon composed of to posterior):pre- marine and estuarinewaters of North America alone. segmental (indistinguishable)acron, protocerebral Becauseof their taxonomic diversity and numerical segment (lackingappendages), antennularseg- ment, antennal segment, mandibular somite, maxil- abundance,it is often said that crustaceansare the "in- lularysomite, and maxillarysomite; one or more sectsof the sea."We prefer to think of insectsas "crus- anteriorthoracomeres may fuse with the head in taceansof the land." And indeed, there is now strong members of some classes (e.9., Remipedia,and phylogenetic evidencethat the insectsarose from a Malacostraca),their appendages forming maxillipeds branch within the Crustacea. 3. Cephalic shield or carapace present (highlyreduced Despitethe enormousmorphological diversity seen in anostracans, amphipods, and isopods) among crustaceans(Figures 21.1" to 21..20),they display 4. Appendages multiarticulate,uniramous or biramous a suite of fundamental unifying features (Box 21A). In 5. Mandibles usually multiarticulatelimbs that function an effort to introduce both the diversity and the unity as biting, piercing, or chewing/grinding jaws of this enormousgroup of arthropods,we first present 6. Gas exchange by aqueous diffusion across special- a classificationand synopsesof the major taxa.We then ized branchial surfaces, either gill-likestructures or discussthe biology of the group as a whole, drawing specialized regions of the body surface examplesfrom its various members.As you read this 7. Excretion by true nephridial structures (e.9., anten- chapter,we ask that you keep in mind the general ac- nal glands, maxillaryglands) count of the arthropodspresented in Chapter20. B. Both simple ocelli and compound eyes occur in most taxa (not Remipedia),at least at some stage of the life cycle; compound eyes often elevated on stalks Classificationof The Crustacea 9. Gut with digestive ceca Crustaceans have been known to humans since ancient 10. With nauplius larva (unknown from any other arthro- times and have provided us with sources of both food pod subphylum); development mixed or direcl and legend. It is somewhat comforting to carcinolo- gists (those who study crustaceans) to note that Can- PHYLUM ARTHROPODA Crustacea:Crabs, Shrimps, and TheirKin 765 (A) Tail Abdomen---l fan I l--cephal dllFt|F"L l t- Carapace ,l Propodus Swimmerets (= pleopods) Dactyl Figure 21 .2 Crustacean external mor- phology: A crayfish (Malacostraca, Astacidea). (A) External form. Note the fully developed carapace covering the Compound cephalon and thorax. (B) The typical mala- Antennule eye costracan tail fan (ventral view). Note the. position of the anus on the telson. Legs (= pereopods) cer, one of the two invertebrates represented in the zodiac, is a crab (the other, of course, is Scorpio-an- other arthropod). Our modem view of Crustacea as a taxon can be traced to Lamarck's scheme in the early nineteenth century. He recognized most crustaceans as such, but placed the barnacles and a few others in separate groups. For many years barnacles were clas- SUBORDERONYCHOCAUDATA CIam shrimps, sified with molluscs because of