(Bowerbank, 1866)
Species Overview
Plocamionida ambigua (Bowerbank, 1866) is a thinly encrusting pale red or orangy sponge, which can only identified with certainty by microscopic examination. It has acanthostrongyles in combination with arcuate isochelae as tell-tale characters. It is a wide-spread deep-sublittoral species.
Taxonomic Description
Colour: Pale red (Mediterranean specimens), orange, yellow or ochre. Grey-brown in alcohol, light brown dried.
Shape, size, surface and consistency: Thin sheets, up to ca. 20 cm across. Surface even, hispid; reported as tuberculate in Mediterranean specimens. Oscules numerous and small. Consistency soft, but difficult to determine because very thin.
Spicules: (Plocamionida ambigua spics) Megascleres : The ectosomal spicules are straight, very slender tornotes, ca. 200-420 µm in length. They may have short and simple mucrons at their ends. The principal spicules of the main skeleton are large acanthostyles, measuring 500-750 µm (Arndt: to 2000 µm) x 28 µm. They are minutely and basally spined. The auxiliary spicules are smaller, entirely spined acanthostyles, ca. 155-400 µm. In juvenile specimens these auxiliary acanthostyles echinate the substrate, and they echinate the indistinct plumose columns of large acanthostyles in mature forms. (Note that for Mediterranean specimens, the lengths of these two categories of acanthostyles are given as up to 375 µm, and 110-130 µm respectively—i.e. much shorter than quoted above). Acanthostrongyles form a basal layer on the substratum. They are entirely spined, curved, possess (often unequally) swollen ends, and can be distorted and variable: 70-180 µm in length.
Microscleres are chelae arcuatae with slightly curved shafts, ca. 19-37 µm.
Skeleton: (Plocamionida ambigua skel) A basal layer of acanthostrongyles lying tangentially to the substratum or forming an irregular thin basal reticulation ("plocamiid" skeleton) 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.
Ecology: On bivalve shells, stones, bedrock and saddle oysters, 40-360 m. Not uncommon.
Distribution: W. Scotland, N. and S.W. Ireland, Channel (Straits of Dover, Roscoff), Shetland, Faroes, Norway, Sweden, and more generally in the N.E. Atlantic. Mediterranean.
Etymology: The name refers to the fact that young stages of the species apparently differ rather dramatically from mature stages, and might easily have led to assignment to different genera.
Type specimen information: The type is in the Natural History Museum, London, BMNH 1910.1.1.65 (dry), Shetland. Norman Collection + slides unregistered (Bk.435). MCS voucher BELUM Mc1528, Ardnoe Point, Sound of Jura, Scotland.
Remarks
For a certain identification examination of the skeletal architecture and the spicule is necessary. A basal layer of fairly distinctive asymmetrical acanthostrongyles in combination with oxea-like tornotes and arcuate isochelae immediately suggests this species, but beware of deep-water species Plocamiancora arndti , which is not uncommon in the Sound of Mull and around Rathlin Island, NW Ireland. It has polydentate anchorate chelae, more robust symmetrical acanthostrongyles, and ectosomal tylotes.
The various Antho species, which also have a "plocamiid" skeleton, are easily distinguished by their palmate isochelae and toxas.
Source: Ackers et al., (1992) (D. Moss, B.E. Picton, G. Ackers).