Leucosolenia somesi (Bowerbank, 1874) is a recently revived species found growing on man-made structures in England (in the 19th century) and in Holland (2006). It is a giant form of Leucosolenia, most closely related to L. variabilis; smaller specimens may look indistinguishable from that species. It spicules (microscopic examination) are different in the predominance of T- and V-shaped malformations of the triactines.
Shape, size, surface, and consistency: Mass of anastomosed tubes, in preserved condition at least 12x12 cm, but alive it is considerably more expanded. Oscular tubes at least 8 mm in diameter, closed tubes may be as thin as 1 mm or less. Tubes increasingly branching towards the periphery of the body, at right or sharp angles, but with few or no diverticula.The consistency is fragile.
Spicules: Triactines, large, relatively thin, with many aberrant T- and V-shapes, paired rays on average longer than unpaired ray, but both longer and shorter occur; paired rays 23-188 x 2-7 µm, unpaired rays 103-155 x 2-6 µm. Tetractines, likewise relatively large and thin, paired rays and unpaired ray similar in length and thickness, 108-171 x 4-5 µm, apical ray 20-32 x 2-4 µm. Oxeas: large oxeas, smooth, irregular outline and diameter, ends more or less pointed, not lance-shaped; long thin, more or less straight oxeotes found in moderate quantities are considered young growth stages, 450-672 x 7-11 µm; intermediate oxeas, smooth, irregular in outline and diameter, ends more or less pointed like large oxeas, not lance-shaped at one of the ends, 165-360 x 5-8 µm; small, straight, spined oxeas (invisible or barely visible by light microscopy, 81-96 x 1-2 µm.
Skeleton: Tube wall consists of a layer of triactines and tetractines supported by larger and smaller oxeas. Triactines are arranged rather irregular, with rays of the regular ones tangential to the surface. Apical rays of tetractines and unpaired rays of irregular T-shaped triactines protrude into the tubar cavity. Large oxeas and intermediate oxeas both tangentially and at sharp or right angles to the surface. The latter make the surface hispid to varying extents. Small oxeas mostly arranged perpendicular to the surface.
Distribution: S coast of England, Oosterschelde (Netherlands)
Ecology: Only known from man-made structures in sheltered habitats.
Etymology: named after Mr Somes, who collected the type material.
Type material: BMNH 19220.127.116.11
Bowerbank's Leuconia somesi was assigned to Leucosolenia variabilis by Minchin (1905) largely on the basis of spicule characters, whereby its extreme size and shape was ignored. The species L. somesi was subsequently ignored in the literature, but its name is revived on the basis of a comparison of its characters with a large number of L. variabilis specimens from the Netherlands and adjacent countries. The type of L. somesi shares most of its features with recent specimens from the SW of The Netherlands (ZMA 17572 and 11389): large body size and compact shape, the large oxeas, and the high frequency and diversity of shapes of malformed tri- and tetractines, including V-shaped spicules. The type of Leuconia somesi and the Dutch specimens, however, also share features of shape and spiculation with Leucosolenia variabilis. An extensive comparison was made with Leucosolenia variabilis specimens in the ZMA collection (van Soest et al. 2007 table 1). Scanning electron microscopy images of the smallest oxeas revealed extensive spines and at first this was considered evidence of specific distinctness of the mentioned Dutch specimens. However, SEM images subsequently made of Dutch and other western European Leucosolenia variabilis specimens demonstrated that this spination is a common, though up till now undetected feature of L. variabilis individuals. Nevertheless, compared to specimens ZMA Por. 11389 and 17572, and the type of L. somesi, the large oxeas of typical Dutch L. variabilis are distinctly and significantly smaller and thinner and usually have lance-shaped endings at one or both ends, the intermediate oxea size-category is indistinct and overlapping with the larger oxeas. No V-shaped large malformed triactines were found. In other locations in Europe, oxeas of up to 520 µm are found in exceptional cases (ZMA Por. 05218 from northern Spain), and Topsent (1936) also gives 500 µm as an upper size in a specimen from Concarneau (Bretagne), but these values are extremes and the average size is much lower. Large oxea sizes in most of the other locations outside the Netherlands are mostly larger, but do not reach the average size of ZMA 11389, 17572 and the type of L. somesi (average Belgium: 332 µm; France: 244 µm; Spain: 455 µm; Ireland: 377 µm; Germany: 267 µm; Norway: 340 µm).
Source: Van Soest et al. 2007