We at Earth Touch News Network love a good “nudi pic,” but this one deserves a double take. Meet Phylliroe bucephalum — one of the most bizarre nudibranchs we’ve ever seen.

Unlike their brightly-coloured, bottom-dwelling kin, the beautifully transparent Phylliroe nudibranchs have a distinct, laterally flattened tail, much like that of a fish. But why does a sea slug need a fish-like tail? For swimming, of course.

P. bucephalum belongs to a small group of pelagic nudibranchs that spend their days drifting or swimming through the water column. Because they're not spending time on the substrate, slurping up anemones and other cnidarians, they’ve turned to a new method of finding food - parasitism. 

Juvenile Phylliroe are planktonic, that is, tiny drifters that cannot navigate against the current. Drifting can be perilous for a larval sea slug, but not for crafty Phylliroe. The tiny larvae slip inside the bells of passing hydromedusae using their pedal gland, a modification of the muscular foot, to latch on.

Over the next few weeks, Phylliroe drifts with the jelly, feeding on the manubrium, a nutrient-rich tubular structure that connects the jelly’s stomach to its mouth. Not only does the nudi feed on the jelly, but in sucking on the manubrium, it absorbs the nutrients from everything the jelly intends to eat. 

nuddi drawing 1
A) A larval P. bucephala attaches to a hydromedusa. B) the near-adult nudibranch begins to feed on the remaining body parts. Image Redrawn from Martin and Brinkman (1963) by Carol M. Lalli, Ronald W. Gilmer

This little nudi is a real 'grow-getter', observed to increase its size nearly 700 percent in the first ten days of attachment. Once the nudibranch reaches a size larger than its new mobile-home, and is ready to swim on its own, it will consume the tentacles and radial canal of the medusa, and be on its way to feed on plankton, jellies, and other small organisms. 

Adult Phylliroe have the ability to produce light through bioluminescence, which make them quite a spectacle for divers. 

“They are great animals. One of my favourites to see in the wild, and so much personality,” MBARI scientist Steven Haddock told Earth Touch. “They can be as abundant as flies down the Gulf of California. Makes for fun blue-water dives”.

Nudibranchs never cease to amaze ... how this one snuck by us for so long is a mystery!

Ref: Lalli, C.M. & Gilmer, R.W. (1989). Pelagic Snails. The Biology of Holoplanktonic Gastropod Mollusks. Stanford: Stanford University Press. 

Martin, Rainer. and Anita Brinckmann. 1963. Zum brutparasitismus von Phylliroe bucephala Per & Les (Gastropoda, Nudibranchia) auf der Meduse Zanclea costata (Hydrozoa, Anthomedusae). Pubbl. staz. zool. Napoli, vol 33 pp. 206-23.