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Topsent, 1896

Species Overview

Hexadella racovitzai Topsent (1896) is a pale rose, finely conulose encrusting sponge, similar to Aplysilla but differing from it in the absence of any fibres. It is a Mediterranean species recorded once from Roscoff.

Taxonomic Description

Colour: Rose-coloured, with light yellowish tinges.
Shape, size, surface and consistency: Thickly encrusting. Lateral expansion may be considerable, but thickness not exceeding 5 mm. Surface smooth but demonstrating conules arranged in paralallel ridges, similar to Aplysilla or Darwinella. The "skin" may be rather easily peeled off and has a characteristic reticulate appearance. Consistency soft.
Spicules: Absent.
Skeleton: Absent. The skin is collagen-reinforced, and may act in some measure as external skeleton but otherwise the body is unsupported, with many channels and lacunae.
Choanocyte chambers: Large and sac-shaped, 65-75 µm in length (histological preparation necesary).
Ecology: On rocks, on pebbles, in deep water 9-85 m (Roscoff: 60 m).
Distribution: Mediterranean, Roscoff, Canary Islands.
Etymology: Named after Topsent's friend Mr. E.G. Racovitza.
Type specimen information: Presumably in the Muséum National d'Histoire Naturella, Paris.

Remarks

This is a typical Mediterranean species, recorded only once from the west coast of France. It resembles Aplysilla or Darwinella, but is easily distinguished from them by the absence of any skeletal structures. Other such species are Chondrosia reniformis, but this is shiny-smooth and has a distinct cortex, Oscarella rubra, but this has a lobulate surface, and Halisarca dujardini, but this has a smooth punctate surface. None of these species has conules nor are they rose-coloured.
In deep-waters off the west coasts of Ireland, Scotland and Norway, a second species of Hexadella is present, H. dedritifera Topsent (1913). This has greenish yellow colour in life, but turns dark violet when exposed to air or in preservation. It favours coldwater coral reefs.
Source: Topsent, 1896.

Hexadella racovitzai