Hymedesmia (Hymedesmia) umbelliformis, Goodwin & Picton, 2009

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 : 900-901

HYMEDESMIA (HYMEDESMIA) UMBELLIFORMIS SP NOV. ( FIG. 3A, B )

Type material: Holotype: specimen in IMS, section and spicule preparation from tissue sample ( Rathlin Island sponge biodiversity project; Loch Garry, 55°15.956 ′ N, 06°10.411 ′ W; water depth, 32–35 m; Mc 2645). Collected by J. Jones and L. Scally, 9 June 2005.

Etymology: Named from the Latin for umbrella, umbella, in reference to the rays of spongin on the surface, which look rather like umbrella spokes.

Comparative material examined: Hymedesmia mammilaris Fristedt, 1885 , spicule preparation prepared by Alander (1942), from material from Skagerack station 13, 9 July 1934, and from Skagerack, 5 August 1937 (subspecies).

Hymedesmia simillima Lundbeck, 1910 , spicule preparation of specimen from Ingolf expedition station 81. ZMUC.

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

Hymedesmia basispinosa Lundbeck, 1910 , spicule preparation of specimen from Ingolf expedition station 28. ZMUC.

External morphology: This specimen is white, with distinctive rays of spongin showing on the surface, it formed a small (<3-cm maximum diameter), thin encrustation on a boulder.

Skeleton: Basal layer of acanthostyles, evenly dispersed, in which the small acanthostyles are much more abundant than the large ones. Because of the thickness of the sponge, even the largest acanthostyles do not reach the sponge surface. It has ascending columns of ectosomal spicules, of 10–15 spicules in width. Chelae are present throughout the tissue, but are most abundant at the surface, where they form a dense layer. The sponge is 900–1200-Mm thick.

Spicules:

1. Large acanthostyles: 300–525 Mm (434 Mm) by 12–14 Mm at the head, or by 10 Mm on shaft. Long slender acanthostyles that have a well-developed tylote head. The majority are spined only on the head, but in some the shaft is spined with very small spines, which give a roughened texture, for up to half of their length. The spines on the head are rather short, and may have rounded points.

2. Small acanthostyles: 130–210 Mm (167 Mm) by 12 Mm (head), or by 6–10 Mm (shaft). Entirely spined with medium-sized spines. The head is slightly tylote, and is marked by more numerous spines. There is often a small unspined gap on the shaft, just above the head.

3. Ectosomal spicules: 270–380 Mm (336 Mm) by 6–10 Mm. Long tornotes that are style-like in form. The majority are formed into a fine point at one end, with the other end either abruptly pointed or rounded. They are slightly fusiform, and tend to taper towards the more sharply pointed end.

4. Chelae: 20–25 Mm (23 Mm), with a broad shaft; abundant.

Remarks: The size of the spicules roughly equate with those of H. mammilaris , which has large acanthostyles (300–400 Mm), small acanthostyles (120– 200 Mm), oxeote ectosomal spicules (up to 360 Mm), and chelae (19–25 Mm). However, the large acanthostyles of H. mammilaris are shorter, and are spined for at least half of their length, and both the large and small acanthostyles lack a tylote head. Additionally, this species is red when living.

Hymedesmia simillima is also similar. However, its acanthostyles are longer (410–650 Mm), the small acanthostyles have more spines, and the ectosomal spicules are true oxeas, tapering to a fine point, rather than being like styles, as in H. umbelliformis sp. nov. The chelae are more strongly curved, and are larger in size (28–37 Mm).

Hymedesmia proxima can be distinguished from H. umbelliformis sp. nov. by its fusiform ectosomal spicules, and by the differences in spination on its acanthostyles: both categories are more spined than H. umbelliformis sp. nov., and the spination on the large acanthostyle extends further up the shaft. The larger acanthostyles also reach a greater length (620 Mm).

Hymedesmia basispinosa can also be differentiated, as its ectosomal spicules are clearly oxea, and are often mucronate at the ends, and are longer, reaching up to 500 Mm in length.

Figures

Figure 3. A, Hymedesmia (Hymedesmia) umbelliformis sp. nov. A, large acanthostyle; B, small acanthostyle; C, ends of ectosomal spicule; D, chelae. Scale bars: 10 Mm. B, Hymedesmia (Hymedesmia) umbelliformis sp. nov., surface.