Onychopoda are a specialised suborder of branchiopod crustacean

Crustacean

Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, belonging to the order Cladocera

Cladocera

Cladocera is an order of small crustaceans commonly called water fleas. Around 620 species have been recognised so far, with many more undescribed. They are ubiquitous in inland aquatic habitats, but rare in the oceans. Most are long, with a down-turned head, and a carapace covering the apparently...

.

The suborder Onychopoda is "one of the most morphologically distinctive groups of cladocerans". They have only four pairs of legs

Arthropod leg

The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

, compared to five or six pairs in Ctenopoda and Anomopoda

Anomopoda

Anomopoda is a suborder of the order Cladocera. These crustaceans, a type of water flea, are members of the Class Branchiopoda. The Anomopoda typically have five pairs of thoracic limbs, but sometimes have six pairs. The head of the Anomopoda lacks a clear separation from the trunk and the...

. Unusually among branchiopod crustaceans

Branchiopoda

Branchiopoda is a class of crustaceans. It is the sister group to the remaining crustaceans, and comprises fairy shrimp, clam shrimp, Cladocera, Notostraca and the Devonian Lepidocaris...

, Onychopoda share with Haplopoda the presence of segmented appendages, which are used for grasping prey.

Most species of Onychopoda live in the waters of the Ponto-Caspian basin (Caspian Sea

Caspian Sea

The Caspian Sea is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. The sea has a surface area of and a volume of...

, Aral Sea

Aral Sea

The Aral Sea was a lake that lay between Kazakhstan in the north and Karakalpakstan, an autonomous region of Uzbekistan, in the south...

, Black Sea

Black Sea

The Black Sea is bounded by Europe, Anatolia and the Caucasus and is ultimately connected to the Atlantic Ocean via the Mediterranean and the Aegean seas and various straits. The Bosphorus strait connects it to the Sea of Marmara, and the strait of the Dardanelles connects that sea to the Aegean...

 including Sea of Azov

Sea of Azov

The Sea of Azov , known in Classical Antiquity as Lake Maeotis, is a sea on the south of Eastern Europe. It is linked by the narrow Strait of Kerch to the Black Sea to the south and is bounded on the north by Ukraine mainland, on the east by Russia, and on the west by the Ukraine's Crimean...

), in remnants of the ancient Paratethys

Paratethys

The Paratethys ocean, Paratethys sea or just Paratethys was a large shallow sea that stretched from the region north of the Alps over Central Europe to the Aral Sea in western Asia. The sea was formed during the Oxfordian epoch as an extension of the rift that formed the Central Atlantic Ocean and...

 ocean. Some other species live in fresh water

Fresh Water

Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

 or in the ocean

Ocean

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

s, where they can be widespread.

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

• 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 embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

s are protected by a brood pouch, which also secretes nutrient

Nutrient

A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

s 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

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

.

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