Podotreme or ‘Primitive’ Crabs

The ‘true’ crabs belong to the decapod crustacean Infraorder, Brachyura, with more than 7000 species, 1200 genera and 100 families. The predominant ‘true’ crabs in the deep sea are known as podotreme or ‘primitive’ crabs, which are placed in 92 genera in 10 families. They are often captured in deep-sea samples or seen in underwater images. Some of the best known podotreme crabs are the sponge crabs, which usually carry a live sponge on their back for camouflage and protection. Many other types of podotreme crabs also carry camouflage in the form of sponges, live corals or debris.

The ‘true’ crabs can be viewed as having three broad levels of anatomical organisation, based on the position of the reproductive structures, the gonopores. The podotreme crabs belong to the ‘first level’, with gonopores on the first segment of the walking legs (3rd leg in females, 5th leg in males). The podotreme crabs are also particularly interesting scientifically because they represent the ancient lineages and thus give much insight into the early evolution of crabs.

The taxonomy of the podotreme crabs has been extensively studied over the past several decades, although identification can still be difficult. This site enables identification of the genera and families. Several points should be noted about the presentation of the keys.

1. The family Poupiniidae is identified in the key to families but there is no key to Poupiniidae. This is because only one genus and species is so far known in the Poupiniidae, Poupinia hirsuta. If your specimen keys out as Poupiniidae, it is either P. hirsuta or something new to science.

2. The key to Cymonomidae includes the genus Cymonomus but not Cymonomoides. This is because the two genera are currently indistinguishable, and they will quite likely be synonymised (merged) in the future.

3. The family Dromiidae includes several subfamilies and some researchers treat one subfamily, Sphaerodromiinae (which includes Sphaerodromia and Eodromia), as an independent family. At present, we use Dromiidae in the wider sense to simplify identification.

Development of this website was supported by the Census of Marine Life project COMARGE (Continental Margin Ecosystems). The Delta databases available for download from this site are in TEST phase and will be regularly updated. For further information regarding these databases please contact Dr. Shane Ahyong Shane.Ahyong@austmus.gov.au or Dr. Joanne Taylor jtaylor@museum.vic.gov.au. Your feedback is welcome.

Delta Keys to Primitive Crabs

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