Pachastrella monilifera Schmidt (1868) is a light coloured formless massive sponge with very rough, hispid surface and hard consistency. It is a deep-water species with a large distribution.
Colour: Whitish with dark greenish patches; in alcohol pale yellow with pinkish tinges.
Shape, size, surface and consistency: Thick formless masses with very rough and hispid surface. Size may be considerable, 20 cm or more in diameter. Oscules distinct: 0.5-1.5 mm in diameter, not raised. Consistency: hard.
Spicules: (Pachastrella monilifera spics) Megascleres: Oxeas, long and thin, almost cylindrical and straight, invariably broken, difficult to obtain complete specimen: 2500-3500 µm long; calthropses, variable in size, the larger ones may have one ray longer than the rest and thus resemble plagiotriaenes, with rays often curved at the end, whereas the longer rhabd is straight: rays 200-835 long, rhabd up to 1030 x 95 µm.
Microscleres: Microstrongyles, variable in shape, straight or centrotylote, surface finely spined: 10-15 x 4-5 µm; spirasters, amphiasters and metasters, passing into each other, with finely spined rays: 11-12 µm.
Skeleton: Ectosomal skeleton a mass of microscleres. Cortex collenchymatous, about 1-1.5 mm in thickness. Choanosomal skeleton largely a confused arrangement of calthropses with occasional oxeas arranged perpendicular to the surface and sticking through it to cause the hispidation.
Ecology: On dead corals, 30-1500 m.
Distribution: Norway, SW Ireland, Portugal, Spain; Mediterranean, Azores.
Etymology: monile (Latin) = necklace, referring to the necklace-shape of the type specimen.
Type specimen information: The type specimen is in the Natural History Museum, London: BMNH 1910:1:1:854 (dry), Norman Collection, BMNH: 18126.96.36.199 (slide).
The combination of a rough hispid mass with predominantly calthrops megascleres make this is a distinctive species. Poecillastra species may be similar in spiculation, but have relatively smooth flattened shape.
Source: Topsent, 1894.