Antho (Antho) inconstans (Topsent, 1925a) is one of the many red encrusting microcionids distinguishable only by microscopic examination. Antho differs from Clathria spp. in the isotropic triangular or quadrangular arrangement of the skeleton. The other encrusting Antho species are very similar and differences are subtle: A. (A.) involvens has smooth surface and only sharp pointed acanthostyles; A. (Acarnia) coriacea has predominantly acanthostrongyles; A. (Antho) brattegardi has the croca microscleres; A. (Antho) granditoxa has very large toxas. Like A. inconstans they have a lumpy irregular surface. This is a southern species reaching its northern limit on the south coasts of the British Isles.
Shape, size, surface and consistency: Thin sheet forming extensive patches on rock surfaces or on other sponges (e.g. Stelletta grubii). Surface lumpy and irregular, with a slightly velvety appearance, similar to Clathria (Microciona) spinarca. Oscules inconspicuous, scattered, irregular in shape and size. Excurrent channels are generally conspicuous, converging on the oscules and forming irregular stars. Consistency fairly tough, holds together well when scraped off.
Spicules: (Antho inconstans spics) Megascleres: The ectosomal spicules are fine subtylostyles (sometimes styles) with microspined heads (this may not be visible with a light microscope) ca. 150 µm in length, slightly fusiform with a constriction above the head. Megascleres of the main skeleton are subectosomal smooth styles of ca. 200 µm, acanthostyles and acanthostrongyles. The acanthostyles have a region of denser spination near the head and near the tip, and the smooth point itself is very short and abrupt. Typically ca. 135-155 µm in length. Microscleres: Palmate isochelae, ca 17 µm, which may be rare, and toxas which may be abundant.
Skelelon: An isotropic reticulation, with a triangular or quadrangular mesh of megascleres (acanthostyles) of varying sizes, quasi-echinated by smooth megascleres at the nodes of the net. Longer smooth megascleres (styles) pierce the surface but these are much shorter than the corresponding spicules in Antho involvens. Fine brushes of ectosomal spicules are present.
Ecology: Vertical or steeply inclined rock faces in both sheltered and exposed places. May be found encrusting other sponges, such as Stelletta grubii and Stryphnus ponderosus.
Distribution: S coast of British Isles (recorded recently from Lough Hyne, SW Ireland, Wales, N Cornwall, Jersey, Guernsey), France, Spain; Mediterranean.
Etymology: inconstans (Latin) = changeable, referring to the range of colours recorded for this species.
Type specimen information: The type is in the Muséum National d'Histoire Naturelle, Paris. MCS voucher BELUM Mc1349, Skomer Island, Wales.
Many microcionids form red sheets, but the spicule complement is quite different from that of Clathria spp. The characters given here and for Antho (A.) involvens should distinguish these two species, which have often been synonymized in the past. Antho (A.) inconstans is easily confused with Antho (Acarnia) coriacea which forms bumpy sheets and may also grow on other sponges. Indeed the two species can be found growing alongside each other. Acanthostrongyles are the prominent megascleres of the main skeleton in A. (Acarnia) coriacea, and the acanthostyles do not show the band of spines near the tip as in A. (A.) inconstans, looking more like those in A. (A.) involvens. Otherwise the spicule complement is similar for these three species (with slight size differences).
Source: Lévi, 1960; Ackers et al., 1992.