What is branchial plume?
branchial plume meaning a respiratory structure or external gills, usually located on the dorsal side toward the posterior of dorid nudibranchs. This plume surrounds the anus and in some species, it may be retracted. The branchial plume is the major respiratory structure in nudibranchs
The feathery structure toward the posterior of this nudibranch gastropod (marine sea slug) is its respiratory organ.
reference: Coral Reef Information System – Glossary