Isops phlegraei Sollas (1880) is a globular, yellowish grey, hispid sponge, often encrusted or sedimented. Oscules small and scattered, through which it may be distinguished from similar Geodia-like sponges which have them organized in groups, often in depressions or cavities. It has a hard consistency. It is a northern deep-water sponge.
Colour: Pale yellowish grey.
Shape, size, surface and consistency: More or less spherical, size up to 20 cm in diameter. Surface hispid due to projecting spicules, often overgrown or sedimented. Oscules small, 1-2 mm, and scattered, mostly conspicuous. Consistency hard.
Spicules: (Isops phlegraei spics) Megascleres: Oxeas up to 6000 x 65 µm; plagiotriaenes up to 4000 x 425, with clads 600 µm, occasionally dichotriaene modifications occur; protriaenes likewise long, but size not recorded, thickness 20 µm; anatriaenes, ditto. Microscleres: Sterrasters, ellipsoid, maximal size 90 µm; spherasters in two size classes: cortical ones with conical rays, 12 µm, choanosomal with longer thinner rays, up to 20 µm.
Skeleton: Radiate; cortex about 650 µm in thickness, consisting chiefly of the sterrastral layer, below that there is a thin fibrous layer. Inner skeleton confused.
Ecology: 85-900 m
Distribution: Sweden, Norway.
Etymology: phlegraeus (Greek) = from the ancient city of Phlegra in Macedonia; its connection with the present species in unknown.
Type specimen information: The type is in the Natural History Museum, London. Type locality: Kors Fjord, Norway, 364 m.
Several other Isops species have been recorded from the area (I. globus Schmidt, from Portugal, I. pallida Vosmaer from the Arctic, and I. sphaeroides Vosmaer, likewise from the Arctic) but the status of these is unclear; it is likely that at least the latter two are synonyms of I. phlegraei.
The Mediterranean Isops intuta differs from the present species in overall smaller size of the megascleres (e.g. oxeas are only up to 2500 x 28 µm), and the absence of anatriaenes and protriaenes.
The genus Isops is distinguished from Geodia by the scattered oscules, which are grouped in Geodia. With Geodia and Pachymatisma it shares the sterrasters. Pachymatisma johnstonia are dark greyish masses; the spiculation includes microstrongyles, which are absent in both Geodia and Isops. The validity of a separate genus Isops remains debatable.
Source: Sollas, 1880, 1888.