Carter, 1879
Species Overview
Alectona millari Carter, 1879 is a deep-water excavating sponge, occurring in Lophelia corals and shells. It is recognizable as tiny pale-rose plate-like papillae provided with pore-sieves, and as similarly tiny conical tubes. Inside the substrate there are big holes bored by this sponge and the tissue is brownish. It has very distinctive spicules, including thick, spined oxeas and amphiasters. Fairly common in Norway, Sweden, and along the west coasts of the British Isles, but also known from NW Spain, the Mediterranean and the Azores.
Taxonomic Description
Colour: Pale rose papillae; brownish endolithic parts. White in alcohol.
Shape, size, surface and consistency: A deep-water excavating sponge occurring in corals, shells, and other calcareous material (Alectona in Lima). There are two types of structures outside the substrate, viz. flattened inhalant papillae, provided with pore-sieves and conical-erect exhalant papillae with an apical opening, diameters of both between 0.1 and 1 mm. The consistency of the papillae is soft. The endolithic parts are much larger, several mm to more than 1 cm in diameter, consistency soft, slimy.
Spicules: (Alectona skel) Very characteristic, consisting of larger and smaller ones, possibly not homologous with mega-and microscleres of other sponges. Larger spicules are heavily spined or warty diactines, conveniently named oxeas, but with a distinct sharp bend in the middle which in some case bears a rudimentary third actine, indicating a possible tri-or tetractine origin; with bluntly rounded apices; the spines are coarse and conical: 215-370 x 15-20 µm. Microscleres are basically amphiasters, with two whorls of six rays, but often these are more or less irregularly placed, or partially reduced; the rhabd and the rays are spined, variable: 10-70 x 1-5 µm.
Skeleton: The diactinal spicules are arranged parallelly in the papillae, strengthening their upright position; elsewhere they are irregularly and singly distributed as are the microscleres.
Reproduction: Sexual reproduction: embryos of 330 x 285 µm were abundantly found in a specimen dredged off Banyuls in May; they were colourless, transparent without spiculation. Asexual reproduction: this species produces very peculiar fortified planktonic stages budded of asexually (A. millari asexual larva). These have a very distinct shape with six long projections, strengthened by long styles and a central rounded body covered by discotriaene-like spicules of up to 600 µm in diameter. During its planktonic life it changes into a smaller ovoid bud which throws off both discotriaenes and styles. Just before settlement its size is about 220 µm and its interior is filled with amphiasters. The same type of planktonic asexual larvae is found in the related tropical-subtropical genus Thoosa .
Ecology: Boring into deep-water corals (Lophelia a.o.) and shells (Lima a.o.) at depths of 85-1200 m.
Distribution: Norway, Faroes, Scotland, Ireland, NW Spain; Azores, Mediterranean.
Etymology: Named after the collector of the type material.
Type specimen information: The type is BMNH 1885.12.22.1 (dry).
Remarks
This is the commonest excavating sponge of the deep-water corals beds along the coasts of Western Europe. Microscopic examination is necessary to tell this species apart from the equally tiny papillae of Aka infesta , which has very different spicules (only smooth small oxeas) and belongs to the Haplosclerida.
The systematic position of Alectona is uncertain, along with that of the genus Thoosa . Both are traditionally assigned to the Hadromerida, family Clionidae, on account of their excavating habit. However, since many decades it is accepted that excavating calcareous substrates is not an exclusive property of Clionids, and this was probably developed several times in the course of sponge evolution. Therefore the considerable discrepancy between the spiculation of Cliona and Alectona /Thoosa should be the basis for a reassignment of the latter two to a possible separate family among the Tetractinellids. This course has been followed so far only by Alander (1942), who assigned both to the family Thoosidae Cockerell, 1925 in the "sub-order Astrophora".
Source: Topsent, 1900.