Form: A thin sheet typically 4mm thick which covers rock surfaces in patches 15cm or more in diameter.

Colour: Yellow to pale orange.

Smell: None

Consistency: Difficult to determine in thin sheet.

Surface: Slightly irregular and velvety with many small round inhalant pores and excurrent channels running to oscules.

Apertures:

• Oscules are inconspicuous, with slightly raised transparent margins. The large hole in the photograph is really a dead barnacle although oscules are associated with it.

Contraction:

• Oscules and excurrent channels collapse on removal from water.

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Internal characters

Skeleton: Typical Microciona skeleton - see description for M. atrasanguinea. Plumose columns of the principal skeletal megascleres are supported by spongin and echinated by smaller auxiliary megascleres (acanthostyles). Slender ectosomal spicules are also present.

Spicules: Principal megascleres are basally-spined styles (a), 340-480 μm. Echinating auxiliary megascleres are entirely-spined acanthostyles (b), 80-205 μm. The slender accessory ectosomal spicules are subtylostyles (c), ca. 200-225 μm. Microscleres are palmate isochelae (d), ca. 6 μm and 11 μm (which may be very rare), and thick toxa (e) (ca. 200 μm) with strongly spined tips. These may be common.

Identity: Difficult to distinguish from other encrusting orange species but the details of the surface may enable provisional identification. Examination of the spicules is essential and even then it may be difficult to distinguish from Microciona fallax, though toxa seem to be very scarce in the latter species.

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

iNaturalist: Microciona spinarcus at iNaturalist World Species Observations database.

WoRMS: Microciona spinarcus at World Register of Marine Species. Accepted name: Clathria (Microciona) spinarcus (Carter & Hope, 1889). AphiaID: 167684.

Classification: Biota; Animalia; Porifera; Demospongiae; Heteroscleromorpha; Poecilosclerida; Microcionidae; Microcioninae; Microciona

Voucher: BELUM : Mc1742. Calf, Isle of Man.

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