Stylochoplana inquilina
photos: Dave Leonhardt. Inside broken hermit crab's shell, Kihei, Maui. 40 ft.

Dave writes:
Found in Kihei inside a broken shell of a jewel eyed anemone hermit crab, about 40 ft down and it was roughly 25mm. It would shrivel up, spin in a circle, and then lengthen itself back out and travel from one side of the shell to the other and repeat....
The crab was nowhere to be found. The shell was broken into pieces and the worm was crawling around on the biggest piece. The anemones on it were still alive so it was most likely a recent kill from whatever eats anemone hermits. The inside of the shell was sparkly clean too.

This acotylean flatworm is commensal with, or perhaps a parasite of, the Jeweled Anemone Crab (Dardanus gemmatus) and its associated anemone Calliactis polypus. The worm's colors resemble those of the anemone's column, which is pale with brown streaks. The subject of Dave's photos was probably left stranded after something ate its hermit crab host. The worm was first formally described from Hawaiian specimens by Libbie Hyman of the American Museum of Natural History in 1950. Her short and very readable paper is available online:

http://hbs.bishopmuseum.org/pubs-online/pdf/op20-4.pdf

The hermit and its associated anemone have wide Indo-Pacific distributions, but as far as I know the flatworm has been recorded only from Hawaii. It could well occur elsewhere as well -- pehaps no one has looked.

Worms in the genus Stylochoplana are often commensal with larger organisms, typically molluscs. The tissues of one New Zealand species that associates with the sea slug Pleurobranchaea maculata are loaded with tetrodotoxin (TTX) the same deadly poison found in pufferfish.

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