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(Bowerbank, 1866)

Species Overview

Clathria (Microciona) armata (Bowerbank, 1866) (also known as Microciona armata) is one of the red encrusting Microcionidae which cannot be recognized reliably without examining a microscopic preparation of skeleton and spicules. Its spiculation is distinguished from the other species by the heavily spined echinating acanthostyles and a large size range of toxas. This species is wide-spread along all European coasts from the intertidal down to 180 m.

Taxonomic Description

Colour: Light red, reddish-orange, blood-red. Brown when dried.
Shape, size, surface and consistency: Thin sheet covering rock or bivalve shells, to thin crust 1-5 mm thick. Surface smooth to casual inspection, but very finely hispid. Oscules have transparent converging excurrent channels. Consistency difficult to determine in thin specimens, but rather brittle in thicker ones.
Spicules: (Clathria armata spics) Megascleres: Slender (sub-)tylostyles with microspined heads are present in the surface (it may be difficult to see these spines): ca. 150-450 µm x 3-4.5 µm. The principal megascleres are almost smooth acanthostyles, slightly spined around the head: ca. 200-780 x 8-15 µm; secondary echinating acanthostyles are entirely and heavily spined, with larger, recurved spines towards the tips: ca. 100-200 µm. Microscleres include palmate isochelae: ca. 7-23 µm, and toxas of variable sizes: ca. 45-260 µm.
Skeleton: There is a surface layer of subtylostyles. Main skeleton typically microcionid with ascending plumose bundles of large, sparsely spined acanthostyles, echinated by smaller acanthostyles. These bundles rarely branch, so thicker specimens tend to break with a right-angled edge, and bundles separate easily. Toxas and palmate isochelae are frequent throughout the skeleton.
Ecology: On bivalve shells on muddy sediments, rocks and boulders in sheltered sites, and also on bedrock in strong tidal streams. Intertidal to 180 m.
Distribution: N Atlantic: White Sea to Mediterranean. Coasts of Norway, Sweden, Faroes, Ireland, southern and western Britain, Channel Isles, Roscoff area.
Etymology: The name refers to the acanthostyles echinating the skeletal columns.
Type specimen information: Type: BMNH 1930.7.3.210 (dry) (Bk.1527) Strangford Lough. Slides Unregistered (Bk.426) "from Dr. Dickie, Strangford Lough".

Remarks

The validity of the large range in spicule sizes given above is uncertain (but see Lévi, 1960: 73, from where it is apparent that spicule sizes tend to vary considerably in this species). In any given specimen such wide variation is unlikely to occur.
Separation of the various microcionid 'red crusts' is difficult and needs careful microscopic examination. Lévi (1960: 66) gives a key to the genus Microciona. This species has extremely large, recurved spines on the smaller acanthostyles, which are quite distinctive.
Sources: Lévi, 1960; Van Soest and Stone, 1986; Ackers et al., 1992.

Clathria armata