Acrocnida

Background

The genus Acrocnida was not officially classified until 1926 by T. Gislén. It contains three species: Acrocnida brachiata, Acrocnida semisquamata, and Acrocnida spatulispina. Hybridization has been observed between Acrocnida brachiata and Acrocnida spatulispina. The species Acrocnida brachiata was recently found to actually be two species—Acrocnida brachiata and Acrocnida semisquamata—by Stöhr & Muths in 2010.

Members of Acrocnida prefer intertidal and subtidal habitats, though they are less likely to be found in intertidal areas. They generally inhabit sandy bottoms, specifically fine or muddy sand, in shallow coastal zones. Acrocnida brachiata is widespread, with its spread still being recorded as far as the 1970s in the North Sea; it was found in the Aegean Sea for the first time in 2007 after being noted on the Mediterranean coast of Turkey in 1995. Acrocnida semisquamata is primarily located off the coast of Ghana, and Acrocnida spatulispina has only been observed in the English Channel.

Morphologically, Acrocnida follows the body plan of other Amphiuridae, with a central disk and five long, thin arms. They possess only one outer mouth papilla, removed from the infradental papillae, and have mucous glands described as 'multicellular aggregations of pyriform cells with elongate necks.' Like other Ophiurida, they can part with limbs and regenerate new ones.