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Marine Fossils and their Living Relatives

Fossil: Keyhole limpet 
Diodora species

Limpets are snails with a cap-shaped shell, instead of a coiled shell. Intertidal limpets are very rarely preserved in the fossil record because their shells get broken up by the pounding waves. However, whole specimens are occasionally washed into deeper water, often in small underwater landslides, and are preserved in the muds. This limpet, an unnamed Diodora species, comes from the outer coast of the Olympic Peninsula and is about 4 million years old. This is the only keyhole limpet collected from these rocks, and it was found with other rocky intertidal snail shells such as whelks and top shells

keyhole limpet
Fossil keyhole limpet, Diodora species.
Photo by Ron Eng

Live keyhole limpets
Live keyhole limpets.
Photo courtesy of PTMSC

Rough keyhole limpet 
Diodora aspera

The rough keyhole limpet is the only keyhole limpet found on the intertidal beaches of northwestern Washington today. It lives in the low intertidal to subtidal of rocky shorelines. As its name implies, it has a small oval opening at the top of its short, conical shell. Although the keyhole limpet superficially resembles "true" limpets, its soft anatomy reveals an important difference. True limpets draw water into their mantle cavity on the left side, pass it over a single gill and discharge it on the right side. Keyhole limpets draw water in both sides, where it flows over paired gills before flowing out through the "keyhole" aperture at the peak of the shell.

The rough keyhole limpet feeds on algae and tiny encrusting animals that grow on rock surfaces, preferring sponges and bryozoans. When approached by its main predator, a sea star, the limpet responds by raising its mantle up over the outside of its shell, a behavior that may prevent the sea star from gripping the shell surface. The rough keyhole limpet nearly always has a symbiotic polychaete worm, Archonoe villata, living under the edge of its mantle. The worm may help protect the limpet by biting predators that attack it.