''Acrocnida brachiata'', the sand burrowing brittlestar, is a species of brittle star in the family

Amphiuridae Amphiuridae (commonly called long-armed burrowing brittle stars or burrowing brittle stars) are a large family of brittle stars of the suborder Gnathophiurina. Some species are used to study echinoderm development (e.g. '' Amphipholis kochii'' an ...

. It occurs on the seabed in the northeastern Atlantic Ocean and the North Sea, living semi-buried in the sand with only its arm tips projecting.

Description

''Acrocnida brachiata'' has a flat disc up to in diameter and five slender, clearly-demarcated, articulated arms up to in length. It is greyish-brown in colour. It can be distinguished from other similar species by the transverse furrows in the plates at the bases of the arms, the ventral scales bearing small tubercles and by the presence of an outer mouth papilla which is quite distinct from the paired papillae inside the mouth. The arms, which like other brittle stars flex sideways rather than up and down, have a pair of tentacle scales on each joint as well as numerous spines.

Distribution and habitat

''A. brachiata'' occurs in the northeastern Atlantic Ocean and North Sea. It is common round the coasts of the British Isles where its depth range is from the lower shore down to about . It is a

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

species, living on sandy bottoms and burying itself in the sediment, with only the tips of its arms projecting. It often occurs in association with the burrowing sea urchin ''

Echinocardium cordatum ''Echinocardium cordatum'', also known as the common heart urchin or the sea potato, is a sea urchin in the family (biology), family Loveniidae. It is found in sub-tidal regions in the NE Atlantic. Other un-named species have been identified as t ...

'', commonly known as the sea potato.

Taxonomy

There is a marked difference in morphology between populations in the intertidal zone and those thought to be the same species living in the subtidal zone. Study of these differences led to the describing of a new species ''

Acrocnida spatulispina ''Acrocnida'' is a genus of brittle stars in the family Amphiuridae. The genus contains three members: '' Acrocnida brachiata, Acrocnida semisquamata,'' and ''Acrocnida spatulispina.'' There has also been observed hybridization between both '' ...

'' by Sabine Stöhr and Delphine Muths in 2009, for the deeper water individuals. The brittle stars in the genus ''

Acrocnida ''Acrocnida'' is a genus of brittle stars in the family Amphiuridae. The genus contains three members: '' Acrocnida brachiata, Acrocnida semisquamata,'' and ''Acrocnida spatulispina.'' There has also been observed hybridization between both '' A ...

'' show morphological affinities with '' Amphiura chiajei'', but less with ''

Amphiura filiformis ''Amphiura filiformis'' is a species of brittle star belonging to the family Amphiuridae. It is found on the seabed in the north east Atlantic Ocean and adjoining seas to a depth of . It digs itself a shallow burrow in the sand and waves its arms ...

''.

References

{{Taxonbar, from=Q481986 Amphiuridae Animals described in 1804