Atlas of Crustacean Larvae

31. Introduction to the Malacostraca

31 The crustacean class Malacostraca is an enormous assemblage that traditionally has been treated as consisting of 3 subclasses: the Phyllocarida (containing the relatively species-poor order Leptostraca, whose species are also known colloquially as sea fleas), the Hoplocarida (containing the entirely predatory stomatopods, or mantis shrimps, in the order Stomatopoda), and the Eumalacostraca, which contains the majority of all crustacean species within the megadiverse superorders Peracarida (with 9 extant orders ) and Eucarida (containing the krill, decapods, and the monotypic Amphionidacea). Also included within the Eumalacostraca is the superorder Syncarida, containing the small orders Bathynellacea and Anaspidacea. Malacostracans differ from other crustaceans in possessing 19 or 20 body somites, divided into a five-segmented cephalon, an eight-segmented thorax, and a six-segmented abdomen (pleon) (the Phyllocarida, with a seven-segmented abdomen, are the exception), all bearing distinctive appendages. The terminal somite of the pleon typically bears a telson, which, in combination with the uropods (the last pair of pleopods), forms a functional tail fan (also see Richter and Scholtz 2001). Historically, most authors have agreed on the unity (monophyly ) of the Malacostraca, although some (e.g., F. Schram 1986) have excluded the Phyllocarida, and relationships within the Eumalacostraca have long been controversial. Some of the competing views of malacostracan phylogeny are found in the works of Siewing (1963), F. Schram (1981, 1984, 1986), Watling (1983, 1999), Mayrat and Saint Laurent (1996), F. Schram and Hof (1998), Wills (1998), Jarman et al. (2000), J. W. Martin and Davis (2001), Richter and Scholtz (2001), Spears et al. (2005), and Meland and Willassen (2007). A few generalizations can be made for larval development within the Malacostraca (table 31.1). Phyllocarids are direct developers, as are many of the Eumalacostraca (including all of the Peracarida), whereas stomatopods and decapods have distinct and well-defined metamorphic larvae. Naupliar eclosion also occurs in some of the Eucarida (krill and dendrobranchiate shrimps), and these nauplii differ notably from those of most other crustaceans in being completely lecithotrophic (see Scholtz 2000). A few fossils of malacostracan larvae exist, which are almost exclusively fossils of stomatopod and decapod larvae from Jurassic and Cretaceous limestones, and these are described in chapter 32. In subsequent chapters, each of the above-mentioned groups is treated. Joel W. Martin Joel W. Martin Introduction to the Malacostraca Table 31.1 Summary of larval development in the Malacostraca Taxon Direct Metamorphic Chapter(s) Phyllocarida (leptostracans) direct1 33 Hoplocarida (stomatopods) antizoea, pseudozoea, erichthus, alima2 34 Eumalacostraca Syncarida (2 orders) direct (Anaspidacea) parazoea, bathynellid3 (Bathynellacea) 35 Peracarida (9 orders, including amphipods, isopods, tanaids, cumaceans, and others) direct4 manca, nauplioid, post-nauplioid, some others4 37–41 Eucarida Euphausiacea (krill) nauplius, metanauplius, calyptopis, furcilia5 43 Amphionidacea direct6 44 Decapoda (crabs, shrimps, lobsters, and their relatives) some species nauplius, zoea, decapodid7 46–54 1 Embryonic and post-embryonic growth in phyllocarids, however, is often divided into two phases. 2 Types and names of larvae vary according to their stomatopod lineage. 3 Bathynellaceans are only slightly metamorphic at most; the parazoea has been called post-embryonic and the bathynellid phase is essentially a juvenile. 4 Much variation exists in peracarid development, and a developmental stage called a manca occurs in many groups. Additionally, post-hatching stages are referred to as nauplioid and post-nauplioid larvae in the Mysida and Lophogastrica, and some developmental stages have received separate larval names in the Amphipoda (e.g., pantochelis, protopleon, physosoma) and in some parasitic isopod groups (e.g., praniza, zuphea). 5 Development in krill proceeds along different pathways, depending on the type of egg brooding employed by the species in question. 6 Although larval terminology has been applied in this group (e.g., the name amphion; see text), development is actually very gradual and without marked metamorphosis. See Kutschera et al. (2012). 7 A nauplius phase occurs in the Decapoda only in the Dendrobranchiata (penaeoid and sergestoid shrimps). A variety of terms have been proposed for both the zoeal and the decapodid phases in different groups of decapods. Direct or abbreviated development also occurs in some decapods, such as crayfish and freshwater crabs, and some deep-sea species of other groups. ...