Atlas of Crustacean Larvae

9. Spinicaudata

9 general: Spinicaudatans are a group of branchiopods that (along with the laevicaudatans and cyclestheriids) are commonly known as clam shrimps (see chapters 8 and 10). They are a relatively species-poor group of branchiopods (ca. 150 species; Brendonck et al. 2008), characterized by their bivalved and growth-line-bearing shells (carapaces), somewhat resembling those of bivalved mollusks. They are further characterized by their small and unsegmented antennules, biramous swimming antennae, up to about 32 pairs of serially similar foliaceus trunk limbs (decreasing in size and complexity from front to back), and laterally compressed body end, which bears a pair of large claws. All trunk segments bear appendages (the abdomen is absent), which is unusual for crustaceans. Spinicaudatans occur worldwide, except in Antarctica, but their distribution is scattered, since all species are adapted to live in temporary waters of various kinds (e.g., desert pools after rain, snowmelt ponds, and rice paddies). A diverse fossil record is present, mostly of the carapace only (Tasch 1969), but sometimes limb details are preserved (W. Zhang et al. 1990; Orr and Briggs 1999). Currently 3 extant families—the Cyzicidae , Limnadiidae, and Leptestheriidae—are recognized by most authors (Olesen et al. 1996; J. W. Martin and Davis 2001; Olesen 2007; Rogers et al. 2012), but this may change, as relationships within the Spinicaudata are controversial (Naganawa 2001; Hoeh et al. 2006; Schwentner et al. 2009). Spinicaudata used to be grouped with the other clam shrimps (the Laevicaudata and Cyclestherida) in the “Conchostraca,” but this latter, traditional taxon has proven to be artificial (paraphyletic ) (Olesen 1998; Braband 2002; Richter et al. 2007). The life cycle begins with eggs carried beneath the carapace, attached to trunk limb exopods of the female, before deposition in the sediment (figs. 9.1A, B; 9.2L; 9.3K). After hatching, there are five to seven larval (naupliar) stages, but possibly a different number of true instars, before metamorphosis to the bivalved juvenile (example shown in fig. 9.3J). The reproductive pattern of the Spinicaudata exhibits a wide range of breeding systems, including dioecy, androdioecy, parthenogenesis, and hermaphroditism (Sassaman 1995; Weeks et al. 2009). larval types Nauplius: The first part of the post-embryonic development of spinicaudatans is the naupliar phase, which consists of five to seven stages, not all of which clearly represent molt-separated instars. The nauplii are characterized externally by a generally dorsoventrally flattened overall appearance, bud-like antennules , the presence of naupliar processes on the antennae, uniramous mandibular palps, non-articulated caudal claws, and (only in later larvae) non-functional lobate trunk limbs and a small dorsal carapace covering only the anterior pairs of these limbs (figs. 9.1; 9.2; 9.3). Several naming schemes have been applied to the relatively few larval stages of the Spinicaudata . In a number of species—such as Leptestheria saetosa, L. dahalacensis, and Imnadia yeyetta—some authors have recognized five larval stages, which have been termed nauplius I and II, metanauplius, peltatulus, and heilophore (Botnariuc 1947; Petrov 1992; Eder 2002). Other authors have not followed this terminology, instead referring more generally to larvae in the post-embryonic (but pre-juvenile) phase as larvae, stages, or nauplii (D. Anderson 1967; Olesen and Grygier 2003, 2004; Pabst and Richter 2004). Here we use the second set of terms, a nauplius being defined by the presence of a naupliar feeding apparatus, including a pair of distinct antennal naupliar processes. The first stage(s) of all spinicaudatan taxa can be termed orthonauplii, since no post-mandibular limb buds are present externally. There is no clear taxon-related pattern in the number of naupliar stages. Five, six, and seven stages have been reported for both the Limnadiidae and Cyzicidae, while five is the highest number reported for the Leptestheriidae (table 9.1). morphology: The state of development of a nauplius within a spinicaudatan larval series is typically distinguished by combinations of characteristics: (1) degree of lecithotrophy Jørgen Olesen Mark J. Grygier Jørgen Olesen and Mark J. Grygier Spinicaudata 52 Jørgen Olesen and Mark J. Grygier (yolk and undeveloped feeding parts), (2) number of small sensilla on the antennular limb buds, (3) morphology (unbranched or branched) of the antennal naupliar processes, (4) number of pairs of limb buds and their state of development , (5) presence and size of the early carapace, (6) shape and size of caudal lobes/claws, and (6) overall size. Most differences in larval development between various spinicaudatans reflect variability in the timing of the development of the structures mentioned...