(Montagu, 1808)
Description:
This is a robust species, despite its superficial appearance of frailty. It may reach 27 mm in length, translucent white in colour, with tinges of yellow and pink on the dorsum. There are patches of opaque white pigment and also areas of especially transparent skin (notably around the bases of the rhinophores and behind the gills, forming in the latter case a false 'pore'). The mantle edge is fairly well developed but does not form an ample skirt. A low keel runs down the middle of the back and on either side of this are small conical tubercles; a median keel is also visible on the dorsal surface of the metapodium or 'tail'. The lamellate rhinophores are tinged with yellow. There are up to thirteen gills around the anal papilla. The head bears a pair of flattened oral tentacles.
Habitat:
When young, G. nodosa feeds upon encrusting polyzoans, such as Alcyonidium gelatinosum, Callopora dumerili and Flustrellidra hispida , but the adults transfer their attentions to ascidians, especially Diplosoma listerianum, Botryllus schlosseri and Dendrodoa grossularia.
Distribution:
It is a common species on shores and in the shallow sublittoral all around the British Isles, down to 20 m, although in other waters it has been reported down to 120 m. Outside Britain, this species has been recorded from Norway to northwest Spain (Distr. G. nodosa).