"Keyhole Limpets"

This post accompanies my previous one about “Shells with a Selenizone.” This new post, however, focuses on gastropod shells with only a single natural perforation (hole), rather than a spiral band of them.

Gastropod shells with only a single natural hole belong to family Fissurellidae (common name of this family is “keyhole limpets"). They have flattened, oval shells with an excurrent opening on their dorsal (top) surface. Their early stages have a spiral shell, but as the shell grows, the spiral shell is lost. Three examples of fissurellids are shown below. All three live attached to rocky surfaces, just like their selenizone-bearing relatives, the abalones.

 Megathura crenulata Sowerby. This shell is 7.5 cm long, but some individuals can secrete shells up to 15 cm long. During life, the shell is covered mostly by a fleshy mantle. The shell is radially striated, with a large, nearly central perforation. This gastropod lives in the low tide to subtidal depths on rock surfaces in the cool-coastal waters from Monterey Bay, California to central Baja California. The animal attaches itself to rocks. The common name of this gastropod is the “Giant Keyhole Limpet.”

Diodora aspersa. This shell is 5.5 cm long and is from the intertidal zone on the Olympic Peninsula, Washington State. The common name of this shell is “Rough Keyhole Limpet.” The shell is heavy, elevated, and has rough, coarse, radial ribs. This species ranges from Alaska to northern Baja California, Mexico. It is most abundant in the northern part of its range, where individuals can reach a size of 7 cm.

Fisssurella nodosa. This shell is 2.3 cm long and is from the West Indies. It is like other Fissurella shells in having an elevated conical shell, but the fleshy animal is unable to retract within the shell. Its natural opening in the shell is definitely keyhole-shaped. Fissurella gastropods are intertidal species that live primarily only in tropical waters. There is only one species found in the cool waters of California.