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40. Isopoda and Tanaidacea
- Johns Hopkins University Press
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40 general: Isopods and tanaidaceans are often elongate, usually dorsoventrally depressed peracarid crustaceans. Combined , these 2 orders include an estimated 11,500 described species (Blazewicz-Paszkowycz et al. 2012; J. Williams and Boyko 2012). The majority of isopods and tanaidaceans exhibit direct development, passing from embryo to manca stages (the latter lacking the posterior pair of thoracopods, called pereopods in these taxa) to juveniles to adults. The oostegites of females are proximal plate-like limb branches of pereopods 2–5, and sometimes the maxilliped (thoracopod 1) (for a maximum of limbs 1–6; see Boxshall and Jaume 2009), which, together with the corresponding sternites, form an egg receptacle (the marsupium, but see Harrison 1984 for variations in brood-pouch formation). These, along with the gonopods (modified first pleopods) of males, are added in the molt(s) before the final molt to adulthood. Isopods, but not tanaids, molt biphasically, using body and/or limb movements to shed the anterior half of their exoskeleton prior to the posterior half (W. Johnson et al. 2001). Most of the parasitic isopods (e.g., the Cymothooidea) also show direct development, but members of the Gnathiidae and the epicarideans (the Bopyroidea and Cryptoniscoidea) have complex life cycles, with one or more juvenile stages differing radically from the adult forms. Before the life cycles were understood, some of these stages (e.g., praniza and microniscus) were once thought to represent taxa distinct from the adults. Additionally, males and females in these groups show the greatest amount of sexual dimorphism among isopods. Some species of terrestrial isopods (the Oniscoidea ), and marine arcturid and antarcturid isopods, show parental care for their young after hatching. The life cycle consists of the following stages in the groups treated in this chapter: manca I–III, juvenile, adult (most Isopoda ); zuphea I, praniza I, zuphea II, praniza II, zuphea III, praniza III, non-feeding adult (the Gnathiidae); epicaridium, microniscus, cryptoniscus, bopyridium, adult (the Bopyroidea ); epicaridium, microniscus, cryptoniscus, sac-like female or cryptoniscoid-type male (the Cryptoniscoidea); and manca I–IV, juvenile, adult (the Tanaidacea). larval types Embryonic Development: Like other peracarids, isopods and tanaidaceans do not have true larvae. Instead, they hatch as miniature versions of the adult, with some exceptions (see below). Some details of their embryonic development are provided here for comparison with the development of other peracarids and with other crustaceans. Manca: In most isopods, the embryo (fig. 40.1A–F) hatches into the first manca stage (fig. 40.1G) and then becomes a second, more fully developed manca (fig. 40.1H), although the latter stage still lacks the seventh pereopods (the last thoracopods ). It is typically this second manca that emerges from the marsupium. Complete development of the posterior (seventh) pair of pereopods and differentiation of secondary sexual characteristics requires an additional third manca stage (Tomescu and Craciun 1987; Brum and Araujo 2007). Subsequent juvenile stages have seven pairs of fully formed pereopods and pleopods, except in those neotonous taxa lacking the seventh pereopods in adults (some anthuroids, and all gnathiids). Further differentiation of secondary sexual characters occurs during subsequent molts, and juveniles develop into either males or females, although protogyny and protandry are common (e.g., Araujo et al. 2004). Praniza: The larval form in gnathiids is termed the praniza (figs. 40.2A; 40.3B–D, F–I), of which there are three postmarsupial instars. The praniza is a specialized stage for ectoparasitic feeding on fish hosts (fig. 40.3C). Zuphea: An unfed praniza larva is sometimes called a zuphea larva (figs. 40.2B; 40.3A, E), with the term praniza reserved for instars swollen with blood from the host (Smit and Davies 2004). There are no juvenile stages between the three zuphea/praniza instars and the adult forms, which is unusual in isopods. Adult females are more similar to the praniza Christopher B. Boyko Carsten Wolff Christopher B. Boyko and Carsten Wolff Isopoda and Tanaidacea Isopoda and Tanaidacea 211 larvae but are readily distinguishable from them. Adult males are modified as non-feeding forms, with greatly enlarged mandibles used to defend their burrow openings and the resident female(s) (K. Tanaka 2007). Epicaridium: Bopyroids (the Bopyridae, Dajidae, and Entoniscidae ) and cryptoniscoids (7 families) exhibit the most complex isopod life cycle. In all species, the egg hatches into an epicaridium larva (fig. 40.2C, G) that moves into the water column to seek out its first host, a calanoid copepod (Boyko and Williams 2009). Microniscus: Once it is attached to the host, the epicaridium larva molts into the...