Spongosorites difficilis (Lundbeck, 1902) is a grey lumpy deep-water sponge, which turns dark violet or black after it is brought above water. It has been recorded only rarely and must be considered insufficiently known.
Colour: Alive grey; in spirit it varies from dirty dark gray or bluish gray to black violet; in the interior it is dirty gray. Apparently there is a colour change from grey to black when the specimen is brought above water.
Shape, size, surface and consistency: This species is of a very irregularly lumpy or lobate form; deep grooves or furrows may be found and projecting lobes seem sometimes to coalesce, so that narrow hollows may arise, stretching far into the sponge. The largest specimen recorded has a length of ca. 11 cm and a breadth of 8. 5 cm. The surface is otherwise smooth. Oscules have not been observed. The pores seem in most places to be rather scarce, only here and there they are seen more closely gathered in greater numbers; in such places the spicules in the skin may show a coarse, net-like arrangement, which, however, is only seen under the microscope. The sizes of the pores 0.03-0.5 mm; often they do not go perpendicularly through the skin but more or less obliquely, and consequently they are easily overlooked, when the skin is examined from above they continue as fine canals without any formation of larger sub-dermal cavities. The consistency is rather firm, fleshy and somewhat elastic.
Spicules: (Spongosorites difficilis spics) Oxeas only; they are slightly curved or straight and long tapering; a few oxeas may show a swelling in the middle, and this peculiarity seems to be more frequent in some specimens than in others. The spicules are very much varying in length: 60-370 x 4-10 µm; the thickness is generally proportionate with the length; in the shorter spicules the tapering is comparatively shorter than in the longer ones.
Skeleton: Ectosomal: The outermost layer of the sponge is marked off as a layer with a thickness of ca. 200 µm; this layer has its origin in the fact that the spicules here are closely packed; they are lying in all directions, often, however, in such a way, that they are arranged somewhat like bundles, the spicules of which are parallel to each other (palisade). Choanosomal skeleton: Besides the above mentioned closely packed layer of spicules found in the dermal layer, the skeleton consists of spicules spread through the tissue in all directions without any order, or sometimes arranged somewhat like bundles; here and there they may be gathered into larger nutnbers forming something like fibres.
Ecology: Deep water coming up to as shallow as 75 m.
Distribution: Roscoff, Norway, SW of Iceland.
Etymology: difficilis (Latin) = difficult, presumably referring to the vexing problem of classing a sponge with such elusive characters.
Type specimen information: The holotype is in the Zoologisk Museum, Copenhagen.
This species is included in this file on the basis of Cabioch's (1968) record of it from 75 m off Roscoff, the third record after its original description by Lundbeck from over 1000 m depth SW off Iceland and Burton's deep water record off Norway. Cabioch does not provide enough details to be certain that his material and Lundbeck's are the same species. It could also be S. genitrix, a very similar species with larger and more irregular spicules.
Source: Lundbeck (1902)