(Grant, 1826)
Species Overview
Clathria seriata (Grant, 1826) (also known as Ophlitaspongia seriata ) is a firm, smooth, bright orange-red flat crust with regularly spaced perfectly round holes (oscules). Freshly exposed specimens have a distinct gleamy aspect; dried out they become corky-crumbly. It is common under intertidal boulders and on Laminaria holdfasts.
Taxonomic Description
Colour: Deep orange-red. The pigment squeezes out readily.
Shape, size, surface and consistency: Thin sheets, usually 2-3 mm thick, but can develop into cushions of uniform thickness up to 10 mm thick. Diameter of an individual may be up to 10 cm. Surface very finely granular, even. The oscules (Clathria seriata MCS2) are conspicuous, numerous and evenly distributed in a regular fashion between 5 and 10 mm apart over the surface. Neat, round and mostly flush with the surface, but the margins can be raised slightly above the surface. No smell. Consistency moderately firm and elastic. Compressible, resilient. Breaks somewhat in the manner of a soft biscuit.
Spicules: (Clathria seriata spics) Megascleres : The ectosomal spicules are thin subtylostyles; they measure: 86-130 x 2 µm. The principal megascleres of the main skeleton are short fat styles: 60-160 x 5-12 µm.
Microscleres : Slightly curved robust toxas with smooth tips: 10-150 µm. Chelas are absent.
Skeleton: Very characteristic. In cross-section a ladder-like skeleton of spongin can be seen, which forms an anisotropic reticulation of well-developed fibres. The primary ascending fibres are semi-cored by plumosely arranged megascleres; the same spicules echinate the fibres at variable angles. The secondary connecting fibres usually do not contain spicules. Accessory subtylostyles are usually interstitial, rather than at the surface.
Reproduction: Larvae are produced in August and September; they are orange and have a paler coloured bare posterior pole (Fry, 1971; Wapstra and Van Soest, 1987)
Ecology: On rock, commonly under boulders on the lower shore and also in the shallow sublittoral (to 5 m) and on Fucus and Laminaria stipes in areas of strong water movement (either tidal or wave action).
Distribution: British Isles, France and Spain.
Etymology: The namepresumably refers to the regularly spaced oscules.
Type specimen information: No type material in BMNH; possibly in Royal College of Surgeons or Edinburgh Museum. (Ophlitaspongia seriata Bow); MCS voucher: BELUM Mc 588, Rutland Harbour, Donegal, Ireland.
Remarks
Superficially it could be confused with several other species. However the spiculation is very distinctive and the form, colour and habitat make it readily identifiable. It could be confused with Esperiopsis fucorum , but when alive the strong smell of Esperiopsis can be used as an initial indication. It could also be confused with Clathria (Microciona) atrasanguinea (a shallow sublittoral species which also forms thin sheets), but whereas Clathria seriata can be peeled off the rock, C . atrasanguinea usually crumbles and tears. The spiculation is also very different. Fry (1971) did research on this species from the Menai Straits and Anglesey.
Sources: Lévi, 1960; Ackers et al., 1985, 1992 (B.E. Picton, S.M. Stone, R. Earll, D. Moss).