Hymedesmia HIBERNICA STEPHENS 1916

HYMEDESMIA (STYLOPUS) HIBERNICA STEPHENS, 1916 (FIG. 10A, B)

Specimens: Specimen in IMS, section and spicule preparation from tissue sample (Rathlin Island Sponge Biodiversity Project; specimen 1, west of Derginan Point, 55°18.283 ′ N, 06°16.774 ′ W; water depth, 28.5–31.5 m; Mc 2766). Collected by B. Picton and C. Goodwin, 15 August 2005. Specimen 2: White Cliffs (55°17.546 ′ N, 06°14.518 ′ W; water depth, 32–35 m; Mc 2950). Collected by J. Jones and C. Goodwin, 7 August 2005.

Comparative material examined: Hymedesmia hibernica Stephens, 1916 type (W141 14.1916). NMI.

External morphology: A yellow sponge with large pore sieves, which is slightly translucent in appearance. Both specimens formed thin encrustations on rock: these are very small patches of less than 5 cm in maximum diameter.

Skeleton: Basal layer of acanthostyles, in which the smaller category are more abundant, and in which acanthostyles are spread evenly but fairly sparsely, with some space in between them. The ascending columns of anisostrongyles are 5–8 spicules thick. The sponge is 500–800-Mm thick.

Spicules:

1. Large acanthostyles: 250–325 Mm by 5–12 Mm. These acanthostyles have a slightly tylote head, which is more obvious in thinner specimens, and are spined on the head and up to half of the way up the shaft, although with much smaller spines. The head spines are often strongly curved up towards the shaft.

2. Small acanthostyles: 110–130 Mm by 10–12 Mm. These are entirely spined with large recurved spines on the shaft, which become progressively sparser towards the tip; the head is not tylote, but is marked by denser spines that are often curved up towards the shaft.

3. Ectosomal spicules: 200–250 Mm by 2–4 Mm, with thin anisostrongyles. One end is usually thicker than the other, and neither end is tylote.

4. Microscleres: none present.

Remarks: The spiculation agrees almost exactly with that described by Stephens (1916), apart from the tendency for the tylostyles to become subtylote. This species was originally described from 74 m off Reenacry Head, County Kerry, Ireland. It has since been recorded from the English Channel, from Roscoff (Cabioch, 1968).