Form: Thin sheet to thick cushion to massive-lobose (?), with smooth, irregular lobes.

Colour: A more or less bright carmine-red," rarely rose-red", very conspicuous.

Smell: None.

Consistency: Fairly soft but quite tough. "Resilient."

Surface: Distinctive - a closely packed series of dots can be seen under close examination or slight magnification. Porous, with dark spaces under the surface membrane. "Smooth, uneven."

Apertures:

• Oscules are few, large, conspicuous, "scattered and varying in size". They are slightly raised above the surface, and the ends of the excurrent channels can be seen as membranes inside each oscule.

Contraction:

• Slight - oscules close.

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Internal characters

Skeleton: Plumoreticulate. The main skeleton consists of an irregular reticulation of multispicular fibres (of subtylostyles). The ectosomal skeleton is a tangential, triangularly meshed reticulation of multispicular fibres. Spongin reinforces the skeleton fibres to a varying degree. The ectosomal membrane is easily detached.

Spicules: Megascleres are stout, straight-shafted subtylostyles (a), 240-(280)-300 μm long. Loose interstitial subtylostyles lying between the fibres are often flexuous. The microscleres are palmate anisochelae of 3 sizes (b) (ca. 30 μm, 25 μm and 15 μm). None of these form rosettes. In other Mycale spp. the largest anisochelae usually do form rosettes. Also present are sigmata (c) of two sizes, ca. 65-70 μm and 20-25 μm.

Identity: The characteristic surface, once seen, is very distinctive. There is potential confusion with many other red encrusting species, but most are much thinner crusts. The spicule complement will immediately confirm Mycale spp. and the particular microsclere combination present confirms Mycale rotalis. Note the lack of rosettes of anisochelae in this species.

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

iNaturalist: Mycale rotalis at iNaturalist World Species Observations database.

WoRMS: Mycale rotalis at World Register of Marine Species. Accepted name: Mycale (Aegogropila) rotalis (Bowerbank, 1874). AphiaID: 168544.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Mycalidae; Mycale

Voucher: BELUM : Mc475. Tiree, Hebrides.

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