Species Focus - Pelagic Nudibranch

For a gastropod mollusc, Phylliroe bucephala lives a rare pelagic existence. Most nudibranchs live on the seafloor, be it on reefs or soft sediment, and use their extended foot to move around.

The foot of the pelagic nudibranch however, has been modified so much that it is a vestige that now just surrounds the mouth, and which is used to hang on to the jellyfish upon which it feeds.
Young P. bucephala prey upon a single species of hydromedusa (Zanclea costata) several times its own size, attaching to the bell of the medusa and slowly eating it alive. There is not much inside these animals, with the reproductive and digestive glands being the organs most visible through the transparent body. Once the jellyfish prey is digested, the remains are expelled from an anus located in the middle of the right side of its body.

Also modified are the rhinophores. In nudibranchs these are sensory organs used to ‘taste’ chemicals in the water, be it prey, predators or mates. Living now in more three dimensional space, these rhinophores have developed into long horns, half as long as the body, to better find prey in the inky open ocean.

When seen underwater, the pelagic nudibranch can easily be mistaken for a small fish. The laterally compressed body tapers to an expanded tail, and undulates in a fish-like fashion. Living in a pelagic environment, this functional convergence (fish-like swimming mode) provides rapid locomotion and the ability to hunt jellyfish through the boundless surface waters of the sea.

The pelagic nudibranch is also bioluminescent, one of the very few nudibranchs known to produce its own light. Whether this is an attractant or a means of open water camouflage is not entirely clear, though in combination with its largely transparent body, the latter is more likely. Nonetheless, these nudibranchs are rarely seen, and even more so rarely studied, so there is much yet to learn.

Fast Facts

1. Up to 50mm in length.
   
2. Swims in the water column like a fish.
   
3. Juveniles only feed on one species of jellyfish.
   
4. The white internal lines are the digestive gland, used to liquefy jelly prey.
   
5. Its anus is only halfway along its body.
   
6. They are simultaneous hermaphrodites, meaning both ladies and gents at the same time.
   
7. Have the ability to bioluminesce.
   
8. Circumglobal distribution.