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(Bowerbank in Johnston, 1842)

Species Overview

Pachymatisma johnstonia (Bowerbank, in Johnston, 1842) is a massive lobose purple-or slate-grey sponge with a hard consistency. Inside it is yellowish and in shaded localities it may be like that on the outside, too. The oscules are often in rows, evenly spaced. It is common in the lower intertidal and shallow subtidal of rocky coasts.

Taxonomic Description

Colour: Purple-grey to slate grey to off-white; tends to be darker along rows of oscules (this is caused by microsymbiont algae). It may be white in dark niches such as caves. The interior is dull greyish yellow.
Shape, size, surface and consistency: (P. johnstonia Mario) (Pachymatisma Bowerbank) (Pachymatisma Topsent) Massive-lobose, hemispherical to irregularly rounded, up to 30 cm or more across. On overhangs it may become pendulous. Surface clean-looking, even, granular-smooth, without projections. Just beneath the surface is a hard layer, the 'sterrastral layer' packed with sterraster microscleres. Oscules rounded, evenly spaced, often in rows, flush with the surface along the tops of the ridges, 2-3 mm in diameter. Consistency: firm-hard, but slightly compressible. Contraction not noticeable. Smell: none.
Spicules: (P. johnstonia spics V) Megascleres are strongyloxea (sometimes one, or both of the ends may be pointed to resemble oxeas, and sometimes they may be blunt resembling strongyles), size 450-3000 by 13-45 µm (Sponge V gives 550-1000). Triaenes are short-shafted orthotriaenes, 350-1100 by 13-26 µm, clads 85-300 µm; ana-and protriaenes are usually lacking.
Microscleres: Choanosomal euasters are oxyasters with large numbers of finely acanthose rays, 17-80 µm in diameter, with 3-12 rays; ectosomal euasters are sterrasters, characteristic of the family Geodiidae, elliptic, size 60-130 by 50-110 µm; ectosomal microrhabds are microstrongyles, microspined, generally centrotylote, 13-32 by 4-6 µm.
Skeleton: Radiate, with a well developed cortex of up to 1 mm thickness consisting of an outer layer of densely packed microrhabds, below which is a dense layer of sterrasters. Subradially arranged tracts of strongyloxeas and triaenes are most apparent near the surface.
Reproduction: Parfitt (1868) described groups of 1 to 6 "little gemmae springing from sides" of parent sponge, "small, elliptical bodies, the colour of the parent and semitransparent". This asexual buddding needs verification.
Ecology: On bedrock or stable boulders, often on cliff walls projecting into water currents. Can also tolerate a degree of sediment on rock surfaces. Littoral (ELWS) to 300 m. A common species in sites with strong water movement, but also found on overhangs in extreme shelter.
Distribution: Atlantic coasts of Europe; Sweden; Shetland; Orkney; south to Spain. Recorded recently from Orkney and many sites on the western and southern coasts of the British Isles. Absent from North Sea coasts of British Isles(?).
Etymology: Named after George Johnston, celebrated author of the first monograph on British sponges in which microscopic information was collected.
Type specimen information: BMNH 1877:5:21:144, Torquay, intertidal (wet). Also 1 slide unregistered (Bk 348), marked "perhaps from type". MCS voucher: BELUM Mc95, Mulroy Bay, NW Ireland.

Remarks

The slate grey colour, rounded lobes and evenly-spaced oscules (neat looking), flush with the surface give the sponge a characteristic appearance. If the specimen has a hard sub-surface layer identification by eye is fairly reliable.
Sollas (1888) erected a second Western European Pachymatisma species, P. normani, for Norwegian specimens with a "relatively thicker sterrastral layer in the cortex". This has been recently recognized as a distinct species.
Sources: Topsent, 1894a; Van Soest et al., 1981; Ackers et al., 1985, 1992; Stone, in litteris.

Pachymatisma johnstonia