Atlas of Crustacean Larvae

4. Introduction to the Branchiopoda

4 The Branchiopoda is a relatively species-poor assemblage —estimated at approximately 1,000 extant species (J. W. Martin and Davis 2006; Brendonck et al. 2008)—of small, typically phyllopodous (leaf-limbed) crustaceans. Somewhat unusually among crustaceans, they are known almost exclusively from ephemeral ponds, lakes, and other bodies of fresh water on all continents; only a few species are marine (Brendonck et al. 2008; Rogers 2009). Most are small (5–30 mm long as adults), but some predatory anostracans (e.g., Branchinecta gigas) can reach adult lengths of 150 mm. Although traditionally divided into four main groups—the Anostraca, Notostraca, Conchostraca, and Cladocera—it is now known that this simple division masks a far greater diversity than was formerly recognized. The group today is treated as the three groups Anostraca, Notostraca, and Diplostraca. The Diplostraca contains the Laevicaudata, Spinicaudata, and Cladoceromorpha (clam shrimps and water fleas). The Spinicaudata and Cladoceromorpha have recently been suggested to constitute the Onychocaudata (Olesen and Richter 2013). The Cladoceromorpha, in turn, contains the Cyclestherida [monotypic] and the cladoceran groups Anomopoda, Ctenopoda, Haplopoda, and Onychopoda (the latter two united in the Gymnomera) (following Olesen and Richter 2013). Branchiopods are also known from the fossil record, including the Devonian †Lepidocaris rhyniensis from the Rhynie chert and the fascinating Cambrian †Rehbachiella from the Orsten fauna (Walossek 1993; also see chapter 3). Anostracans (commonly called fairy shrimps or brine shrimps) are easily recognizable crustaceans that are archaic looking in some respects. They are graceful upside-down swimmers that lack a carapace and bear paired, stalked compound eyes; both characters are unique in the Branchiopoda. Notostracans, commonly called shield shrimps or tadpole shrimps (the latter because of their superficial resemblance to frog larvae), are large (up to 100 mm long in some cases, although most species are in the 30–50 mm range as adults) predatory and scavenging branchiopods easily recognizable by their sizeable, horseshoe-shaped dorsal shield and long multisegemented abdomen bearing filamentous caudal rami (cercopods). “Conchostracans” (now partitioned among the Spinicaudata, Cyclestherida, and Laevicaudata) are commonly called clam shrimps, because of their superficial resemblance to bivalved mollusks; all species have a bivalved carapace. Cladocerans (sometimes called water fleas) are recognized today as encompassing many disparate morphological forms with unclear relationships. Most species have a recognizable head and trunk, a bivalved carapace that does not enclose the head, a single compound eye, four to six trunk limbs, and a limbless recurved post-abdomen with a pair of strong terminal claws. Branchiopods have long played an important role in discussions of crustacean phylogeny, with various workers stressing their presumed primitive morphology. Monophyly of the group has rarely been questioned, but relationships within the Branchiopoda are still not entirely understood and are the subject of much study (e.g., Fryer 1987a; J. W. Martin 1992; J. W. Martin and Davis 2001; Walossek 1993; Olesen et al. 1996; Olesen 2000, 2004, 2007, 2009; Olesen and Grygier 2004; Stenderup et al. 2006; Richter et al. 2007; Regier et al. 2010). Enough is known, however, to state that, whereas the Cladocera appears to be a natural group, the grouping of large branchiopods is not. Larval developmental modes range from nearly completely anamorphic development (e.g., in some anostracans) to extreme metamorphosis (e.g., in the Laevicaudata). Nauplii (where known) are distinct from those of all other crustaceans in having short unsegmented antennules, large antennae in which the length of the protopod is more than half the length of the entire limb, and uniramous mandibles (Olesen 2007). All groups are treated in the ten chapters that follow, with the exception of the Cambrian †Rehbachiella, which is mentioned in chapter 3. Among the chapters on extant branchiopods, we have Joel W. Martin Jørgen Olesen Joel W. Martin and Jørgen Olesen Introduction to the Branchiopoda 28 Joel W. Martin and Jørgen Olesen included a chapter on some fascinating Devonian fossils from the Rhynie and Windyfield cherts (see chapter 6). Our reason for doing so is that three of these Devonian fossils— †Lepidocaris rhyniensis, †Castracollis wilsonae, and †Ebullitiocaris oviformis—have been identified as branchiopods (Scourfield 1926; Fayers and Trewin 2003; L. Anderson et al. 2004). Additionally , the unnamed fossils of orthonauplius larvae from the Windyfield chert were originally considered to have branchiopod affinities (C. Haug et al. 2012). Although †Lepidocaris is still considered a branchiopod, the branchiopod affinities of the Windyfield nauplii are now under discussion (see chapter 6). ...