Atlas of Crustacean Larvae

13. Cladocera: Haplopoda

13 general: The Haplopoda is a taxonomically small group of large (typically about 12 mm), nearly transparent predatory water fleas (cladocerans). It consists of only 1 genus, Leptodora, which contains 1 widespread species, L. kindtii, described more than 150 years ago in an 1844 newspaper article by Focke (see Dumont and Hollwedel 2009), and another species, L. richardi, described recently (Korovchinsky 2009). Leptodora kindtii is fairly widespread in fresh water, and sometimes occurs in brackish water (e.g., parts of Chesapeake Bay) (W. Johnson and Allen 2012), but it is most often found in the deeper parts of lakes across the Northern Hemisphere. The species was recently shown to have had a rather complicated evolutionary history (Xu et al. 2010). Unlike most other cladocerans, Leptodora is an obligate carnivore, feeding on other planktonic crustaceans such as smaller water fleas and copepods, and much of its peculiar morphology is explainable as adaptations to this lifestyle. This includes its nearly hyaline body; large eye; two rows of segmented legs, forming a large feeding basket; and modified mouth region, housing a pair of styliform mandibles and a peculiar trilobed lower lip. The large abdomen is also possibly (at least in part) an adaptation to a predatory lifestyle. Live prey (e.g., Diaphanosoma ) are seized by the long first pair of legs and maneuvered into the feeding basket, while the long limbless abdomen is temporarily bent forward to assist in trapping the prey. The styliform mandibles are pushed deeply into the tissue of the prey, and the pharynx starts suctioning movements (Sebestyén 1931; Herzig and Auer 1990; Browman et al. 1989). Another specialization in Leptodora, unseen in other cladocerans, is the presence of free-living metanauplii in the sexual part of the life cycle. The unusual appearance of Leptodora has resulted in much attention being paid to its phylogenetic position. Most of the recent morphological and molecular evidence suggest that Leptodora (Haplopoda) is the sister-group to the Onychopoda, another group of water fleas to which this species bears some resemblance in its morphology (segmented trunk legs) and feeding behavior (raptorial feeding, in contrast to filtration) (J. W. Martin and Cash-Clark 1995; Olesen 1998; Richter et al. 2001, 2007, Swain and Taylor 2003). As in other cladocerans, the life cycle of Leptodora is heterogonic (alternating parthenogenesis and sexual reproduction). larval types Metanauplius and Direct Development: The life cycle is heterogonic. It begins in the early spring, when free-living metanauplii hatch from the resting eggs (winter eggs). The first worker to report free-living larvae (metanauplii) of Leptodora was Sars (1873), and apparently there have been only two later treatments (Warren 1901; Sebestyén 1949). In all cases the larvae were collected in early spring (February–April). According to Sebestyén (1949), who studied their development based on specimens kept in petri dishes, the larval phase is passed exceedingly fast. By the third day after hatching the legs assume their final position, being ready to act as a feeding basket . This short duration may explain the few reports of freeliving larvae. Based on Sebestyén (1949), three larval stages seem to be passed before the feeding basket has been achieved. Sars (1873), however, depicted a larval stage apparently not accounted for by Sebestyén (1949), so the number of stages seems to be at least four, or even higher. This topic needs more study. When the larvae have become adults, parthenogenesis, which is by far the dominant part of the life cycle, starts, and it lasts from spring to autumn. Varying numbers of eggs are deposited in the dorsal brood pouch (modified carapace) via the dorsally extending oviduct. T. Andrews (1948) reported that egg deposition required 5–10 minutes, starting immediately following ecdysis at the beginning of the first adult instar. All early developmental stages (embryos) are passed within the brood pouch (direct development), and the offspring are not released before they assume the shape of the adults. Both T. Andrews (1948) and Boikova (2008) reported that the brooding period is around 70–90 hours (at room temperature). According to T. Andrews (1948), the free-swimming juveniles/ adults are released a few hours prior to the following ecdysis Jørgen Olesen Jørgen Olesen Cladocera: Haplopoda 74 Jørgen Olesen of the mother. At the end of the parthenogenetic part of the life cycle, when conditions become unfavorable (in autumn), males appear and resting eggs are produced, which serve to carry the species through the winter. Resting eggs were studied briefly by...