Phorbas punctata Picton & Goodwin, 2007

Order : POECILOSCLERIDA

Family : Hymedesmiidae

Form: Encrusting species, forming patches up to 10 cm in diameter on bedrock.

Colour: Yellow to pale orange in colour.

Surface: Oscules surrounded by 5–8 radiating oscular channels, these channels may be grouped in pairs. The ostia seem to be grouped in patches which gives the surface a spotted appearance.

Contraction: Out of water the oscules and excurrent channels collapse.

Internal characters

Skeleton: The choanosome has a basal layer of acanthostyles and columns of large and small acanthostyles. There is a thick ectosomal layer of tornotes and chelae, the chelae are very abundant.

Spicules: The primary acanthostyles are 250-(366)-430 μm long and 10–15 μm thick. They taper fairly abruptly at the end to a sharp point and are spined only in bottom third to half of their shaft with small spines. The head is not tylote but may be marked by heavier spination than the rest of the shaft, which may be straight or slightly curved. The echinating acanthostyles are 80–(120)-160 μm long and 6–10 μm thick. They are entirely spined with larger spines than the primary acanthostyles and there is no development of the head. These spicules taper to a sharp point and may be straight or slightly curved. The tornotes measure 165–(170)-180 by 2-4 μm thick. The ends tend towards being mucronate, one, occasionally both, ends are usually slightly swollen. There are two size-classes of chelae present, the larger 30–40 μm and the smaller 12-15 μm.

Distribution: This species is currently known from only two specimens, from Rathlin Island, Northern Ireland.

Identity: The closest described species is Phorbas lieberkuehni (Burton, 1930), however, this is described as a bright red brown crust; although it has two classes of acanthostyles of a similar appearance the larger class range only from 250–360 μm in length; the tornotes are larger and have a larger size range (180–270 μm); and although the chelae occur in two size categories they are much smaller (10–15 μm and 18–22 μm respectively). There is no information on the form of the skeleton of this species. Most other species of Phorbas lack two size-classes of chelae, in those where there are two sizes present either sigmas are present (e.g. Phorbas roemeri (Hentschel, 1929)) or the spicules are much larger (e.g. Phorbas perarmatus Bowerbank, 1866).

Voucher: BELUM:Mc2445, holotype.

Editors: Claire Goodwin & Bernard Picton