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(Johnston, 1842)

Species Overview

Myxilla (Myxilla) incrustans (Johnston, 1842) is a yellow or pinkish orange thickly encrusting sponge, with a distinctive open structure at its surface due to a system of internal channels shining through. Oscules are large and elevated, often on ridges. The surface feels crisp. It may be difficult to tell apart from Myxilla (M.) rosacea without microscopic examination (tornote ends !). Common in the shallow sublittoral, especially in NW Europe.

Taxonomic Description

Colour: Usually sulphur yellow (also described as primrose-yellow), but can be orange, pink, red or white.
Shape, size, surface and consistency: (Myxilla incrustans 2) (Myxilla incrustans MCS3) (Halichondria incrustans) A thick, spreading cushion, up to 20 cm across, 5 cm high. Exudes a large amount of slime when taken out of the water. Surface distinctive, consisting of numerous deep labyrinthine channels across which run cobweb—like strands of tissue. The appearance varies somewhat depending on whether or not the channels are covered by a surface membrane. Oscules are variable in size, numerous and scattered. They are obvious, circular and typically in lines along the raised ridges. The surface feels crisp due to vertical spicule bundles supporting the channel walls. Consistency moderately soft, elastic, crumbly. Contraction not noticeable. No smell.
Spicules: (Myxilla incrustans spicules) Megascleres: The ectosomal spicules are tornotes with spear-shaped, microspined ends, which are occasionally unequal, size typically: 150-200 x 3-5 µm, but may range from 130 to 260 µm. The ends of the tornotes differ from sympatric Myxilla species in having numerous spines (as opposed to one or three spines). The tornotes lie in vertical brushes. The megascleres of the main skeleton are acanthostyles, typically: 180-230 x 11-15 µm, but may range from 124 to 350 µm (Arndt, 1935).
Microscleres are spatuliferous anchorate chelae (Myxilla incrustans chSEM) of two sizes: 32-71 and 17-28 µm, and sigmas of two sizes: 15-25 and 30-75 µm.
Skeleton: Ectosomal skeleton a palissade of erect tornotes. Choanosomal skeleton a regular, tight-meshed, isodictyal reticulation with multi-spicular fibres, without echinating spicules. Spongin is scarce.
Reproduction: Embryos are commonly found in specimens of M. incrustans during the months of August and September. These are globular, measuring 0.3 mm to 0.5 mm (largest diameter).
Ecology: In clear water. Low water mark to 400 m on stones, shells, sand and mud.
Distribution: Arctic; Bear Isl.; Faroes; Norway; Atlantic coasts of Europe south to Gibraltar; Mediterranean. Recently known from many sites around the British Isles.
Etymology: The name refers to the persistently massively encrusting habit.
Type specimen information: The type is in the Natural History Museum, London: BMNH 1847.9.7.46 (as Halichondria saburrata ?). MCS voucher: BELUM Mc564, Malin Beg, NW Ireland.

Remarks

This is a very common massive yellow sponge in exposed sites on vertical or clean rock. Identification needs to be checked with a microscope. Having checked that it is Myxilla (with its network of acanthostyles, without echinating spicules, accompanied by spatuliferous anchorate isochelae, and with tornotes at the surface), it is necessary to examine the tornote ends with a microscope lens of good resolution (x40 objective is adequate). The tornote ends of three species commonly found in the UK are sketched in an accompanying figure (Myxilla tornotes). A number of spicules may have to be examined before it can be decided unambiguously which type is present.
Sources: Ackers et al., 1985, 1992 (S.M. Stone, J.D. Guiterman, D. Moss); Hiscock et al., 1983.

Myxilla incrustans