Body (excluding appendages)
Up to 28mm long. Translucent white, sometimes part of dorsum tinted pink. Ovotestis lobules may show as pink or white blobs in the rear half [2]. No surface markings.
Cerata
Numerous, but usually most of dorsum of body exposed. Arranged in crowded rows on each side of the body. Extend in front of rhinophores on specimens over 8 mm long [3]. Translucent white revealing reddish brown [1] or brownish internal digestive gland. Those fed on female Hydractinia reported to be darker [3] than those fed on male Hydractinia. Powdery white spots on surface, concentrating into a band at distal end [2], except translucent white apex containing white cnidosacs.
Rhinophores
Smooth. Translucent white, often with white surface pigment on distal half.
Head & oral tentacles
Translucent white oral tentacles, often with white surface marks [1], arise near edge of head in large specimens, but more dorsally in juveniles. Head and mouth unusually wide when seen from below [5].
Foot
Expanded at anterior, no propodial tentacles [5]. Translucent white sole.
Key identification features
- Smooth translucent white rhinophores, often with white surface marks.
- Cerata numerous, extend forwards of rhinophores in specimens over 8mm long [3].
- Powdery white dots on cerata concentrate into band distally [2].
- White surface pigment often on oral tentacles. [1].
- On Hydractina echinata, usually on shell of Hermit crab, but sometimes on hard substrate where strong current, or wave surge (EML).
Similar species
- Wide area of powdery surface pigment covers distal part of cerata; more extensive than on C. nana.
- Translucent smooth rhinophores tinted orange-brown.
- Not found on Hydractina on Hermit crabs.
Ecology and behaviour
Low water spring tide to 30m. Usually on Hydractinia echinata on shells of Hermit crabs [4] on muddy sand in full salinity waters sheltered from waves. Sometimes on H. echinata growing on hard substrate where very strong currents or wave surge. Simultaneous hermaphrodite. Convoluted spiral of spawn laid on H. echinata. Veliger larvae drift as plankton for over two weeks before settling on the sea floor and transforming into the adult form. Breeds most months, with maximum in spring. Several generations each year.
Distribution and status
Brittany to Spitzbergen (GBIF map). All round Britain and Ireland where H. echinata occurs (UK distribution map).
The map provided here shows the distribution of the species based on Conchological Society data.
References and links
Alder, J. & Hancock, A. 1845-1855. A monograph of the British nudibranchiate mollusca. London, Ray Society.
Thompson, T.E. & Brown, G.H. 1984. Biology of opisthobranch molluscs 2. London, Ray Society.
Irish distribution maps & prey organisms:
Picton, B.E. & Morrow, C.C., 2010. Encyclopedia of Marine Life of Britain and Ireland (EML).