Tethya hibernica Heim, Nickel, Picton & Brümmer, 2007

Order : TETHYIDA

Family : Tethyidae

Form: Spherical, attached by the base. Typically 1-2cm in diameter.

Colour: Yellow to brown, often partially covered with adhering silt.

Consistency: Firm, slightly compressible.

Surface: Covered with flattened tubercles which tend to coalesce when the animal is contracted.

Apertures: Oscule or oscules inconspicuous, at centre of upper surface.

Contraction: Capable of considerable expansion and contraction, as in other Tethya species.

Internal characters

Skeleton: The radiate bundles of styles (strongyloxeas) are dense and terminate in cortical tubercles, forming terminal fans. Groups of auxiliary megascleres are present in the choanosome in between the main bundles. The megasters (oxyspherasters) are evenly and densely scattered throughout the whole cortex. Micrasters form a discrete layer allocated to the exopinacoderm surface and the endopinacoderm of the lacunae. In addition, micrasters are evenly distributed throughout the whole cortex as well as the choanoderm.

Spicules: Main megascleres are strongyloxeas, 880-1250 μm in length, 8-18 μm in diameter. Auxiliary megascleres are also strongyloxeas, 365-820 μm in length, 7-14 μm in diameter.

Megasters are oxyspherasters, 31-60 μm in diameter, with 13-18 rays which often slightly curved towards the tip, R/C = 0.40-0.78. Micrasters fall into three catgories: mainly oxyspherasters, 10-19 μm in diameter, with 10-13 rays; strongylasters 8-18 μm in diameter, with 11-14 rays; few small tylasters, 8-10 μm in diameter, with 7-9 slightly spinulated rays.

Habitat: Found attached to the sides of boulders at 30-35m depth in a sponge-rich habitat. Associated species include Axinella damicornis, Axinella pyramidata and a wide diversity of encrusting sponges. The site is affected by periodic tidal streams and is adjacent to deep, well-mixed water and strong tidal streams.

Distribution: At present this species is only known from a small bay on Rathlin Island, Co Antrim, Northern Ireland. As it has only been recently distinguished from the common Tethya citrina it will probably be found at other sites.

Identity: Small Tethya which could be mistaken for juvenile Tethya citrina. It is very similar in external appearance to Tethya norvegica and can only be reliably distinguished from that species by its spicules.

Editors: Isabel Heim, Bernard Picton.