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(Bowerbank, 1864)

Species Overview

Myxilla (Myxilla) fimbriata (Bowerbank, 1864) is an orange or yellow, encrusting to cushion-shaped sponge occurring in deeper water. The surface is slightly hispid with small oscules with slightly raised rims. The consistency is rather firm. It is a northern species.

Taxonomic Description
Colour: Bright orange in surface layers, yellow in deeper layers. Specimens from less clear water conditions may be beige in colour. The sponge goes black or brown in alcohol, staining the label, but may fade to a dirty, pale brown.
Shape, size, surface and consistency: (Myxilla fimbriata MCS2) Cushion, usually 6-7 mm thick; can be 30 mm thick. Found as distinct specimens 2-5 cm across, with an irregular outline. The specimens are rounded in cross section with steep sides. Surface not 'open' as in M. incrustans though underlying porous areas are clearly visible through transparent 'skin'. Slightly hispid, smooth to touch. The oscules are small, with slightly raised transparent rims. They are placed towards the central, thickest parts of the sponge, or raised on ridges. There are no oscular channels. Consistency quite firm, tears easily. Resilient. Smell slight. No slime. Contraction slight.
Spicules: (Myxilla fimbriata spics) Megascleres: The surface tornotes have abruptly terminating ends, ending in a short point, often mucronate: 202-290-325 µm. The ends are sometimes slightly unequal Iying in vertical brushes. Different development stages of tornote ends can be present in any one specimen, making recognition difficult. The megascleres of the main skeleton are long, sparsely-spined acanthostyles: 265-300-350 µm.
Microscleres are spatuliferous anchorate chelae, of two sizes: 60 µm and 27 µm. Sigmata are infrequent or absent, thin, and easily overlooked.
Skeleton: The reticulation of multispicular fibres is less regular and isodictyal than in other Myxilla spp. There are no echinating spicules associated with these fibres. Spongin is scarce.
Ecology: On rocks in lower infralittoral and upper circalittoral.
Distribution: Arctic, Faroes, Norway, Sweden, British Isles, Roscoff. Recorded recently from various sites on the W and SE coasts of Ireland, where this sponge is a characteristic circalittoral species in sites exposed to the Atlantic.
Etymology: The name refers to the clearly visible fimbriae of the large anchorate chelae.
Type specimen information: The type is in the Natural History Museum, London. MCS voucher BELUM: Mc575, Skird Rocks, Galway.

Remarks

Superfcially there might be confusion with Amphilectus fucorum but there are distinct spicule differences. The blackening of the sponge in alcohol is rapid and striking, and this combined with a lack of slime in what is otherwise a typical Myxillid skeleton makes this species distinctive from other myxillid species such as M. (M.) rosacea and M. (M.) incrustans.
Note from B.E. Picton: "There is definitely another form/species of this sponge with distinct oscular channels, which goes dark brown in alcohol, doesn't slime and has a similar spiculation. However the acanthostyles in this form are much thicker."
Myxilla fibrosa Levinsen (1893) is similar in spiculation and is here treated as a synonym; it has sigmas 25-50 µm.
Source: Ackers et al., 1992 (B.E. Picton, S.M. Stone, R. Earll).

Myxilla fimbriata