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(Bowerbank, 1866)

Species Overview

Clathria (Microciona) fallax (Bowerbank, 1866) is one of the red crust microcionids which need to be microscopically examined for identification. It is very similar in spiculation to Clathria spinarcus, and may have been confused with that species, but it differs in the pronounced bumpy surface, the rarity of toxas and the flexuous subtylostyles. It is reliably known only from the coasts of the British Isles.

Taxonomic Description

Colour: Red to reddish-orange.
Shape, size, surface and consistency: Thin sheet or cushion covering rock in patches to 15 cm or more in diameter. Surface bumpy-rough. Excurrent channels may be obvious. The oscules are fairly regularly distributed and reasonably obvious because of the presence of converging channels. Contraction: some, especially of the oscules. Consistency crumbly.
Spicules: (Clathria fallax spics) Megascleres: Thin ectosomal subtylostyles are frequently flexuous, and microspined on the heads: 180 x 3-4 µm; main styles spined on the head only: ca. 240 x 8 µm; echinating acanthostyles especially spined in the lower half and on the head: ca. 100 x 8 µm. Microscleres: Palmate isochelae: ca. 5-11 µm; toxas which taper to points and are sparsely spined near their ends: 25-300 by 2.5 µm.
Spiculation is similar to that of Clathria spinarcus.
Skeleton: Typically microcionid with plumose columns of large nearly smooth styles echinated by smaller entirely spined acanthostyles. There is a surface layer of subtylostyles and very abundant minute chelae. Toxas may be rare and not present in all specimens.
Ecology: On vertical rock faces in exposed places with strong wave action or tidal streams.
Distribution: South coasts of the British Isles: England: Hastings; Ireland: Saltee Is., Co. Wexford, Sherkin Is., Co. Cork, Rathlin O'Birne Is., Co. Donegal.
Etymology: fallax = deceitful, referring to the similarity to the more common Clathria spinarcus.
Type specimen information: BMNH 1910.1.1.71 (dry) + slides BMNH 1910.1.1.2307 and 2308. Norman Collection. MCS voucher BELUM: Mc260, Rathlin O'Birne Island, Co Donegal, NW Ireland.

Remarks

This sponge is very similar to C. spinarcus. The presence of flexuous subtylostyles and scarce microspined toxas seems to be characteristic. This, combined with the very bumpy surface, may be diagnostic.
The conspecificity of this species with Bowerbank's Microciona fallax is based on Ackers et al.'s decisions. Bowerbank (1874)'s pl. XXIII figs. 12-16 does not not picture the microscleres.
Source: Ackers et al., 1992 (B.E. Picton, D. Moss)

Clathria fallax