Myxilla (Styloptilon) ancoratum (Cabioch, 1968) is a thinly encrusting deep-water species, identifiable only on its microscopic skeletal features. It has been recorded only from Roscoff, NW France, and Rathlin Island, Northern Ireland.
Colour: Yellowish white in alcohol.
Shape, size, surface and consistency: Thinly encrusting, about 1 mm in thickness, agglutinating shell debris. The surface is finely hispid.
Spicules: Megascleres are acanthostyles and tornotes. The tornotes are smooth and have pointed, but unequal ends, one being slightly swollen; they may be somewhat polytylote or have an irregular outline: 140-200 x 2.5-5 µm. The acanthostyles are entirely spined and perhaps dividable in two size categories: 100-135 x 5-8 µm and 190-280 x 5-10 µm, but they do not constitute two functional categories (main and echinating).
Microscleres: Spatulate anchorate isochelae in two size categories: 35-48 µm and 11-15 µm; sigmas in a single variable size category: 32-50 µm.
Skeleton: The dermal membrane is charged with microscleres and carried by bouquets of tornotes, which also form short tangential bundles. The bouquets of tornotes are in turn erected on the peripheral ends of the choanosomal spicule tracts, which consist of acanthostyles. The skeletal columns are thick and form a plumoreticulate skeleton, in which the longitudinal elements dominate.
Ecology: On pebbles and vertical cliffs at 30-85 m.
Distribution: Known only from Roscoff, NW France, and Rathlin Island, Northern Ireland.
Etymology: The name refers to the anchorate chelae, which are unusual in combination with a plumose skeletal structure.
Type specimen information: The type is in the collections of the Station Biologique de Roscoff.
This species has characters inbetween those of Phorbas (plumose columns, pointed tornotes, two sizes of acanthostyles) and Myxilla (anchorate chelae). For that reason it is assigned to its own subgenus in the Myxillidae.
Source: Cabioch, 1968.