Pleraplysilla minchini Topsent (1905) is a thin conulose encrustation of which the colour is reported as chocolate brown or pale yellow. There are no spicules, and the dendritic spongin fibres are filled with sand grains and/or broken spicules (microscopic examination).
Colour: Light chocolate brown or pale yellow.
Shape, size, surface and consistency: Large (up to 25 cm in diameter) thinly encrusting patches, barely 1 mm in thickness over most of its body, but may thicken here and there to form lumps of 5 mm in thickness. Strongly conulose surface, with fibres sticking up to 2 mm out above the surface; conules up to 2 mm apart. Oscules in the form of craters of 1-2 mm diameter.
Skeleton: (Pleraplysilla minchini fibres) Single fibres rise up from a basal plate of spongin, they are 3-4 mm long and 100-130 µm in diameter at the base, tapering to about 70 µm near the surface; occasionally they are branched near the surface, but not anastomosing. The center of the fibres is filled with debris of spicules and sand grains; the enveloping spongin is stratified. Choanocyte chambers large and elongate, 90 x 35 µm.
Ecology: In the sublittoral on bare rocks, down to 50 m.
Distribution: North Sea (Helgoland), W coasts of France, Mediterranean.
Etymology: Named after E.A. Minchin, specialist of Calcareous sponges around the turn of the 20th century.
Type specimen information: The type is in the Muséum National d'Histoire Naturelle, Paris.
This is an easily recognized species, sharing the encrusting habit and "goose flesh" surface with the similar rose-or yellow coloured Aplysilla rosea, the yellow coloured Darwinella intermedia and the dark purple Chelonaplysilla erecta. None of these have their fibres filled with foreign material.
There is a disagreement among authors about the conspecificity of P. minchini and the Mediterranean P. spinifera (Schulze, 1879b). Vacelet (1959) considers them synonyms, but Pulitzer-Finali and Pronzato (1977) maintain they are separate species. However, the alleged differences between the two species are largely quantitative and may be subject to regional variation. For the time being, both are considered different, and consequently the northern representatives are called P. minchini. If they turn out to be conspecific with the Mediterranean P. spinifera, then the latter name has priority.
Source: Topsent, 1905.