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  • José H. Leal

Fish or Mollusk?

Phylliroe bucephala, off Palm Beach County, Florida. "Black-water" photo: Susan Mears.

A sea slug that lives in open water, looks and swims like a fish, and glows in the dark? That is Phylliroe bucephala, a transparent, bioluminescent, three inches-long nudibranch gastropod. The sea slug swims in the upper layers of the open ocean, throughout the world’s tropical and subtropical regions.

Phylliroe bucephala, off Palm Beach County, Florida. "Black-water" photo by Linda Ianniello.

This great image above, by black-water diver and photographer Linda Ianniello, shows the sea slug feeding on a small jellyfish, possibly an Aequorea species. (Zoom in, and you can see three amphipod crustaceans within the bell of the jellyfish.) Young Phylliroe bucephala attach to the inside of the bell of jellyfish, which they parasitize before becoming adults. And don’t miss the fantastic video clip by Pedro Valencia that emphasizes the "fish-like" behavior of P. bucephala, at

And many thanks to Linda and Susan for letting me show their photos!


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