About Simnia spelta (Linnaeus, 1758)
Simnia spelta reaches a length of approximately 20 mm (0.8 in). Its shell is roughly egg-shaped or cigar-shaped with a flattened base. The shell is strong, thick, smooth, and glossy, with inrolled edges along the sides of the long aperture on its underside. The shell’s colouring varies depending on the species of gorgonian that the individual lives on. The shell’s background colour can be white, yellow, pink, or orange, and it often displays white spots, dark spots, marbling, or streaks that work as camouflage. Like most other Simnia species, the colour of all visible soft parts of Simnia spelta usually matches the colour of the gorgonian the snail grazes on. This happens because pigment from the gorgonian is retained in the mollusk’s mantle.
Simnia spelta was once thought to be endemic to the western Mediterranean Sea and the Adriatic Sea. It is now known to also occur in the eastern Atlantic Ocean, ranging between the Bay of Biscay, the Canary Islands, and Angola. Its typical habitat is living on gorgonians (sea fans) at depths down to about 10 metres (33 ft).
Simnia spelta is a specialist predator that feeds on gorgonians from the genera Eunicella, Paramuricea, and Lophogorgia. It eats the gorgonian’s polyps and coenenchyme, the living connective tissue that surrounds the horny skeleton. It does not graze excessively, so it does not kill its host gorgonian. This species has separate sexes, and fertilisation is internal. Eggs are laid on the branches of the host gorgonian, and they match the naturally warty appearance of the gorgonian stems. The white gorgonian Eunicella singularis is one of the sea fans that this snail associates with. When living on this host, the snail is normally white and mimics the appearance of the gorgonian’s twigs. It feeds and lays its eggs on the gorgonian’s branches. After the snail feeds, epibionts grow on the branches that have been stripped of tissue.
