Introduction to Cladocera (water fleas)
In current classification Cladocera is considered a suborder of the crustacean class Branchiopoda (meaning "gill feet"), order Diplostraca (Martin & Davis, 2001), although the higher classification of Branchiopoda is still undergoing changes (Olesen, 1998; Richter et al., 2007). The majority of water fleas are freshwater species with only a few species living in marine waters, but quite a large number particularly adapted for the brackish waters of the Ponto-Caspian region (the Black Sea, Caspian Sea and the Sea of Azov). Very few cladocerans are parasitic. Most species are small, <5 mm long.
Most cladocerans have a carapace covering most of the body, except the head. No segmentation is visible on the carapace, but in many species the carapace forms a spine posteriorly. Sometimes there is also a spine on top of the head. The second antennae are very well developed and muscular and are used for swimming. The swimming strokes of these appendages make the animals move in tiny jumps, which is probably why they are called "water fleas". Cladocerans have a large, sessile compound eye formed by the fusion of paired eyes. The eye is capable of rotating in different directions. Typical cladocerans have 5-6 pairs of "trunk" appendages, and their bodies are not divided into a separate thorax and abdomen. The tip of the trunk forms a "postabdomen", which is bent towards the ventral trunk surface and is equipped with claws and spines for cleaning the carapace.
Most cladocerans are suspension feeders, filtrating food particles with bristles on the trunk appendages and transporting them to the mouth in a mid-ventral food groove (Geller & Müller, 1981).
Many cladocerans, including the common freshwater Daphnia, have complex life cycles, including stages that reproduce parthenogenetically and also cyclomorphosis, i.e., different body-shapes at different seasons. The reproductive pattern depends on environmental factors, and often an entire population appears to consist of only females. Eggs are brooded inside the carapace. Some species may produce resting eggs which are highly resistant and therefore survive long-distance transportation under adverse conditions.
The infraorder Onychopoda have a rather aberrant morphology. The carapace is reduced to forming just a dorsal brood chamber, and the thorax and abdomen are distinctly separated. They have only 4 pairs of thoracic appendages. The abdomen may be long and equipped with long caudal spines, as seen in some of the Ponto-Caspian species that have become invasive in Europe and North America. The cuticle of these spines is not completely shed during moulting, and hence the length increases with every moult. Onychopods are predatory and have lost the ventral food groove.
In Nordic waters the "fish-hook water flea", Cercopagis pengoi, has invaded the Baltic Sea from the Ponto-Caspian region (see separate fact sheet). Three other Ponto-Caspian species, Evadne anonyx G.O. Sars, 1897, Podonevadne trigona (G.O. Sars, 1897) and Cornigerius maeoticus (Pengo, 1879) have been introduced to some oligohaline parts of the Baltic Sea (Panov et al., 2007). Another species, Penilia avirostris, has been found to be common in Kattegat since 2002 (Johns et al., 2005). The species has an almost cosmopolitan tropical-subtropical distribution, so possibly it should be considered cryptogenic. It should also be mentioned that the North European freshwater species Bythotrephes longimanus (=B. cederstroemi) has been introduced and become invasive in the Great Lakes of North America
Geller, W. and Muller, H. 1981. The filtration apparatus of Cladocera: Filter mesh-sizes and their implications on food selectivity. Oecologia 49: 316-321.
Johns, D.G., Edwards, M., Greve, W. and SJohn, A.W.G. 2005. Increasing prevalence of the marine cladoceran Penilia avirostris (Dana, 1852) in the North Sea. Helgoland Marine Research 59: 214-218.
Martin, J.W. and Davis, G.E. 2001. An updated classification of the recent Crustacea. Science Series 39, Natural History Museum of Los Angeles County, Los Angeles, California (USA), vii + 123pp.
Olesen, J. 1998. A phylogenetic analysis of the Conchostraca and Cladocera (Crustacea, Branchiopoda, Diplostraca). Zoological Journal of the Linnean Society 122: 491-536.
Olesen, J. 2007. Monophyly and phylogeny of Branchiopoda, with focus on morphology and homologies of branchiopod phyllopodous limbs. Journal of Crustacean Biology 27(2): 165-183.
Panov, V.E., Rodionova, N.V., Bolshagin, P.V. and Bychek, E.A. 2007. Invasion biology of Ponto-Caspian onychopod cladocerans (Crustacea: Cladocera: Onychopoda). Hydrobiologia 590: 3-14.
Põllupüü, M., Simm, M., Põllumäe, A. and Ojeveer, H. 2008. Successful establishment of the Ponto-Caspian alien cladoceran Evadne anonyx G.O. Sars 1897 in low-salinity environment in the Baltic Sea. Journal of Plankton Research 30(7): 777-782.
Richter, S., Olesen, J. and Wheeler, W.C. 2007. Phylogeny of Branchiopoda (Crustacea) based on a vombined analysis of morphological data and six molecular loci. Cladistics 23: 301-336.