•59 Decapoda (Crustacea) of the Gulf of Mexico, with Comments on the Amphionidacea Darryl L. Felder, Fernando Álvarez, Joseph W. Goy, and Rafael Lemaitre The decapod crustaceans are primarily marine in terms of abundance and diversity, although they include a variety of well- known freshwater and even some semiterrestrial forms. Some species move between marine and freshwater environments, and large populations thrive in oligohaline estuaries of the Gulf of Mexico (GMx). Yet the group also ranges in abundance onto continental shelves, slopes, and even the deepest basin floors in this and other ocean envi- ronments. Especially diverse are the decapod crustacean assemblages of tropical shallow waters, including those of seagrass beds, shell or rubble substrates, and hard sub- strates such as coral reefs. They may live burrowed within varied substrates, wander over the surfaces, or live in some Decapoda. After Faxon 1895. special association with diverse bottom features and host biota. Yet others specialize in exploiting the water column ment in the closely related order Euphausiacea, treated in a itself. Commonly known as the shrimps, hermit crabs, separate chapter of this volume, in which the overall body mole crabs, porcelain crabs, squat lobsters, mud shrimps, plan is otherwise also very shrimplike and all 8 pairs of lobsters, crayfish, and true crabs, this group encompasses thoracic legs are pretty much alike in general shape. It also a number of familiar large or commercially important differs from a peculiar arrangement in the monospecific species, though these are markedly outnumbered by small order Amphionidacea, in which an expanded, semimem- cryptic forms. branous carapace extends to totally enclose the compara- The name “deca- poda” (= 10 legs) originates from the tively small thoracic legs, but one of several features sepa- usually conspicuously differentiated posteriormost 5 pairs rating this group from decapods (Williamson 1973). The of thoracic legs that originate below the carapace. Vari- amphionidaceans are represented by only a single glob- ous of these pairs may terminate in pincers (chelae) or be ally distributed planktonic species, Amphionides reynaudii otherwise modified for feeding, crushing, defense, cling- (H. Milne Edwards, 1832), which almost certainly ranges ing, walking, swimming, or in some cases cleaning of the into GMx oceanic waters, but it is not specifically recorded gill chambers; in a few cases, the posteriormost pairs may as such in literature of which we are aware; it is thus not be reduced to varied degrees or carried so as to be hidden treated in this volume beyond present mention. from obvious external view. This differs from the arrange- Among those decapod crustaceans commonly termed 1019 1020 ~ Decapoda (Crustacea) Decapoda. After A. Milne-Edwards and Bouvier 1923. Decapoda. After A. Milne-Edwards and Bouvier 1923. shrimps are the commercially harvested or cultured spe- include deepwater marine forms resembling the familiar cies of penaeoideans and palaemonids, as well as an abun- New England lobster as well as the speciose freshwater dance of less familiar but diverse groups widely distrib- crayfish, the latter of which are not treated further in uted in planktonic, pelagic, and benthic environments this work (despite the slight encroachment of some into from grassbeds and reefs to muddy burrows, some living extreme upper estuaries). However, among marine forms, as symbiotic forms. Overall, these groups constitute a tre- the term lobster is also applied to the spiny lobsters with mendous biomass and are of inestimable importance in long thorny antennae, most commonly found in shallow food chains and other ecological functions. Because many tropical waters (also commonly termed “crayfish” in parts of the shrimps are adapted for swimming, or at least have of their range), and to the wide- ranging marine slipper well- developed swimmerets on their abdomens, they are lobsters or shovel- nose lobsters, so named for their over- sometimes called the natant decapods, alluding to their all depressed body shape and broadly flattened spade- natatory adaptations. The rest of the group is sometimes like antennae. Finally, the true crabs are those species in pulled together loosely as the reptant decapods, alluding which the thorax is comparatively inflated and its cu- to the reptatory or crawling habits as exhibited by varied ticle thickened, while the abdomen has become reduced kinds of lobsters, crabs, and their relatives. Among these, in relative size and tightly flexed beneath the thorax. the hermit crabs (or simply “hermits”) are commonly Although some members of this group can occupy sem- recognized as those crablike forms having a poorly calci- iterrestrial and freshwater