Atlas of Crustacean Larvae

36. Introduction to the Peracarida

36 The Peracarida are an incredibly diverse group of mostly small crustaceans (with some notable exceptions ). They are found in nearly all habitats and exhibit a wide range of lifestyles. Most are marine, but freshwater and terrestrial forms are common, as are parasites. Estimates of the number of described species are in the range of 20,000 to 25,000; there are undoubtedly thousands more still undiscovered and undescribed (e.g., see Ahyong et al. 2011; Poore and Bruce 2012). Peracarids appear to be united by the fact that development occurs within a female’s brood pouch, formed by plates of the female’s thoracic legs. Thus larvae per se are lacking. Traditionally, the group has been divided into 9 or 10 extant orders, including the highly diverse Amphipoda and Isopoda (together containing perhaps 300 extant families) and the much less diverse Tanaidacea, Cumacea, two groups of mysid shrimps (now treated as the Lophograstrida and Mysida), Spelaeogriphacea, Thermosbaenacea, and “Mictacea ” (probably paraphyletic). Another order (the Bochusacea) has been proposed for one of the “mictacean” families and is accepted by some workers (Gutu and Iliffe 1998). These orders contain perhaps 350 families collectively (see J. W. Martin and Davis 2001; Ahyong et al. 2011). Acceptance of the Peracarida as monophyletic is by no means settled. Analyses continue to question the group’s monophyly and, if it is recognized as natural, debates would still remain over which taxa are contained within it. Some of the many studies devoted to this question include the works of Watling (1983), Pires (1987), H. Wagner (1994), Mayrat and Saint Laurent (1996), Gutu and Iliffe (1998), Hessler and Watling (1999), Watling (1999), Jarman et al. (2000), Richter and Scholz (2001), Jaume et al. (2006), and Meland and Willassen (2007) (much of which was summarized by Spears et al. 2005). Peracarids do not have true larvae. Instead, in some species the eggs within the female’s brood pouch each hatch as a miniature version of the adult, which is called a manca. The manca differs from the adult mostly in the fact that the final pair of thoracic legs is not yet present. Strictly speaking, then, we could be excused for excluding the peracarids from this atlas. Yet many display developmental patterns that can have a bearing on our understanding of relationships both within the Peracarida and of peracarids to other groups of Crustacea. Additionally, some peracarids exhibit unique morphological stages that have been called larvae in the past (e.g., the “Cystisoma larvae” of some hyperiid amphipods) or have juveniles that differ significantly from the adult (e.g., the praniza and microniscus forms of some parasitic isopods). For these reasons , we have included not only images of peracarid mancae and other post-hatching morphological stages, but also, for some groups, more developmental (late embryological) information than we have allowed for other crustacean taxa. In the following chapters, all extant orders are treated, with the exception of the “Mictacea” (and thus the Bochusacea), for which no developmental studies exist (but see chapter 37 for some developmental information on “mictaceans”). Partly because of the paucity of information on some of these groups, we have combined into one chapter each what is known for the Thermosbaenacea, Spelaeogriphacea and “Mictacea” (chapter 37); Mysida and Lophogastrida (chapter 38); and Isopoda and Tanaidacea (chapter 40). Joel W. Martin Joel W. Martin Introduction to the Peracarida ...