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(Bowerbank, 1866)

Species Overview

Stryphnus ponderosus (Bowerbank, 1866) is a large, hard, greyish brown sponge which is not easily detected because it is usually almost entirely overgrown by other sponges and other sessile organisms. It has a hispid surface (where free from encrusters) and a thin cortex (visible when opened up). It may be confused with Stelletta grubii but this has a clearly thicker cortex. Both are easily distinguished upon microscopic examination. It occurs along the oceanic coasts of Europe and also into the Mediterranean.

Taxonomic Description

Colour: Pale greyish brown on the surface and ochre in the interior; dark brown. Areas of free surface may have a purple-brown coloration.
Shape, size, surface and consistency: Massive-lobose, with rounded lobes, or irregular in outline; occasionally goblet or cup-shaped. Often there is a central hole the size and shape of an orange in large specimens. Up to 40 cms in diameter. Almost invariably coated with other sponges (e.g. Antho inconstans, Antho coriacea, Desmacella annexa) or bryozoans, and conseqnently not obvious to the eye when seen in situ. Areas of free surface are small, tending to be adjacent to the substrate, where coatings species have not extended. Surface even; smooth or more commonly hispid, with occasional conulate patches, high friction, rough to the touch, like sandpaper (because of the heads of the megascleres penetrating the surface). Groups of 3 to 8 oscules are occasionally present on free surfaces. The oscules are small, up to 1 mm in diameter, and flush with the surface. Characteristically there is a slightly sharp smell. Consistency very firm and incompressible.
Spicules: (Stryphnus ponderosus spics) Megascleres are oxeas and short shafted dichotriaenes (sometimes modified to orthotriaenes). The oxeas are curved to a greater or lesser extent, 1000-1150-1250 (Topsent: 1200-2500) x 20-50 µm. The dichotriaenes have shafts of 340-850 x 25-40 µm. The ratio of shaft length divided by clade diameter is between 1.6 to 2.3.
Microscleres: Amphiasters 10-13 µm, and oxyasters 18-26 µm in diameter.
Skeleton: The cortex is thin, less than 0.5 mm. It consists of radially arranged, densely packed bundles of dichotriaenes and oxeas. Here amphiasters are numerous, oxyasters occasional. The choanosome consists of irregularly arranged oxeas, with occasional dichotriaenes. In places the oxeas are ordered into radial bundles directed towards the cortex. Oxyasters are numerous, and amphiasters occasional.
Ecology: On vertical or overhung rock faces, sand and shells, away from direct light, from 1-740 m.
Distribution: Arctic, Atlantic coasts of Europe, Mediterranean. Fairly common on vertical cliff faces and limestone caves on north coast of Rathlin Island, N Ireland.
Etymology: ponderosus (Latin) = heavy, weighty.
Type specimen information: BMNH 1910.1.1.4 (dry) Saints Bay, Guernsey, Norman Collection. Also slide "probably from type" unregistered (Bk 351). MCS voucher BELUM Mc1038, Rathlin Island, N Ireland.

Remarks

This species is very similar to Stelletta grubii in appearance and habit. The skeletal characterstics are different, most obviously the presence of short shafted dichotriaenes in Stryphnus ponderosus. Also, the distinctive hard, pale grey, 'rind' of Stelletta grubii is several millimetres thick, and obvious; that of Stryphnus is only 0.5 mm thick, and inconspicuous. This can be used as a distinguishing character in the field, if a small portion of the sponge is broken off. However, beware of mistaking an epifaunal sponge for 'rind'.
Sympatrically, a second species of Stryphnus is found, which may be easily distinguished by the lack of dichotriaenes and much larger oxyasters (up to 70 µm).
Source: Ackers et al., 1992 (B.E. Picton, D. Moss).

Stryphnus ponderosus