Hymedesmia rathlinia Goodwin & Picton, 2009


Family : Hymedesmiidae

Form: A distinctive sponge, which may be recognised in situ. Forms small patches of 35 cm in diameter, often with several patches adjacent to one another; each patch bears one or two oscules. It turns a dark brown/black in alcohol.

Colour: The colour of the sponge is bright yellow, sometimes with darker yellow or orange lines radiating from the papillae.

Apertures: The oscules are combined with inhalent pore sieves at the tip of inflated papillae, which project from the sediment that the sponge is frequently covered in.

Internal characters

Skeleton: Basal layer of acanthostyles, in which the small acanthostyles are very abundant. The larger acanthostyles tend to be surrounded by the columns of ectosomal spicules. There are ascending columns of ectosomal spicules that are variable in thickness: between 5 and 12 spicules in width. Chelae are present in the surface layer, but are not very abundant. The sponge is ~900 μm thick.

Spicules: Large acanthostyles: (A,B) 175390 μm (282 μm) by 1014 μm. Spined for between half and two-thirds of their length, with small straight spines. The head is not tylote. Small acanthostyles: (C) 65125 μm (103 μm) by 1012 μm. Spined for their whole length, again the head is not tylote. The spines are very large, and are usually straight (although some are very slightly curved). Ectosomal spicules: (D) tylotes 220335 μm (280 μm) by 46 μm, most with both ends tylote, but in some, only one end is swollen. Chelae: (E) arcuate chelae 22.527.5 μm (25 μm) along shaft, with fairly short alae.

Distribution: Currently known only from the North Channel, ie. Rathlin Island, the Maidens and Sanda Island.

Identity: External appearance with inhalent pore sieves surrounding osculae is distinctive and make this sponge identifiable in situ.

Editors: Claire Goodwin & Bernard Picton.

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