Thymosia guernei Topsent (1896) is a whitish massive sponge with a broad attachement to the substrate. It has only small scattered oscules and a smooth-slippery but irregular surface, which may be raised into angular projections. Consistency firm-rubbery. The identification can be made certain by examining sections which reveal the total absence of spicules, and the presence of rare peculiarly shaped spongin fibres. It is a southern species reaching its northermost extension in NW Ireland,
Colour: Off-white surface, pale grey-orange interior.
Shape, size, surface and consistency: Thin sheets to low, spreading, massive-lobose forms, forming patches up to 60 cm across. Firmly attached by a broad encrusting base. Overall appearance like cold mashed potato. Surface clean, smooth and undulating. Sometimes raised up into angular projections where other sedentary organisms have become engulfed (e.g. hydroids, barnacles, etc.). Often, double openings of the mud-lined burrows of the polychaete worm Polydora are seen at the surface. A few small oscules, 1-2 mm diameter, are scattered singly along the top of ridges and lobes; these are often occupied by crustaceans and brittle stars. Countless microscopic pores cover the surface except in the vicinity of the oscules. In mature sponges these pores are concentrated at tbe base of the deep hollows formed by the steep-sided lobes. They are difficult to see with tbe naked eye on the living animal, especially when the 'skin' is tightly stretched. They are mone easily seen as puckered wrinkles in the hollows, when the surface is relaxed on preservation. A thin covering of slime is present, which only becomes apparent when silt falls out of suspension onto tbe surface of the sponge, where it becomes trapped and forms mucous 'threadlets', which are then caught in the passing current and removed. It is very slippery to handle immediately after collection. There is a slight smell. Consistency a solid, rubbery texture.
Skeleton: There is no mineral skeleton. Instead it has a thick cortex which is plentifully reinforced by fibrillar collagen which serves to strengthen the otherwise soft matrix. Spongin fibres, with unusual kidney-shaped swellings, further strengthen the matrix, by running vertically through the body to the surface. These branch and anastomose at intervals and are very distinctive in sections.
Associations: Polydora seems to form a regular association with the sponge. The brittle star Ophiothrix fragilis also often occurs with it.
Ecology: This sponge favours flowing water conditions in habitats which are semi-exposed to waves and current. It is usually found on fissured, vertical rock (gullies and cliff faces), set back in recesses, or under overhangs or inside caves. It is only occasionally found out on open rock faces, which are exposed to the full force of the moving water. Very occasionally it is found on horizontal surfaces. Littoral to ca. 30 m (optimum depth range between 15 and 25 m).
Distribution: Considered to be rare. Recently found off a number of islands off SWBritain and western Ireland: Aran Islands, Clare Island, Skokholm, Lundy. Also occasionally off the mainland: N Pembrokeshire (off S Wales), St. Johns Point (off NW Ireland); Concarneau (France); Algarve (Portugal).
Etymology: The name refers to the type locality, the island of Guernsey.
Type specimen information: The type is in the Muséum National d'Histoire Naturelle, Paris; MCS voucber BELUM: Mc737. St. John's Pt, NW Ireland.
The irregular surface, tiny oscules and white colour in a massive sponge are quite distinctive. The identity is easily confirmed by the lack of spicules and distinctive skeletal fibres.
Source: Hiscock et al., 1983 ; Ackers et al., 1992.