Clathria (Microciona) elliptichela (Alander, 1942) is a deep-water thinly encrusting sponge known from the Skager Rak and recently discovered off the Hebrides (Scotland) and Rathlin Island (Northern Ireland). It can be distinguished from other Clathria crusts only by microscopic examination. Its distinguishing feature is the possession of cleistochelae (closed chelae).
Colour: Reddish grey.
Shape, size, surface and consistency: Thinly encrusting. Surface uneven, hispid, due to projecting spicules. Consistency rather loose, although a basal thin spongin layer gives a certain firmness.
Spicules: Megascleres: The ectosomal styles are long, slender and almost straight. The blunt end is somewhat swollen and crowned with very fine straight spines. The shaft is even and slightly tapering along practically the whole of its length, while the extremity itself is rather sharply pointed. These styles measure 400-850 µm in length and have a greatest diameter of 5-7 µm nearest to the swelling at the head-end. The acanthostyles are of very varying form and size, but it is not possible to classify them clearly in groups. The smallest acanthostyles are straight or bent and generally have a fairly clearly marked head. The shaft is provided with reclined spines along the whole of its length on the smallest spicules, but the longer the spiculum is, the longer is the smooth part of the apical end. The large acanthostyles are slightly bent and the heads are not clearly marked off. They are spined only on the head and on a small portion of the shaft nearest to the head. The spines are comparatively small and in shape straight and broadly conical. The medium sized acanthostyles fall, as to shape and appearance, somewhere between the extremes here described. The smallest acanthostyles that were measured were 140 µm in length and 8 µm in thickness, the latter measurement being taken nearest the head. The largest measured 1800 µm in length and 22 µm in thickness.
Microscleres: Isochelae are of two different kinds. The one sort diverges, as to shape, only slightly from the type of isochelae palmatae generally characterizing the genus Clathria, but is provided with a peculiarity in the shape of a median crest projecting from the inside of the shaft. This crest is very thin, and it is sometimes difficult to see it when the chela is Iying on its side. When one looks al the chela from in front, the crest is clearly visible as a distinct line in the middle of the shaft. The other kind of chela has a very curious structure. It has arisen from chelae palmatae in the following way: the lateral alae have joined along the sides of the shaft, forming a united elliptic plate. In the same way, median alae have merged into one another to form a united plate, though a more or less distinct line shows where they have grown together. As in the case of the previously described chelae, there is a median crest on the inside of the shaft. This is, however, rather more strongly marked, and as it has a thickened band in its free border it is clearly to be seen also from the side. The plate formed by the lateral alae is twisted in a slight spiral round the longitudinal axis, so that median alae do not lie exactly flush against each other. Owing to this asymmetry, these chelae may look like anisochelae under the microscope: the normal chelae measure 14-16 µm. The ring-shaped ones 15-16 µm in length.
Toxas are of two kinds. The largest have very varying appearances. They vary considerably in length, which may be anything between 45 and 300 µm measured between the points. The thickness varies between 0.5 and 4 µm. The small toxas are slightly bent. They measure 6-12 µm in length and have a greatest thickness of 1-2 µm.
Skeleton: The ectosomal skeleton consists of bouquets of styles, partially piercing the skin. They rest on the projecting bundles formed by the long acanthostyles. The choanosomal skeleton consists of acanthostyles firmly attached with their heads in the basal membrane. The largest acanthostyles are found in groups of four and five, surrounded by a large number of small acanthostyles.
Ecology: Deep water (30-400 m).
Distribution: Skager Rak, Mingulay cold water reefs (W coast of Scotland), Rathlin Island (Northern Ireland).
Type specimen information: No data.
This species, which is so well characterized by its peculiar chelae, can scarcely be mistaken for any other species. In the Mediterranean there is a species C. cleistochela Topsent (1925) which also has cleistochelae like the present, but that species has much smaller acanthostyles and only a single category of toxas. C. ctenichela shares the ctenichelate condition of the palmate isochelae, and is generally very similar. Possibly the two are synonyms of a variable species.
Source: Alander, 1942.