Onychopoda Explained

Onychopoda are a specialised order of branchiopod crustaceans, belonging to the superorder Cladocera.

The order Onychopoda is "one of the most morphologically distinctive groups of cladocerans".^[1] They have only four pairs of legs, compared to five or six pairs in Ctenopoda and Anomopoda.^[2] Unusually among branchiopod crustaceans, Onychopoda share with Haplopoda the presence of segmented appendages, which are used for grasping prey.^[3]

Most species of Onychopoda live in the waters of the Ponto-Caspian basin (Caspian Sea, Aral Sea, Black Sea including Sea of Azov), in remnants of the ancient Paratethys ocean.^[4] Some other species live in fresh water or in the oceans, where they can be widespread.^[4]

There are three families, containing 10 genera and around 33 described species, most of which are endemic to the Ponto-Caspian basin:^[1]

• Cercopagididae Mordukhai-Boltovskoi, 1968 – 2 genera (Cercopagis and Bythotrephes), 14 species; Black Sea & Caspian Sea
• Podonidae Mordukhai-Boltovskoi, 1968 – 7 genera, 17 species; Pont-Caspian (Caspievadne, Cornigerius and Podonevadne) and marine (Evadne, Pleopis, Podon and Pseudevadne)
• Polyphemidae Baird, 1845 – 1 genus (Polyphemus), 2 species; fresh water

The embryos are protected by a brood pouch, which also secretes nutrients to aid their development. This may be related to the colonisation of the oceans, since the only other marine cladoceran, Penilia avirostris, has a similar pouch as a result of convergent evolution.^[1]

See also

• Bythotrephes longimanus
• Cercopagis pengoi

External links

• Podon sp. fact sheet - Guide to the marine zooplankton of south eastern Australia
• Evadne sp. fact sheet - Guide to the marine zooplankton of south eastern Australia

Notes and References

1. M. E. A. Cristescu . P. D. N. Hebert . amp . 2002 . Phylogeny and adaptive radiation in the Onychopoda (Crustacea, Cladocera): evidence from multiple gene sequences . . 15 . 5 . 838–849 . 10.1046/j.1420-9101.2002.00466.x . PDF.
2. Book: D. R. Khanna . 2004 . Biology of Arthropoda . . 978-81-7141-897-8 . Segmentation in arthropods . 316–394 .
3. Book: Richard A. Fortey . Richard H. Thomas . 1998 . Arthropod Relationships . Volume 55 of Systematics Association Series . . 978-0-412-75420-3 . A defence of arthropod polyphyly . G. Fryer . 23–34 .
4. Stefan Richter . Anke Braband . Nikolai Aladin . Gerhard Scholtz . 2001 . The phylogenetic relationships of "predatory water-fleas" (Cladocera: Onychopoda, Haplopoda) inferred from 12S rDNA . . 19 . 1 . 105–113 . 10.1006/mpev.2000.0901 . 11286495.