One-tooth Simnias, Simnialena uniplicata (G. B. Sowerby II, 1849), are elongated marine gastropods that may reach 20 mm (about 4/5 inch). They live exclusively on the “branches” of Sea Whips, which are colonial organisms related to corals. One-tooth Simnias feed on the mucus of and materials entrapped by Sea Whips, apparently without damaging the Sea-whip polyps. They spend most of their lives on the hosts (except for the time they spend in the plankton as larvae), and may be yellow, white, red or rosy-brown, with their shells taking the color of the sea whips, which they achieve by ingesting pigment from the hosts. One-tooth Simnias also deposit their egg masses onto the Sea Whip branches.
The One-tooth Simnia, Simnialena uniplicata. From left, shell photos by José H. Leal, live One-tooth Simnia on “branches” of the Sea Whip Leptogorgia virgulata by Amy Tripp.