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 . No surface markings.
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 . Translucent white revealing reddish brown  or brownish internal digestive gland. Those fed on female Hydractinia reported to be darker  than those fed on male Hydractinia. Powdery white spots on surface, concentrating into a band at distal end , except translucent white apex containing white cnidosacs.
Smooth. Translucent white, often with white surface pigment on distal half.
Head & oral tentacles
Translucent white oral tentacles, often with white surface marks , arise near edge of head in large specimens, but more dorsally in juveniles. Head and mouth unusually wide when seen from below .
Expanded at anterior, no propodial tentacles . 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 .
- Powdery white dots on cerata concentrate into band distally .
- White surface pigment often on oral tentacles. .
- On Hydractina echinata, usually on shell of Hermit crab, but sometimes on hard substrate where strong current, or wave surge (EML).
- 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  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.