Hymedesmia, 1864

Goodwin, Claire E. & Picton, Bernard E., 2009, Demosponges of the genus Hymedesmia (Poecilosclerida: Hymedesmidae) from Rathlin Island, Northern Ireland, with a description of six new species, Zoological Journal of the Linnean Society 156 (4), pp. 896-912 : 901-902

HYMEDESMIA (HYMEDESMIA) CRATERA SP. NOV.

( FIG. 4A, B )

Type material: Holotype: specimen in IMS, section and spicule preparation from tissue sample ( Rathlin Island sponge biodiversity project; Duncan’s Bo, 55°18.718 ′ N, 06°15.123 ′ W; water depth, 29–32 m; Mc 2897). Collected by B. Picton and C. Goodwin, 6 September 2005.

Etymology: Named from the Latin Crater, meaning a bowl, or the crater of a volcano, as the raised rims of the pore sieves give a crater-like appearance.

Comparative material examined: Hymedesmia proxima Lundbeck, 1910 , spicule preparation of specimen from Ingolf expedition station 85. ZMUC.

Hymedesmia irregularis Lundbeck, 1910 , spicule preparation of specimen from Ingolf expedition station 10. ZMUC.

External morphology: This species has a distinctive appearance, with pore sieves that have high, raised rims. It is a thin peach-coloured crust on bedrock, and has a maximum patch diameter of 3 cm.

Skeleton: Basal layer of acanthostyles, evenly dispersed, in which the small acanthostyles are more abundant than the larger category. It has ascending columns of ectosomal spicules, 10–15-spicules thick, and the orientation of the ectosomal spicules in these columns is not uniform: some point up and others point down. There is a dense layer of chelae at the surface, and chelae are also present in small numbers throughout the sponge tissue. The sponge is 700– 800-Mm thick.

Spicules:

1. Large acanthostyles: 335–620 Mm (452 Mm) by 12–20 Mm on the head, or by 8–14 Mm on the shaft. The majority are between 400 and 500 Mm in length. These have a very slightly tylote head, and are spined for about two-thirds of their length with very small spines (much smaller than those on the small acanthostyles). On some, the spines are barely perceptible, giving a roughened appearance. The spines on the head are slightly larger than those on the shaft, and often have rounded tips. The spicules are often curved.

2. Small acanthostyles: 130–220 Mm (164 Mm) by 14–16 Mm at the head, or by 8–10 Mm on the shaft. The majority are between 140 and 190 Mm long. These are entirely spined, with a slightly tylote head. The shaft is densely set with small recurved spines, on the head, these are slightly bigger, and even more numerous. The spicules are often curved.

3. Ectosomal spicules: 260–350 Mm (312 Mm) by 6–10 Mm. Most of the spicules are styles, in which one end of the spicule is rounded and the other end tapers to a sharp point. However, a few are more tornote-like in form, with the blunter end very slightly pointed, and the other end coming to a more abrupt point. They are very variable in width, and the fatter ones are sometimes fusiform, and occasionally faintly polytylote.

4. Chelae: arcuate chelae are present, and are of 20–27.5 Mm (23 Mm) in length.

Remarks: H. proxima has spicules that are similar in size to this species. However, the ectosomal spicules of H. proxima are fatter, 8–12 Mm in width, and are uniformly fusiform, rather than variable in form. The small acanthostyles do not have a developed head, or any difference between the spination of the head and the shaft, and the large acanthostyles do not have a tylote head. The chelae are also less robust. Lundbeck also reports that this species is hispid. Hymedesmia irregularis has acanthostyles that are more similar in form, with tylote heads; however, these are not divisible into two size categories. Additionally, its ectosomal spicules are always polytylote, and the chelae are much larger (40–50 Mm).

Figures

Figure 4. A, Hymedesmia (Hymedesmia) cratera. sp. nov. A, large acanthostyle; B, base of large acanthostyle; C, small acanthostyle; D, ends of ectosomal spicule; E, chelae. Scale bars: 10 Mm. B, Hymedesmia (Hymedesmia) cratera. sp. nov., surface.