This species of nudibranch is found in shallow and subtidal waters.
The bodies of nudibranchs in this species are semi-translucent. The body is covered in short cerata. In Janolus fuscus the cerata are orange and white tipped.
Janolus species feed on Bryozoa.
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. If there is a water current, as if often the case, the sea slug may then be passively rolled away from the predator.