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(Haeckel, 1870)

Species Overview

Leucosolenia variabilis (Haeckel, 1870) is a whitish mass of hollow branches, anastomosing and intertwined on the substrate from which rise isolated open-ended (oscular) tubes. Although it is a calcareous sponge the tube walls are so thin that the consistency is limp and soft. It is similar to Leucosolenia complicata, but that species often forms arborescent bushes and the tubes have side-branches. L. variabilis is a common intertidal and shallow-subtidal sponge, found on rocks, algae and on firm sediments.

Taxonomic Description

Colour: White, yellow or grey when alive.
Shape, size, surface and consistency: (L. variabilis Holland) (L. variabilis juv.) (Leucosolenia variabilis Mario) (Leucosolenia variabilis V) (Leucosolenia variabilis BandW) This sponge consists of a basal reticulation of thin-walled tubes which creep and anastomose over the substrate, from which arise numerous very (normally up to 2 cm) long, free-standing, closely-set, oscular tubes showing little or no tendency to throw out lateral diverticula except near their base. It occurs as a spreading form and as a straggly bushy form, never as an arborescent form as in L. complicata. Surface of the tubes is soft and smooth; minutely hispid. The tubes cling together by surface tension when out of the water. Otherwise no noticeable contraction takes place. Oscules are terminal on erect tubes. The oscular rims are long. Oscular tubes show no tendency to anastomose. Consistency very delicate and soft, but not as fragile as L. complicata, which easily breaks when handled. No noticeable smell.
Spicules: (L. variabilis spics MCS) (Leucosolenia variabilis sp.) (Leuc_variab_spics.jpg) Calcareous. Triactines, tetractines and and oxeas.
Triactines have rays 80-100 µm long x 7-8 µm thick; the basal ray generally is shorter, 50-100 µm long x 6-7 µm thick. Peculiarly modified triactines termed "brackets" by Minchin regularly occur which have the short basal ray curved out of the facial plane in which the paired rays lie (same sizes). T-shaped triactines are relatively rare.
Tetractines with rays of similar size to the triactines and with apical ray ca. 10-80 µm long x 7 µm thick.
Oxeas are of 3 sizes; the largest have lanceolate distal ends with more or less straight shafts, 190-375 µm long x 3.5-9 µm thick; the intermediate are less abundant with distinct lanceolate endings, 75-210 µm long x ca. 3-6 µm thick; the smallest oxeas are 35-125 x 0.5-2 µm but differ sharply in being entirely spined (SEM imaging necessary). Occasionally, large, bayonet-like oxeas can occur with a sharp bend in the middle, up to 200 µm long x 7-10 µm thick.
Skeleton: The canal system is asconoid. The skeleton of the chamber layer consists of triactines. The choanosomal skeleton is of tetractines, which also line the atrial cavity where the apical rays project into the atrium. Oxeas are scattered.
Reproduction: July-September (Dubosq and Tuzet, 1942).
Ecology: This sponge tends to grow in the shallow sublittoral, where it is characteristic of mixed sediments, growing upright (?) in small patches on shells, algae, hydroids and ascidians, and on horizontal rock, including undersides of boulders; often found in sea lochs. It has been recorded as often half buried in mud and sediment, which induces longer oscular tubes. On the shore it may be found in situations where it is "left dry at all tides". It is very common.
Distribution: Arctic; North Sea and Atlantic coasts of Europe; Mediterranean. Elsewhere, the species has been recorded from South Africa and Chile, but these records need verification.
Etymology: The name refers to the polymorphic shape.
Type specimen information: BMNH 1906.12.1.40 and 50. 2 slides "collected by Norman, identified by Haeckel", Minchin Collection. MCS voucher: BELUM Mc3, Arranmore, NW-Ireland.

Remarks

Superficially this species can be confused with several other branched species of Leucosolenia, e.g. L. complicata, L. botryoides and especially L. somesi. The first is more fragile and arborescent, and the unpaired angle of the triactines is only slightly thicker than the paired angles; moreover the unpaired ray is always longer than the paired rays. L. botryoides is an erect species resembling a bunch of bananas. Its oscular rims are very short, the spiculation, though similar to that of L. variabilis, is much more robust, and the T-shaped triactines are abundant. L. somesi is much larger in size (up to 12 x 12 cm) and has a dominance of V- or T-shaped spicules, while the larger oxea-category are twice as long as those of L. variabilis).
Source: van Soest et al. 2007; S.M. Stone (Ackers et al., 1985, 1992); Van Koolwijk, 1982.

Leucosolenia variabilis