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(Topsent, 1891a)

Species Overview

Phorbas dives (Topsent, 1891a) is a brownish yellow to cream cushion sponge with distinct venal channel pattern as well as a punctate surface. Its consistency is fairly compressible. It has a rather sistinctive spiculation. Rather a rare southern species.

Taxonomic Description

Colour: Creamy to brownish or greenish yellow, orange.
Shape, size, surface and consistency: Crust to cushions to about 1 cm thick; spreading in patches to more than 1 m across. Surface more or less smooth with conspicuous broad subsurface channels converging on the oscules. Apertures (pore-sieves and oscules) quite numerous, of moderate size. Consistency compressible, fairly resilient; breaks when bent through ca. 90°. A slight spongy/fresh marine smell.
Spicules: (Phorbas dives spics) Megascleres: The ectosomal tornotes may be somewhat inequiended and are 122-216 x 2 µm long. The megascleres of the main skeleton are acanthostyles: 151-299 x 8-10 µm, sparsely spined especially away from the head. The auxiliary acanthostyles are 60-112 µm long, and relatively strongly spined.
Microscleres are arcuate isochelae and sigmata. The isochelae are mostly of two size classes, ca. 8-15 µm and ca. 20-35 µm.The sigmata are usually in two size groups, ca. 15-18 µm and 35-40 µm—the latter are more numerous.
Skeleton: Tornotes are common at the surface, arranged more or less perpendicular to it. Choanosomal skeleton a plumose arrangement of ascending multispicular fibres of acanthostyles, with smaller accessory acanthostyles. Occasionally spicules at the end of the fibres penetrate the surface. Abundant sigmata in the choanosomal tissues, and chelae arcuatae most abundant near the surface.
Ecology: Vertical hard substrate faces, from shallow sublittoral (well-shaded and wave exposed) to 75m; on overhangs and under cobbles and boulders. Among Cystoseira (Topsent, 1891a); may be associated with Axinella dissimilis.
Distribution: Wales, SW Ireland, Roscoff, Channel Isles, NW Spain. Apparently restricted to the south west of the British Isles and southwards along the W coasts of France and Spain.
Etymology: dives (Latin) = rich, referring to the large number of microsclere types.
Type specimen information: The type is in the Paris Museum.

Remarks

The appearance, with conspicuous subsurface channels, is not very distinctive. The spicule complement, however, is fairly distinctive: there are apparently several species of Phorbas with similar spiculation, but differing in the size and relative abundance of the isochelae, and the size, shape and spination of the acanthostyles.
Phorbas bihamiger is very similar to this species, but has abundant chelae of two size classes in a surface layer. The channels are more noticeable in close-up photographs in situ.
Phorbas microchelifer likewise is very close differing only in the sizes of the spicules.
Source: Ackers et al. (1992) (D. Moss, G. Ackers, B.E. Picton).

Phorbas dives