Hymedesmia (Hymedesmia) jecusculum (Bowerbank, 1866) is an orange-red thin encrustation with prominent oval surface areolae (pore sieves), distinct from other Hymedesmia species in colour and spiculation. It may be confused with thin growth forms of Phorbas fictitius. It is the commonest Hymedesmia species off the west coasts of Scotland and Ireland.
Colour: Deep orange or red.
Shape, size, surface and consistency: Thin sheets, up to 10 cm across, but smaller typically. Surface smooth, with subsurface exhalent canals visible. Densely covered with oval pore areas. Oscules may not be apparent. Pores in oval pore-sieve areas with smooth tissue between. Consistency soft.
Spicules: (Hymedesmia jecusculum spics) Megascleres: The tornotes are slightly fusiform, thin, measuring 285 x 3.5 µm. They are symmetrical, pointed at both ends. Primary acanthostyles measure 360 x 7.5 µm. The head is thickly set with spines, whilst small, rather scattered, spines extend along the shaft but are much less frequent in the distal half. There is usually an abrupt curvature just above the head, the rest of the shaft being straight. The smaller acanthostyles measure 120 x 5 µm. They are straight or very slightly curved, and conspicuously spined along the whole length. The spines on the head are fairly long, those on the shaft recurved. Microscleres: Arcuate isochelae, ca. 20 µm in length, with slightly incurved alae.
Skeleton: Hymedesmoid, with large primary acanthostyles and also smaller acanthostyles echinating the substrate. The rest of the skeleton consists of flexuous columns or fibres of many parallel tornotes, which fan out to run parallel with the surface. Arcuate isochelae reinforce the ectosome.
Ecology: On bedrock and boulders, usually in deep water below 25 m depth but originally described from cave in the intertidal zone. On horizontal rock in clear water.
Distribution: West coast of Ireland and Scotland; this is the commonest Hymedesmia species in diving range in these areas.
Etymology: jecusculum (Latin) = small liver, referring to the aspect of the type specimen in dried condition.
Type specimen information: The type is in the Natural History Museum, London. MCS voucher BELUM: Mc1309, Loch Duich, Scotland
External appearance is not particularly helpful, but the pore sieves and thin sheet may suggest a hymedesmid sponge. The spicule complement and sizes are distinct from the other Hymedesmia species in this file, but there are many other Hymedesmia species in deeper water and the complex is currently ill-understood. The spiculation and external appearance are similar to Phorbas fictitius; however, the latter has tornotes which are slightly dissimilar at both ends. The chelae have their alae slightly flaring away from the shaft.
Source: Ackers et al., 1992.