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Bowerbank, 1882

Species Overview

Hymedesmia (Hymedesmia) pansa Bowerbank (1882) is a pale yellow thin encrustation with areolated pore fields on its surface. This combination of characters is sufficient to distinguish it from other thin ecrustations, at least in shallow (sub)littoral waters (similar specimens but thicker and more beige coloured may be Hemimycale columella). It is a common species in the British Isles and along the west coast of France.

Taxonomic Description

Colour: Consistently pale yellow.
Shape, size, surface and consistency: Thin sheets, up to 3 cm across, but smaller typically. Surface smooth, with subsurface exhalent canals visible. Densely covered with oval pore areas. Oscules may not be apparent. Pores in oval pore areas with smooth tissue between. Consistency soft.
Spicules: (Hymedesmia pansa spics) Megascleres: The tornotes are very slender, straight, measuring 202-260 by 2-3 µm. They are strongylote to subtylostylote, one end being rounded and sometimes very slightly swollen, the other tapering in a fine point, slightly fusiform; indeed they could equally be described as styles. Primary acanthostyles measure 162-250 x 5-10 µm. They have a slightly curved shaft and the head is thickly set with spines, whilst small, rather scattered, spines extend along the shaft but are much less frequent in the distal half. The smaller acanthostyles measure 86-110 x 4-7 µm. They are straight or very slightly curved, and conspicuously spined along the whole length. The spines on the head are fairly long, those on tbe shaft recurved.
Microscleres are arcuate isochelae, ca. 14-20 µm in length with less strongly curved shaft than in most Hymedesmia species.
Skeleton: Hymedesmoid, with large primary acanthostyles and also smaller acanthostyles echinating the substrate. The rest of the skeleton consists of flexuous columns or reinforce the ectosome.
Ecology: On rock, stones and bivalve shells, to 300 m. Reported on algae and sponges in the Mediterranean. On horizontal rock in clear water.
Distribution: Channel (Roscoff), W coast of England; Mediterranean, Azores.
Etymology: pansus (Latin) = spread out, extended, referring to the shape.
Type specimen information: The type is in the Natural History Museum, London: BMNH 1910:1:1:91 (dry) + slides BMNH 1910.1.1.2284 (Bk.1133), Norman Collection. MCS voucher BELUM: Mc1576. Rathlin Island, Antrim, N Ireland.


External appearance is not particularly helpful, but the pore sieves and thin sheet may suggest a hymedesmid sponge. The spicule complement and sizes are distinct from the other Hymedesmia in this file, but there are many other Hymedesmia species in deeper water and the complex is currently ill-understood.
Source: Ackers et al., 1992 (D. Moss, B.E. Picton).

Hymedesmia pansa