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(Carter and Hope, 1889)

Species Overview

Clathria (Microciona) spinarcus (Carter and Hope, 1889) is one several thinly encrusting orange Clathria species, which are difficult to distinguish from each other without examining the spiculation. The inconspicuous oscules and thin faint radiating canals are rather characteristic. It favours vertical rock faces in exposed habitats.

Taxonomic Description

Colour: Yellow to pale orange.
Shape, size, surface and consistency: A thin to moderately thick sheet typically 4-8 mm thick which covers rock surfaces in patches of 15 cm or more in diameter. Surface slightly irregular, becoming bumpy in thicker specimens (Clathria spinarca MCS2). Velvety with many small round inhalant pores and excurrent channels running to oscules. Oscules are inconspicuous, with slightly raised transparent margins. Oscules and excurrent channels collapse on removal from water. Consistency: difficult to determine in thin sheet, but crumbly in thicker specimens.
Spicules: (Clathria spinarca spics V) Megascleres: The ectosomal spicules are slender subtylostyles, ca. 200-225 x 2 µm (Arndt, 1935 gives 220-380 x 3.5 µm). Principal megascleres are basally spined styles: 340-480 x 9 µm. Echinating auxiliary megascleres are entirely spined acanthostyles: 100-205 µm. (Microciona spinarcus Lévi) Microscleres are palmate isochelae in two sizes: ca. 5-6 µm and 11 µm (which may be very rare), and thick toxas with strongly spined tips: 25-290 by 2.5 µm (these may be common).
Skeleton: Typical microcionid skeleton. Plumose columns of the principal skeletal megascleres are supported by spongin and echinated by smaller auxiliary megascleres (acanthostyles). Slender ectosomal spicules are also present.
Ecology: On vertical rock faces in exposed places; under boulders in rapids—seems to favour strong water movement. Frequently associated with the coelenterates Alcyonium digitatum and Corynactis viridis. Found growing on Pecten.
Distribution: British Isles, W coasts of France and Spain. Recorded recently from Lough Hyne, SW Ireland; N. coast of Donegal, N. coast of Co. Antrim, N Ireland; Calf of Man; St Kilda; Lundy.
Etymology: spinarcus (Latin) = with spined bow, referring to the spined toxas.
Type specimen information: The type is in the Natural History Museum, London. Type locality is Hastings. MCS voucher BELUM Mc1742, Calf, Isle of Man.

Remarks

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 Clathria (Microciona) fallax, although toxas seem to be very rare in the latter species.
Sources: Lévi, 1960; Ackers et al., 1985, 1992 (B.E Picton, S.M. Stone, D. Moss).

Clathria spinarcus