Radiella hemisphaerica (Sars, 1872), also known as Trichostemma hemisphaericum, is a hemispherical papillate sponge with a characteristic fringe of long spicules; it lives in the Arctic and North Atlantic, in deeper water on soft bottoms.
Shape, size, surface and consistency: (radiella_hemisphaerica2.jpg) More or less hemispherical, with the lower surface circular; the upper surface evenly convex. The circular edge rather sharp, and closely bordered with a ring of numerous radiate long spicules. Of the whole sponge there is only the upper convex surface which projects above the mud. From this upper surface arise a greater or lesser number of obtusely conical, fistular processes, each having at its extremity an osculum. The papillae number may vary from a very few up to 20. The largest specimens have a transverse diameter of nearly 5 cm and a height of 2.5 cm. The smallest specimen found had already the convex surface and only one papilla in the centre. The circular rim of spicula was already completely developed; and its width was equal to 1/3 of the transverse diameter of the sponge. The whole sponge is of rather firm consistency.
Spicules: The spicules are tylostyles of 4 different kinds: (1) long, straight and somewhat fusiform: from 600 to 5400 µm, (2) much thinner and longer, very elastic, bristle-like, not enlarged in the middle, but evenly tapered in their whole length, and terminating in an extremely fine elongated point: up to 8000 µm, (3) very short, with the shaft rather strongly enlarged in the middle: about 240 µm, (4) the same length but different in the small development of the head, and in the scarcely perceptible swelling of the middle part of the shaft.
Skeleton: (r_hemisphaerica_sars.jpg) The cortex is particularly thick and sharply distinguished from the interior. In the exterior part of this cortical layer we find spicules extremely closely packed, standing side by side at right angles to the exterior surface which they penetrate with their extreme point. The interior is of a very loose texture and shows several cavities. The spicules are arranged in long ramified fascicles which radiate from a point. Bristle-like spicule arrangements form the wide border which gives to the sponge its peculiar appearance.
Ecology: Deep water, in fjords, 40-360 m, on soft clay bottom.
Etymology: The name refers to the cushion-shaped body.
Type specimen information: No data.
A second species of Radiella, R. sarsi, is known in deeper waters of the Mediterranean and the neighbouring Atlantic.
Radiella sol sensu Burton, 1930a (Polymastia sol sensu Koltun, 1959) is here considered to be different from Schmidt's (1870) West Indian species, and to represent small specimens with a single papilla of R. hemisphaerica. The generic distinctness of Polymastia and Radiella (as Trichostemma) is discussed by Uriz and Rosell (1990) and Boury-Esnault et al. (1994).
Source: Sars, 1872