Form: A thick, spreading cushion, with raised ridges.

Colour: Usually sulphur yellow.

Smell: None. Slime : Exudes a large amount of slime when taken out of the water.

Consistency: Moderately soft, elastic, crumbly.

Surface: Distinctive, consisting of numerous deep labyrinthine channels, across which run cobweb - like strands of tissue.

Apertures:

• Oscules are variable in size, numerous and scattered. They are obvious, circular and typically in lines along the raised ridges.

Contraction:

• Not noticeable.

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Synonyms: Dendoryx incrustans (Johnston, 1842:122) Gray, 1867:535

Internal characters

Skeleton: A regular, isodictyal, reticulation with multi-spicular fibres, without echinating spicules. There is a surface layer of tornotes, tangential or at an angle to the surface. Spongin is scarce.

Spicules: The megascleres of the main skeleton are acanthostyles (a) 145-(170)-200 μm. The ectosomal spicules are tornotes (b) with spear-shaped, microspined, ends, which are occasionally unequal 180-(190)-200 μm. These lie in vertical brushes. Microscleres are spatuliferous anchorate chelae of two sizes, (c) 40 μm and (d) 18 μm, and sigmata (e) of two sizes, 33 μm and 15 μm.

Identity: This is probably the commonest massive, yellow sponge in exposed sites on vertical or clean rock. Initially identification needs to be checked with a microscope. Having checked that it is Myxilla (with a 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 (b) with a microscope lens of good resolution (x40 objective is adequate). A number of spicules may have to be examined before it can be decided unambiguously which type is present.

Distribution Map from NBN: Myxilla incrustans at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Myxilla incrustans at iNaturalist World Species Observations database.

WoRMS: Myxilla incrustans at World Register of Marine Species. Accepted name: Myxilla (Myxilla) incrustans (Johnston, 1842). AphiaID: 169466.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Myxillidae; Myxilla

Voucher: BELUM : Mc564. Malin Beg, Donegal.

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