Atlas of Crustacean Larvae

11. Cladocera: Anomopoda

11 general: The Anomopoda is a diverse taxon of water fleas (Cladocera). They are commonly found in most freshwater habitats, and even interstitially or in moist habitats (Frey 1980; J. W.Martin1992;DumontandNegrea2002).Forróetal.(2008) estimated 537 currently described species and recognized 11 families. Sizes range from less than 0.3 mm to about 6 mm, and males are smaller than females. The Daphniidae contains the “typical” Daphnia (fig. 11.1A), and most members of this family are open-water forms (or even pelagic), while other families, like the Eurycercidae (fig. 11.2A), Chydoridae (fig. 11.2H), Macrothricidae (fig. 11.2E), and Acantholeberidae, contain lesser known bottom-dwelling or vegetation-associated species. As do most other cladocerans, anomopods have a pair of biramous swimming antennae, small sensory antennules, a carapace covering only the trunk and appendages, and a post-abdomen ending in a pair of claws (e.g., Fryer 1987a; J. W. Martin 1992; Dumont and Negrea 2002). The five to six pairs of trunk limbs in the Anomopoda are not serially similar, as they are in the Ctenopoda; rather, each limb pair is modified to perform various functions in different taxa. These modifications are linked to a diversity in feeding strategies. Most species feed by filtration or scraping, but others are scavengers on other crustaceans (Pseudochydorus), and 1 genus, Anchistropus, is ectoparasitic on freshwater hydras (Fryer 1968, 1974, 1991). The monophyly of the Anomopoda has been supported by most workers (Fryer 1995; Olesen 1998; Negrea et al. 1999; Stenderup et al. 2006; Richter et al. 2007). A variety of phylogenetic positions of the Anomopoda within the Cladocera have been suggested, so this question must be considered unresolved. As in other cladocerans , the life cycle of anomopods is heterogonic (alternating parthenogenesis and sexual reproduction). larval types Stages (Instars): There are no true larvae, but the stages of development (instars) are described here. The life cycle is heterogonic. In temperate regions many cycles of parthenogenetic generations normally occur from spring to late summer or early autumn, followed (after mating) by the release of resting eggs that hatch in the spring and start a new parthenogenetic cycle (e.g., Dumont and Negrea 2002), essentially similar to what is summarized for ctenopod cladocerans in chapter 12. As in ctenopods, development is direct and takes place in a brood pouch formed by the carapace valves. Development from resting eggs is also direct (as opposed to Leptodora kindtii; see chapter 13). Most earlier treatments have dealt with early embryogenesis or single stages (e.g., Samassa 1893; Cannon 1921; Baldass 1941), while important aspects of later development have escaped attention. The most detailed recent treatment is that of Kotov and Boikova (2001), who studied the late embryogenesis of Daphnia galeata and D. hyalina by observing live embryos removed from the brood chambers of females. As for the Ctenopoda (see chapter 12), Kotov and Boikova (2001) recognized four stages (instars) taking place in the female’s brood chamber, separated by the shedding of membranes (molts). An important difference between the Ctenopoda and Anomopoda is that one more instar is incorporated into the egg in the latter, which therefore hatches as instar IV, not instar III (still inside the female’s brood chamber). Hence the development of the Anomopoda appears to be shortened, or more embryonized, compared with the Ctenopoda. In the resting eggs of anomopods (and ctenopods and onychopods) all embryonic stages are incorporated into the egg membrane (Kotov and Boikova 2001). morphology: The development of Daphnia hyalina, about which many details are known, is briefly summarized here, based mostly on Kotov and Boikova (2001). Surprisingly little is known about the late development of other species of the diverse Anomopoda, so my summary of D. hyalina is supplemented by new SEM examinations of the developmental stages of 6 selected anomopod species, representing 5 different families. Daphnia hyalina (Daphniidae) (fig. 11.1B): The laying of parthenogenetic eggs in the dorsal brood chamber in DaphJ ørgen Olesen Jørgen Olesen Cladocera: Anomopoda 64 Jørgen Olesen nia has often been described (Weismann 1876–1879; Baldass 1941; Rossi 1980; Zaffagnini 1987). The first instar lasts until the (first) outer egg membrane is shed, after approximately 16 hours (Kotov and Boikova 2001). It covers early embryogenesis and the early appearance of the antennules, antennae , labrum, maxillary zone, and first thoracic segment. The second instar lasts until the second membrane is shed, after approximately 20 hours (Kotov and Boikova (2001). During this instar the maxillules and maxillae start to differentiate, the...