Phylliroe is a genus of average sized (up to 5.5 centimetres or 2.2 inches), highly transparent pelagic nudibranchs, marine gastropod molluscs in the order Opisthobranchia, that consists of two known species.

[2] It is notable for being an open-ocean hunter that resembles a fish in body plan and locomotion, an example of convergent evolution.

[2] Juveniles of P. bucephalum are found exclusively feeding on the bell of the hydromedusa Zanclea costata (this junior synonym was given precedence over Mnestra parasites).

When the slug is as big as the meduse, it starts to swim, eating the remaining parts, including the tentacles.

[2] Phylliroe has been seen approaching a swarm of the larvacean Oikopleura albicans from below, grabbing a specimen with its paired denticulate jaws and swallowing it in half a minute.