Form: A thin somewhat slimy sheet on rocks or shells. Patches up to 15cm diameter have been seen, 2-3mm thick.
Colour: Usually grey-brown.
Consistency: Firm, rubbery, curls up when scraped off the substratum.
Surface: With pores scattered fairly evenly all over. Smooth. Obvious excurrent channels may be present. Pore sieves absent.
Apertures: Oscules with conical raised edges, at the centre of converging excurrent channels.
Skeleton: In all Hymedesmia spp. there are acanthostyles with their heads standing on the substrate and their shafts sticking up through the sponge. In this species they are short and may easily be missed. The rest of the skeleton consists of flexuous columns or fibres of many parallel smooth tornotes with tylote ends which fan out at the surface. Skeleton is Hymedesmoid, i.e. permanently encrusting with both principal and secondary endosomal spicules orientated perpendicularly to the substrate.
Spicules: The principal megascleres are acanthostyles (a), basally spined, but especially thickly on the heads, 110-135Ám. Accessory megascleres are tornotes (b) 180-(190)-200Ám, with strongylote to subtylote ends, sometimes with unequal ends. This description, although technically accurate may be misleading in practice as the acanthostyles are often few in number and easily missed. Thus the main body of the sponge usually appears to consist of bundles of tornotes. Microscleres are absent.
Habitat: Very common, e.g. on valves of Aequipecten opercularis and other shells; rock surfaces especially in the kelp zone.
Distribution: "Arctic, Atlantic coast of Europe, Mediterranean" Known recently from E. Scotland, St. Abbs; W. Scotland, Tiree; Strangford Lough; Lough Hyne, Co. Cork; Saltee Is., Co. Wexford.
Distribution Map from NBN: Grid map (fast) : Interactive map (slower, requires login to view records) : National Biodiversity Network mapping facility, data for UK.
Identity: The appearance seems to be fairly characteristic, but caution is needed. A Hymedesmoid skeleton with no microscleres is suggestive, but other Hymedesmia spp. from Britain which lack microscleres include Hymedesmia hibernica Stephens 1916 : 237. Hymedesmia radiata is actually a species of Eurypon. BEP has collected several specimens with spiculation and skeleton more or less as described here, but with different external appearances, and it is now clear that more species exist which lack chelae or sigmata. Hymedesmia species which lack microscleres are frequently placed in the genus Stylopus and several other species are described in the continental literature.
Voucher: BELUM : Mc164. Strangford Lough, Down.
Editors: D. Moss, B.E. Picton.
|Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland |
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