(Bowerbank, 1866)
Species Overview
Hymedesmia paupertas (Bowerbank, 1866) is sky blue encrusting sponge with large rounded and slightly raised pore sieves. These characters make it easily recognizable. It is a northern species occurring on the north coasts of Ireland and Scotland and on the coasts of Norway and Sweden.
Taxonomic Descriptions
Colour: Bright blue or green—distinctly unusual.
Shape, size, surface and consistency: Small thin sheets usually covering areas only 1 or 2 cm in diameter, very irregular in outline, giving the appearance of several separate patches (Hymedesmia paupertas MCS). Surface with small pore sieves, similar to Hemimycale columella . This appears to be characteristic of a group of Hymedesmia spp., which may all be related more closely to Phorbas and Hemimycale than to other Hymedesmia species. Oscules with converging excurrent channels and transparent, slightly raised rims, few in number and towards the centre of patches. Consistency difficult to determine.
Spicules: (Hymedesmia paupertas spics) Megascleres : Tornotes of 225-263-275 µm. Acanthostyles of two types are present: long ones with more or less smooth shafts, 360-400-420 µm long; and short entirely spined ones, 130-150-155 µm long. Microscleres : Arcuate isochelae, 25-32 µm, with particularly broad shafts when viewed face-on, and very arcuate from the side. Occasional aberrant spicules apparently derived from the larger acanthostyles, may be present.
Skeleton: A basal layer of acanthostyles of two sizes with their heads on the substrate. The longer ones have smooth shafts, the shorter are entirely spined. Fibres of parallel tornotes, with slightly different ends, run up through the body to the surface. The surface layer of chelae is especially packed in the pore areas.
Ecology: So far recorded from deep rocky areas, sheltered or with moderate exposure, down to 400 m. A circalittoral species, found on cliffs in W. Scotland and on limestone ridges in western Ireland.
Distribution: Along most coasts of Europe, including Norway and Sweden. Recorded recently from Bill's Rocks, Achill Is., Co. Mayo, Rathlin O'Birne Is., Co. Donegal; St. John's Pt., Co. Donegal; west coast of Scotland.
Etymology: Paupertas (Latin) = poverty, referring to the "poor and uninviting" appearance of the (dried) type specimen.
Type specimen information: The type is in the Natural History Museum, London: BMNH 1910.1.1.2323 (slide), Norman Collection. MCS voucher BELUM: Mc513, Lunga, Firth of Lome, Hebrides.
Remarks
The combination of pore sieves and bright blue or green colour seems to be characteristic of this species, as is the shape of the chelae and tornotes. Many Hymedesmia species exist, whose live appearance is currently unknown. Terpios gelatinosa is a blue sponge found in the littoral zone.
NB: the species known as Anchinoe paupertas in the Mediterranean (cf. e.g. Boury-Esnault, 1971) is a red species of Phorbas and is not this species; it needs a new name.
Source: Ackers et al., 1992 (B.E. Picton)