environments, or even move fied abdomen and being adapted to carrying of a gastro- between these inland and coastal marine waters, most pod (snail) shell or occupying some comparable alterna- of the true crabs are marine and estuarine forms, rang- tive protective enclosure. Mole crabs are for the most part ing widely in size and being distributed from supratidal uniquely adapted to burrowing backwards into shallow beaches to deep marine waters. Among them are species marine sediments, keeping their antennae exposed; some variously adapted for cryptic, pelagic, fossorial, or sym- of those on beaches are commonly called “sand fleas.” The biotic habitats. related porcelain crabs look very crablike and, with rare Systematics, taxonomy, and biogeography of GMx exception, live cryptically among debris and rubble in decapod crustaceans have been addressed in a profu- mostly shallow coastal waters. Squat lobsters are related sion of literature since the publication of Behre (1954). to porcelain crabs, exhibiting something between a crab- Even so, available and applicable literature treatments of like and lobsterlike shape; most of these are outer con- that day were more extensive than might be suggested by tinental shelf or deep ocean species. The mud shrimps Behre’s brief summary, which attempted a broadly com- are seldom- seen but abundant obligate burrowing spe- parative biogeographic and ecological overview but did cialists, ranging in distribution from intertidal estuaries not undertake compilation of a comprehensive checklist to outer continental shelf sediments, many digging to a to actually document records and distributions for deca- meter or more below the sediment surface and circulat- pods of the GMx. Only northern GMx decapods were ing water through a burrow system with their uniquely used in tabulating her account of families (26), genera adapted abdominal swimmerets. Lobsters and crayfish (66), and species (113), and major gaps in knowledge Felder et al. ~ 1021 of the regional records were not supported by properly archived specimens. Shortly following the publication of Behre (1954), a flood of valuable new reference materials became avail- able concerning GMx decapod crustacean distributions, systematics, and ecology. Based upon original surveys, or in some cases compilation of existing records, these included such regionally focused works as Hildebrand (1954, 1955, 1958), Wass (1955), Chace, Shoemaker, and Bowman (1956), Menzel (1956), Parker (1956, 1959), Sim- mons (1957), Trott (1960), Tabb and Manning (1961), and Breuer (1962). However, taxonomically focused efforts of Decapoda. After Faxon 1895. that period also added to knowledge of the GMx decapod fauna and included such works as Haig (1956), Proven- were noted for the southeastern GMx, Yucatán, Cuba, and zano (1959), and Holthuis (1960). deep waters of the GMx in general. Supplementary infor- Regional accounts of GMx decapod assemblages have mation on commercial shrimps (Lindner and Ander- continued to accumulate over the last 4 decades, facilitated son 1954) and spiny lobsters (Smith 1954) appeared in in great part by the addition of key taxonomic literature in brief companion chapters for this classic “Bulletin 89” at- the mid- 1960s to mid- 1970s. While Williams’ (1965) gen- tempt to summarize knowledge concerning the GMx. eral reference to Carolinian decapod crustaceans (built While multitudes of new distribution records have been upon earlier efforts of Hay and Shore 1918) did not spe- added and numerous new species described since that cifically encompass the GMx in its geographic coverage, day, some mainstream monographs available prior to it included many species that either ranged into or were Behre’s (1954) account encompassed the GMx region and then thought to be conspecific with those in the GMx. In were richly appointed with voucher-supported records many instances it attempted to clarify the extent to which from specific areas of the GMx. These remain critically distributions for some Carolinian species included the important and useful taxonomic tools to the present, and GMx and provided a ready summary of literature that include such remarkable compilations as the classical tax- might be used to establish taxonomic identities of GMx onomic summaries of selected major groups by Coutière species. Because of the much larger presence of tropical (1909), Rathbun (1918, 1925, 1930, 1937), and Holthuis forms in the GMx than in the Carolinas, along with the (1946, 1951, 1952), as well as a host of papers focused large number of endemics (ever more in evidence today), on smaller taxonomic units. Faunistic surveys and Williams (1965) could not be applied as a definitive refer- accompanying systematic treatments of decapod crus- ence for identification