Microciona spinarcus Carter & Hope, 1889

Order : POECILOSCLERIDA

Family : Microcionidae

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.

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

Habitat: Common on vertical rock faces in exposed places; under boulders in rapids - seems to favour strong water movement. Frequently associated with Alcyonium digitatum and Corynactis viridis. "Found growing on Pecten."

Distribution: "British Isles (type locality is Hastings); France; Spain." Known recently from Lough Hyne; N. coast of Donegal; N. coast of Co. Antrim; Calf of Man; St Kilda; Lundy.

Distribution Map: NBN map : National Biodiversity Network mapping facility, data for UK.

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.

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

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