HYMEDESMIA (HYMEDESMIA) PEACHII BOWERBANK, 1882

(FIG. 7A, B)

Specimens: Specimen in IMS, section and spicule preparation from tissue sample (Rathlin Island sponge biodiversity project; specimen 1, Ruecallan, 55°18.473 ′ N, 06°16.206 ′ W; water depth: 29–32 m; Mc 2562). Collected by B. Picton A. M. Mahon, 5 July 2005 . Specimen 2 (White Cliffs, 55°17.527 ′ N, 06°14.672 ′ W; water depth, 30–33 m; Mc 2733). Collected by C. Goodwin, D. Goodwin, B. Picton, 19 August 2005 .

Comparative material examined: Hymedesmia peachii type specimen, 1877.5.21.1137 (Bk). BMNH.

External morphology: A very thin pale peach to yellow encrustation, with oscules surrounded by radiating veins. Two specimens were taken: one from Ruecallan on the North Wall, and one from the White Cliffs on the south of the island (29–33 m). Both formed fairly large patches (with a maximum diameter of <15 cm) on rock.

Skeleton: Basal layer of acanthostyles, with ascending columns of ectosomal spicules, 3–8 spicules thick. Chelae are present in the surface layer. It is a thin sponge that is 500–600-Mm thick.

Spicules:

1. Acanthostyles: 75–320 Mm by 8–14 Mm; only one size category is present. The smallest acanthostyles are entirely spined, whereas those at the larger end of the size range may be smooth for up to half of their length. The head is not tylote, and the spicules are sometimes faintly or strongly curved. Extremes of this category are shown in Figure 7a; however, there are intermediates in both size and form.

2. Ectosomal spicules: 165–225 Mm by 4–5 Mm; tornotes with conical ends.

3. Chelae: two size categories of chelae are present. The largest chelae (28–30 Mm) have an unusual, broad, flattened shaft, whereas the smaller chelae (20–22 Mm) are normal arcuate chelae.

Remarks: The spicules differ slightly from the sizes reported by Bowerbank (acanthostyles up to 379 Mm in length, ectosomal spicules 144-Mm long, chelae 42- and 13-Mm long, respectively), but are the same in form. The sizes agree more closely with those described by Burton (1930), although he stated that the acanthostyles fall into two size classes, and that only large chelae are present.

Hymedesmia peachii was originally described by Bowerbank (1882) from Wick, and can be separated from other Hymedesmia by the unusual form of the large chelae. It has also been reported from the Mediterranean (Topsent, 1925; Sara, 1964; Pouliquen, 1972), the Azores (Topsent, 1904), the English Channel (Burton, 1930; Borojevic, Cabioch & Lévi, 1968; Boury-Esnault, 1971), the North Sea (Arndt, 1935), and Sweden (Fristedt, 1885). The reported depths vary, from under 3 m (Sara, 1964) to 1022 m (Topsent, 1904).