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Janolus is a genus of small to large sea slugs, or more accurately nudibranchs, marine gastropod mollusks, in the family Proctonotidae.[2]

The name Janolus is derived from the two-headed god Janus, in ancient Roman mythology.

Although these nudibranchs superficially resemble aeolid nudibranchs, this genus is in fact in the suborder Arminina.


Adult individuals of Janolus species can be between 2.5 cm to 8 cm long, depending on the species. They are semi-translucent and the body is covered in short cerata.

The color of the cerata varies according to the species: in Janolus fuscus the cerata are orange and white tipped, whereas in Janolus barbarensis they are orange and blue tipped.


Janolus species are found in many areas world-wide, including Europe, Australia, Japan and Africa.

The species Janolus fuscus is found from the Kenai Peninsula, Alaska to central California and also in northern Japan.[3]



This genus of nudibranch is found in shallow and subtidal waters.

Feeding habits[edit]

Janolus species feed on Bryozoa, moss animals.


In California, Navanax is a known predator of Janolus. Navanax tracks the slime of Janolus by using chemoreceptors. When Janolus is about to be caught, it rolls into a ball, leaving its cerata exposed.


Species in the genus Janolus include:[2]

Species brought into synonymy
  • Janolus costacubensis Ortea & Espinosa, 2000: synonym of Janolus comis Er. Marcus, 1955
  • Janolus nakaza (Gosliner, 1981): synonym of Bonisa nakaza Gosliner, 1981


  1. ^ BERGH, L. S. R. 1884a. Report on the Nudibranchiata dredged by H.M.S. Challenger during the years 1873-1876. Report of the Scientific Results of the Voyage of H.M.S. Challenger during the years 1873-76, under the command of Captain George S. Nares, R.N., F.R.S. and Captain Frank Tourle Thomson, R.N. prepared under the superintendence of the late Sir C. Wyville Thomson, Knt., F.R.S., &c. Regius Professor of Natural History in the University of Edinburgh Director of the Civilian Scientific Staff on board, and now of John Murray one of the naturalists of the expedition, Zoology 10(26):1-154, pls. 1-14.
  2. ^ a b c Bouchet, P.; Gofas, S. (2014). Janolus Bergh, 1884. Accessed through: World Register of Marine Species at on 2014-11-19
  3. ^ Goddard, J.H.R., 2003 (February 1) Janolus fuscus O'Donoghue, 1924. [In] Sea Slug Forum. Australian Museum, Sydney.
  • Vaught, K.C. (1989). A classification of the living Mollusca. American Malacologists: Melbourne, FL (USA). ISBN 0-915826-22-4. XII, 195 pp.


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