Atlas of Crustacean Larvae

45. Introduction to the Decapoda

The Decapoda is a diverse and morphologically disparate group of malacostracan crustaceans. Most decapods are familiar to everyone, and many of them are consumed by humans all over the world. Included within this order are the familiar crabs, lobsters, shrimps, and hermit crabs, as well as many other lesser-known groups. The Decapoda have been referred to as the “pinnacle of crustacean evolution” (e.g., Crandall et al. 2009), because of their great diversity and successful exploitation of so many habitats. They differ from other eucarids in having three pairs of maxillipeds and five pairs of pereopods, the first of which is often (though not always) a heavy chelate appendage. The Decapoda is usually divided into two large suborders: the Dendrobranchiata (containing the commercially harvested penaeoid shrimps), distinguished by having eggs that are not borne on the abdominal pleopods and that hatch as nauplii, and the Pleocyemata (all other groups), in which females carry the eggs on the pleopods and where eclosion is always postnaupliar . Gill morphology is another important distinguishing feature (see J. W. Martin et al. 2007). The Pleocyemata include the colorful stenopodid cleaner shrimps, the true shrimps of the Caridea, the diverse and successful freshwater (and marine ) crayfish and lobster groups (Astacidea), the familiar hermit crabs and their relatives (in the Anomura), the true crabs (Brachyura), and several other large and speciose assemblages: 10 infraorders, 233 families, and 17,635 species all told (e.g., De Grave et al. 2009; Ahyong et al. 2011; Shen et al 2013). Recent compilations have provided useful analyses of the overall diversity of living decapods. We now have relatively current lists of all living species of marine lobsters (T. Chan 2010); caridean, dendrobranchiate, procarididean, and stenopodidean shrimps (De Grave and Fransen 2011); anomurans (e.g., Baba et al. 2008; Boyko and McLaughlin 2010; McLaughlin et al. 2010; Osawa and McLaughlin 2010); and brachyuran (true) crabs (Ng et al. 2008). Even fossil families and genera have been compiled recently (De Grave et al. 2009; Schweitzer et al. 2010), such that our overall knowledge of extinct and extant decapods is better than at any point in history. Yet perhaps the only thing agreed upon in the classification and phylogeny of the Decapoda is that the group is monophyletic. Beyond that, there have always been many competing hypotheses about their internal relationships, what R. Brusca and Brusca (2003) referred to as a “popular carcinological pastime.” Even the lower limit of the Brachyura—the question of what is, and what is not, a crab—has proved surprisingly elusive to resolve. Concerning their larval development, it is fair to say that more is known about decapods than for all other groups of crustaceans combined. Nearly all decapods hatch at a postnaupliar stage and have both zoeal (planktonic) and decapodid larval forms. The latter are morphologically and ecologically transitional larvae that are usually referred to as a megalopa (e.g., in the true crabs and carideans), a puerulus (in achelatan lobsters), or a glaucothoe (in anomurans) (table 45.1). Only the dendrobranchiates(penaeoidshrimps)hatchasanaupliuslarva, a condition assumed to be ancestral to the Decapoda in general. Subsequent development in dendrobranchs—although they may go through orthonaupliar, metanaupliar, zoeal (including protozoeal and mysis), and several decapodid stages—is usually called regular anamorphic (e.g., Anger 2001), referring to the gradual changes in morphology during ontogeny, rather than truly metamorphic, as in other decapod groups. Because of the great morphological diversity of decapod larvae, many early workers mistakenly assumed that some of these forms were adults. As a result, a large number of genus-level names were applied, and many of these names are still encountered in the literature on decapod larvae. The result is a vast body of literature that can be very confusing. We have attempted to clarify the use of all previously employed decapod larval names in table 45.2. The basic morphological progression of the appendages of decapod larvae is summarized in table 45.3. Of the 9 infraorders of decapods currently recognized in the Pleocyemata, larvae remain unknown only for the Procaridoida and Glypheidea. All major groups save these two are treated in the chapters that follow. 45 Joel W. Martin Joel W. Martin Introduction to the Decapoda Table 45.1 Summary of commonly employed terminology for larval phases in the Decapoda Taxon Hatch as nauplius (planktonic) Zoea (planktonic) Decapodid (usually benthic or demersal) Chapter Dendrobranchiata yes protozoea, mysis decapodid 46 Pleocyemata Stenopodidea no zoea decapodid 47 Caridea no zoea decapodid 48 Astacidea no zoea...