Eurylepta leoparda, a cotylean polyclad flatworm, rolls up into a tube and enters C. willmeriana through the oral aperture. It unrolls itself and ingests the branchial basket. It then either leaves the individual through a siphon or continues to feed on the remainder of the internal organs. Other flatworms and polychaetes have been found to eat C. willmeriana. Very young C. willmeriana are frequently overgrown during the winter by colonial ascidian Diplosoma madconaldi. Diatom blooms in springtime sometimes smother adult C. willmeriana.
