Plocamionida ambigua (Bowerbank, 1866)


Family : Hymedesmiidae

Form: Thin sheets, up to ca. 20cms across.

Colour: Orange, yellow or ochre. Grey-brown in alcohol, light brown dried.

Smell: None

Consistency: Soft, but difficult to determine as very thin.

Surface: Even, hispid.

Apertures: Oscules numerous and small.

Contraction: None

Internal characters

Skeleton: A basal layer of acanthostrongyles lying tangentially to the substratum with short, entirely spined acanthostyles oriented more or less at right angles to the substratum with their heads towards the base. Much larger acanthostyles with spines almost confined to the head regions project from this basal layer. There is a surface layer of tornotes arranged more or less tangentially. Chelae arcuatae are scattered sparsely in a layer near the surface.

Spicules: The principal spicules of the main skeleton are large acanthostyles, measuring 365-750μm. They are minutely and basally spined. The auxilliary skeletal spicules are smaller, entirely spined, acanthostyles, ca. 155-300μm. In juvenile specimens these auxilliary acanthostyles echinate the substrate, and they echinate the indistinct plumose columns of large acanthostyles in mature forms. Acanthostrongyles form a basal layer on the substratum. They are entirely spined, curved, possess (often unequally) swollen ends, and can be distorted and variable; 70-180μm in length. The ectosomal spicules are straight, very slender tornotes, ca. 200-250μm in length. They may have short and simple mucrons at their ends. The microscleres are chelae arcuatae with slightly curved shafts, ca. 15-20μm.

Habitat: On bivalve shells, stones, bedrock and saddle oysters, 20-200m.

Distribution: Channel (Straits of Dover, Roscoff), Shetland, Norway, Sweden, and more generally in the N.E. Atlantic. Known recently from Firth of Lorne (W. Scotland), N. and S.W. Ireland. Not uncommon. Considerable confusion with other species of Plocamionida and probably with several undescribed species in the north Atlantic and Mediterranean.

Distribution Map: NBN map : National Biodiversity Network mapping facility, data for UK.

Identity: The diagnosis for the genus Plocamionida is a Microciona like skeleton, with diactinal ectosomal spicules (tornotes), and chelae with curved shafts. This is based on a misconception as this genus truly belongs in the Hymedesmiidae! A basal layer of fairly distinctive asymmetrical acanthostrongyles immediately suggests this species, but beware Plocamiancora arndti, which is not uncommon in the Sound of Mull and around Rathlin Island. It has anchorate chelae and more robust, symmetrical acanthostrongyles. Antho species are often confused, but have toxa and subtylostyles. These subtylostyles are in bouquets in a surface layer and have microspined heads, as with most Clathriidae.

Voucher: BELUM : Mc1528. Ardnoe Point, Sound of Jura, Scotland.

Editors: D. Moss, B.E. Picton.

 Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland