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(Hanitsch, 1895)

Species Overview

Oscarella rubra (Hanitsch, 1895), also known as Oscarella lobularis, is easily recognizable in the field by its lobulate surface. It is an encrusting pale yellow or orange sponge with a slippery-gelatinous surface. Out of the water the lobules-bubbles disappear largely and then it may be superficially confused with Halisarca dujardini, a species also without any mineral or organic skeleton. It may be necessary to make histological sections to make absolutely certain. It is acommon species occurring along most rocky coasts of Europe from the intertidal to a depth of 300 m or more.

Taxonomic Description

Colour: Pale yellow-orange, sometimes with red pigment on surfaces exposed to light or in shallow water.
Shape, size, surface and consistency: (oscarella_rubra_iv.jpg) (oscarella_rubra_v.jpg ) A thin bubbly sheet over boulders or rock faces. Small patches 4 cm across to large patches 30 cm or more across. Surface covered with bubble-like projections—lobulate. These lobules measure 0.5-1 cm in diameter. Translucent when alive. Slippery. Widely scattered large oscules with a delicate chimney-like structure, which collapses and is lost in preservation. Consistency very soft and fragile, gelatinous. When collected it becomes a thin opaque sheet and loses its lobulate appearance.
Spicules: None.
Skeleton: None. Without spicules, spongin fibres or foreign inclusions which might strengthen the soft matrix. Large cavities and lacunae permeate the body.
Ecology: A common species in the littoral down to > 300 m. Covering tops of boulders in sheltered areas of moderate current. On vertical rock faces and on chalk and clay cliff faces of moderate current. Sometimes found in sheltered conditions with little current or wave action. Found abundantly on fully exposed bare rock faces at St. Kilda, overgrowing Lithothamnion.
Development: oscarella_rubra_develop.jpg of the body from earliest settled larvae to adults was followed using histological sections by Sollas (1884).
Distribution: Arctic; British lsles; Belgium; France; Mediterranean; Azores.
Etymology: The name refers to the colour (which is not typical)
Type specimen information: No specimens known. MCS voucher BELUM Mc324, Strangford Lough, N Ireland.

Remarks

The lobulate surface together with delicate oscular chimneys, combined with a delicate texture and lack of skeleton, is diagnostic under water. The only other common sponge in the area which lacks a mineral or spongin skeleton is Halisarca dujardini, which can be distinguished with a microscope from Oscarella by its large and elongate choanocyte chambers, with the flagella sometimes visible. However, differences in external appearance (presence of a cuticle and lack of lobules, giving a smooth, rather slimy, toffee-like appearance) probably are great enough to make confusion unlikely. A species of Hexadella has recently been found near Rathlin Island, N Ireland. It differs from Oscarella in having a surface with radiating fibre-like lines, but also has no spicules. In the south of the area Chondrosia reniformis is a further species lacking a skeleton; it is form and has a cartilaginous consistency.
Source: Ackers et al. (1992) (B.E. Picton, S.M. Stone, D. Moss, G. Ackers, F.A. Dipper).

Oscarella rubra