Atlas of Crustacean Larvae

5. Anostraca

5 general: Anostracan branchiopods are commonly called fairy shrimps or brine shrimps. They are a group of mostly colorful freshwater crustaceans, characterized by their peculiar habit of swimming upside down while using their phyllopodous thoracopods to filter organic particles from the water (although a few species scrape algae from surfaces). Anostracans are among the most diverse groups of branchiopods, with about 300 species divided among 8 families (Brendonck et al. 2008). Different from other extant branchiopods, they lack a dorsal shield (carapace). In the majority of species the body is composed of 19 body segments, the anterior 11 of which each carry a pair of thoracopods, followed by 2 genital segments, 6 limbless abdominal segments, and a telson. Some atypical species may bear as many as 17 or 19 thoracopods (in the subarctic genera Polyartemiella and Polyartemia). Anostracans are medium-sized crustaceans, usually 1–3 cm long, but a few raptorial species (e.g., Branchinecta raptor or B. gigas) can grow significantly larger, the latter up to 10 cm long (e.g., Fryer 1966; Rogers et al. 2006). Anostracans occur worldwide, and are even found in Antarctica, but their distribution is scattered, since all of the species are adapted to special types of water bodies, such as snowmelt ponds or desert pools after rain. The life cycle consists of dioecious (male and female) adults, but some strains of Artemia are parthenogenetic. Eggs are carried in a midventral brood pouch, located behind the last pair of thoracopods (fig. 5.1A). Two types of eggs are laid: (1) thin-shelled summer eggs that continue developing and hatch quickly, or (2) thick-shelled winter eggs (resting eggs) in which development is arrested at about the gastrula stage. Larvae hatch as orthonauplii and—at least in some species— their development is long and gradual (anamorphic). larval types Nauplius (Including Orthonauplius and Metanauplius) and Post-Larval Stages: Some anostracans have a very gradual (anamorphic) development, with only minor differences between the stages (Benesch 1969; Fryer 1983; Schrehardt 1987). As many as 25 pre-adult stages have been reported for Artemia salina (Benesch 1969; also see Walossek 1993), and Fryer (1983) identified 20 instars of Branchinecta ferox before the larvae acquired a full complement of functional trunk limbs. Weisz (1947) and Hentschel (1967) also reported relatively many stages. Other authors have reported fewer stages for various species (e.g., Claus 1873, 1886; Oehmichen 1921; Heath 1924; Cannon 1926; Hsü 1933; Pai 1958; D. Anderson 1967; Baqai 1983; Jurasz et al. 1983), but many of these descriptions may be incomplete (see Fryer 1983). There have been attempts to name various phases of the development of Artemia, but since its development does not naturally fall in distinct phases—in contrast to many other crustaceans—many of these attempts fall short or appear arbitrary. Schrehardt (1987), based on Kaestner (1967), used a quite detailed naming scheme for various phases (orthonaupliar, metanaupliar, post-metanupliar, post-larval) which has been adopted here for convenience, although any attempt to divide the development of anostracans into discrete phases conflicts with its gradual nature. The hatching stage in Artemia salina, and in other taxa where this stage is known (e.g., fig. 5.1B, D), is a lecithotrophic (containing yolk and carrying non-functional mouthparts) larva with no clear external anlagen of the post-mandibular limbs; this stage can therefore be termed an orthonauplius (Schrehardt 1987). The orthonaupliar phase (which lasts for only one stage in A. salina) ends with the formation of post-mandibular limb buds. The metanaupliar phase (which lasts for four stages in A. salina) ends when the first pair of trunk limbs has become functional. The post-metanaupliar phase (which lasts for seven stages in A. salina) ends when complete body segmentation has been achieved and the natatory function of the antennae has been lost. The post-larval phase (which lasts for five stages in A. salina) is characterized by the development of the antennae and genital structures. What complicates attempts to set up an unequivocal division of anostracan ontogeny into distinct and well-defined phases is not only that development is gradual, but also that Jørgen Olesen Jørgen Olesen Anostraca 30 Jørgen Olesen the naupliar (cephalic) feeding system in many intermediate developmental stages operates concomitantly with the adult (thoracic) feeding system (e.g., larvae in fig. 5.3). Many larval aspects, such as the natatory antennae with naupliar processes and the mandibular palps, are retained well into anostracan development, alongside a fully functional adult thoracopodal system...