Atlas of Crustacean Larvae

7. Notostraca

7 general: Notostracans are commonly called tadpole shrimps. They are a species-poor group of distinctly archaiclooking branchiopod crustaceans. The number of species is much debated because of morphological plasticity, and cryptic species may be involved (King and Hanner 1998; Rogers 2001; M. Korn and Hundsdoerfer 2006). Traditionally, 11 species divided in 2 genera (Triops and Lepidurus) have been recognized (Linder 1952; Longhurst 1955; Brendonck et al. 2008). Notostracans , like most other large branchiopods, inhabit inland freshwater (occasionally brackish) sites, mostly temporary pools on all continents except Antarctica. Notostracans are rare in most places, but they can be locally very abundant in most parts of the world. Some species can reach 100 mm long (J. W. Martin 1992) but most are around 30–50 mm. Superficially , notostracans are characterized by a shield-like dorsal carapace and a long limbless tail section, the latter composed of a variable number of cylindrical trunk segments (often termed body rings) mostly extending beyond the carapace and ending in a pair of long, thin, multiarticulate caudal rami. The endites of trunk limb 1 are long and antenna-like. The compound eyes are internalized. In some taxa, the trunk may bear as many as 72 pairs of limbs (J. W. Martin 1992), the posteriormost of which are very small and densely placed, with (unseen in other crustaceans) several limb pairs attached to each trunk segment (polypody) (e.g., fig. 7.3K, showing a juvenile; also see Linder 1952). The trunk limbs vary greatly in morphology along the body axis (reduced serial similarity), but at least the posterior trunk limbs are clearly phyllopodous, while the anterior ones have various sclerotized parts and appear almost segmented . Notostracans are omnivorous (consuming detritus and a variety of small organisms) and predominantly benthic, although they can swim well (Fryer 1988). The monophyly of the Notostraca has never been questioned, but their phylogenetic position within the Branchiopoda is not entirely settled. Morphological data support the Notostraca as a sister-group to the “bivalved branchiopods” (the Diplostraca) (Olesen 2007, 2009; Richter et al. 2007), while molecular data often place them at various positions within the Diplostraca (e.g., Taylor et al. 1999; Spears and Abele 2000; Braband et al. 2002; Stenderup et al. 2006). Reproduction is complicated and known to vary among populations (see the summary in Zierold et al. 2009). In Triops cancriformis, populations can be gonochoric, hermaphoditic, or androdioecious. The latter is a very rare reproductive mode in animals, in which populations are made of hermaphrodites, with a small proportion of males (e.g., Zierold et al. 2009). This lack of males long suggested that Triops was parthenogenetic , but the presence of testicular lobes seems to suggest hermaphroditism (Zaffagnini and Trentini 1980; Zierold et al. 2009). The eggs are released by the female and then carried for a while in pouches on the eleventh pair of trunk limbs (Fryer 1988). Various authors have reported that Lepidurus arcticus attaches its eggs to moss fronds (see Fryer 1988). When dry, the shed eggs enter into a diapause phase before hatching. Their life cycle after hatching is very fast, and individuals of Triops cancriformis become mature about two weeks after hatching. larval types Metanauplius and Later Stages: Larval development is known in most detail for Triops cancriformis (Claus 1873; Fryer 1988; Møller et al. 2003) and for Lepidurus arcticus (Borgstrøm and Larsson 1974), but the development of L. apus and T. longicaudatus is also partly known (Brauer 1874; Fryer 1988), and, at least between Triops and Lepidurus, there are significant differences (see below). Here we will refer to T. cancriformis, the early instars of which have been restudied during the preparation of this chapter. The development of T. cancriformis is essentially anamorphic (gradual), but, in contrast to the extreme gradual development of anostracans (at least in some species) (see chapter 5), large steps are taken at each molt, resulting in substantial morphological differences from stage to stage. For example, at stage 3 of T. cancriformis, the rudiments of about 10 trunk limbs are already present (fig. 7.2), while in stage 3 of Artemia salina only a few anterior limbs are present as rudiJ ørgen Olesen Ole Sten Møller Jørgen Olesen and Ole Sten Møller Notostraca Notostraca 41 mentary limb buds (Schrehardt 1987). As in anostracans, the development of notostracans does not naturally fall into distinct phases. In the first two stages of T. cancriformis, only the naupliar appendages are functional and active (fig. 7.1A–C), while the...