Hymedesmia primitiva Lundbeck, 1910


Family : Hymedesmiidae

Form: Thin encrusting sponge, typically less than 1mm in thickness, but 5-10cm in extent.

Colour: Grey, brown. Brown colour may be due to diatoms in surface of sponge.

Consistency: Thin sheet, flexible.

Surface: Entire surface is covered with circular pore-sieves (cribri) with upstanding rims. Areas between pore-sieves smooth.

Apertures: Pore-sieves are inhalent structures, oscules are less conspicuous, raised and conical.

Contraction: Pore-sieves and oscules close on disturbance.

Internal characters

Skeleton: Skeleton simple, consisting of acanthostyles with their heads resting on the substratum in a basal spongin layer, ascending columns of strongyles support the surface and strongyles support the rims of the pore-sieves.

Spicules: Acanthostyles of variable size, short and entirely spined and longer, becoming smooth towards the tip, 120μm - 350μm. Strongyles are slightly polytylote with both ends rounded, slightly swollen at the ends and slightly assymmetric, 200μm - 280μm. There are no microscleres.

Habitat: On rocks, boulders and bedrock, shallow water specimens in strong tidal streams on the open coast.

Distribution: Originally described from deep water, 100 - 850m, Iceland, Faeroes, Davis Strait, Denmark Strait, but recently discovered to be occasional at Rathlin Island, Co Antrim, Northern Ireland in 30m.

Distribution Map: NBN map : National Biodiversity Network mapping facility, data for UK.

Identity: This is one of a group of similar-looking Hymedesmia species which can only be distinguished by examination of their spicules. Some authors have considered the genus to be composed of a few variable species but recent evidence shows that there are correlating differences in external appearance with spicule characters suggesting that the large number of species in this genus are truly valid.

Editors: Claire Goodwin & Bernard Picton

 Picton, B.E., Morrow, C.C. & van Soest, R.W.B., 2011. [In] Sponges of Britain and Ireland