(Pallas, 1766)
Species Overview
Tethya aurantium (Pallas, 1766) looks like a small orange and even when sectioned mimics this fruit because it has characteristic yellow-orange, somewhat warty surface, an orange cortex looking like a thick skin and a darker orange centre with radial bundles (of spicules). This is a common species in shallow water on rocky surfaces in open water or tidal channels; tolerates some silt. Distribution Mediterranean-Atlantic, reaching its northern limits along the west coasts of the British Isles.
Taxonomic Description
Colour: Most commonly pale to bright yellow, but can be orange.
Shape, size, surface and consistency: (Tethya aurantium MCS1) (Tethya aurantium MCS2) Massive-globose (usually spherical to hemispherical) up to 6 cm in diameter, sometimes with 'rooting' processes. Overall appearance like a small orange. Surface tuberculate ("warty"); the tubercles are separated by contractile pore-bearing grooves. Sometimes 'buds' are present, found on short stalks on top of these tubercles. Often covered by a layer of silt. Appearance variable, dependent on expansion and contraction. When contracted, can appear smooth and even, faintly marked by meandering striations. These can expand into grooves or channels, leaving 'islands of tissue' between, which can be rounded in profile (i.e. tuberculate) or flat (i.e. polygonal in outline). Usually one oscule, apical, almost opposite to the point of attachment. Contracts to less than half fully expanded size when removed from water. There have been reports of the surface layer being expanded so as to appear partially separated from the underlying tissues. Smell: even when fresh, the interior smells of marine specimens which have been allowed to decay. Consistency moderately firm, moderately elastic.
Spicules: (Tethya aurantium spics) Megascleres are strongyloxeas (stylote or tylostylote) 510-680-850 µm in length.
Microscleres are euasters, with spherasters of 30-100 µm diameter (av. 60 µm) forming a distinct layer in the contractile cortex, and with strongylasters (micrasters) with microspined tylote rays, ca. 12 µm in diameter. The micrasters may be divided into thicker-rayed ones occurring in the peripheral part of the cortex (T. aurantium cort.micraster), and thinner-rayed of the inner cortex (T. aurantium choan.micr.) (Sarà and Gaina, 1987).
Skeleton: (Tethya aurantium cross) (Tethya aurantium cortex) (T. aurantium distrib.asters) Radiate, with a well developed cortex. The thick radially arranged tracts of megascleres, which can be seen with the unaided eye in sectioned specimens, run at right angles to the surface, and terminate in a surface tubercle without piercing the surface.
Reproduction: Stalked buds produced between July and September.
Ecology: On rock surfaces usually in open water, although it has been reported in harbours. A common species on horizontal or sloping rocky surfces in clean water but tolerant of silt. Littoral to 930m.
Distribution: E Atlantic; Mediterranean. Apparently absent from the North Sea coasts, but common on western and southern coasts of the British Isles.
Etymology: Aurantium = orange.
Type specimen information: Type probably lost. MCS voucher BELUM: Mc181, Strangford Lough, N Ireland.
Remarks
A similar species, T . norvegica , replaces this species in the north (southernmost records of this species in N Scotland), but no sympatric occurrence has been reported. Thus, in view of the characteristic habit, identification by sight alone stands a good chance of being correct. Suberites species sometimes adopt the same form but have a smooth surface. Tetilla cranium and Tetilla zetlandica are of the same shape and have a rough surface, but are usually bright white in colour. Tetilla species have triaenes and oxeas as their megascleres and T . cranium has distinctive distorted 'S' shaped microscleres (sigmaspires).
Recent studies by Sarà c.s. concluded that the name for this species may have to be changed into T . citrina Sarà, because Pallas' original material may have belonged to a species different from the East Atlantic material, and restricted to the Mediterranean.
Sources: Sarà and Gaino, 1987; Ackers et al. (1992: D. Guiterman, S.M. Stone, D. Moss, B.E. Picton).