Atlas of Crustacean Larvae

54. Brachyura

general: Brachyurans (true crabs) are easily recognized by their typically wide, dorsoventrally compressed carapace, reduced abdomen tucked beneath the cephalothorax, and first legs modified as relatively large chelipeds (claws). They are the most speciose group of decapods, with nearly 7,000 extant species in 93 families and 38 superfamilies (Ng et al. 2008; De Grave et al. 2009). Morphological variation in both adults and larvae can be extreme. Adults inhabit a wide variety of marine, freshwater, and terrestrial habitats, from the deep sea to cloud forests and phytotelmata. Most are marine, but nearly 1,300 species are known from freshwater systems (Cumberlidge and Ng 2009). The vast majority are free living, but some are commensal, and a few are considered parasitic (pinnotherids), because of tissue damage to the host. The life cycle typically consists of a pre-zoea (not known in all species, and possibly an artifact), a zoea, a megalopa, a juvenile, and an adult. larval types Pre-Zoea: A brief non-feeding pre-zoeal hatching stage (fig. 54.1A–E) (also see earlier decapod groups, such as the Penaeioidea, Caridea, Anomura, and Polychelida), considered by some authors to be late embryonic rather than larval, has been reported for several crab families (see review by Hong 1988a), such as cancrids (Quintana 1984a; Quintana and Saelzer 1986), majids (Webber and Wear 1981), epialtids (Guerao and Abelló 1996b), oregoniids (Haynes 1973), panopeids (Montú et al. 1988), portunids (Campbell and Fielder 1987), and atelecyclids (Sasaki and Mihara 1993). Because of its very short duration (often a matter of minutes), this stage probably occurs in nature more widely than is currently recognized (Quintana 1984a). The pre-zoea swims poorly and is not seen in the plankton. Not all laboratory hatchings, even of the same species, exhibit this stage, and some workers feel that its presence is an artifact and indicates suboptimal conditions at hatching (Campbell and Fielder 1987; but see Hong 1988a, who argues that it is a normal part of development). Zoea: Except where larval stages are bypassed during development (e.g., direct development, which occurs in most freshwater species; also see Wear 1967 for a description of embryonic zoeal stages in a marine species), brachyuran crabs are characterized by having a series of planktonic shrimp-like larval stages, each of which is called a zoea (figs. 54.1–54.5). The number of zoeal stages can be as few as one or two (e.g., most majids) or as many as 8 (some portunids) or 12 (e.g., Plagusia), and it is usually consistent within a family (see Rice 1980). These stages are often designated ZI, ZII, ZIII, and so forth. Abbreviated development (loss of some zoeal stages) is also common (Rabalais and Gore 1985; Clark 2000). Megalopa: The zoea is followed by a functionally and ecologically transitional form called a megalopa (figs. 54.6– 54.10)—also referred to as a megalops or megalop (the decapodid of Kaestner 1970; Felder et al. 1985)—a stage known to occur only in the Malacostraca. The megalopa is sometimes unfortunately referred to as a post-larval stage, mostly in the older literature. Rarely, there can be more than one megalopa, or no megalopal stage (e.g., in hymenosomatids). The megalopa is followed by a benthic juvenile first-crab stage that more closely resembles a miniature version of the adult. morphology Pre-Zoea: The pre-zoeal stage (fig. 54.1A–E), if present, is ensheathed by a cuticular covering and is therefore nonfeeding . Pre-zoeae swim poorly, with a jerking motion via abdominal flexing (Campbell and Fielder 1987; Hong 1988a); more often they sink to the bottom. Appendages are identifiable mostly as poorly developed limb buds. Zoeal setae, if visible, are not evaginated, and the spines are folded under the cuticle. The pre-zoeal sheath itself is lightly plumose, especially where it covers the antennae and telson. The pre-zoeal cuticle does not merely reflect the underlying zoeal cuticle (fig. 54.1C–E), and it may represent “a remnant of an ancestral free swimming stage” (see Hong 1988a; Ingle 1992). 54 Joel W. Martin Joel W. Martin Brachyura 296 Joel W. Martin Zoea: All zoeae possess a carapace that covers the head and the anterior part of the thorax and its limbs (figs. 54.1F, G; 54.2). The carapace has free (unattached) ventral and posterolateral borders, and it typically has a prominent dorsal spine, a rostral spine (the rostrum), and a pair of lateral spines. Other combinations exist, however, and dorsal and lateral...