The giant keyhole limpet, Megathura crenulata, is the largest of all species of limpets found along the coast of California and south along the coast of Baja California. Limpets are snail-like animals with flattened, cap-like shells, rather than the spiral shells characteristic of true snails. The family of limpets to which it belongs all have an opening of some sort near the apex of the shell that serves as the exit for water that has passed over the gills, and in the case of the giant keyhole limpet, an exit for feces. For the giant keyhole limpet, this opening accounts for the perception by many people that it looks like an eyeball when viewed in an aquarium, or a touch tank.
While I've described the shell of the giant keyhole limpet, you can't actually see much of it in today's photo. The only trace of it is the thin reddish-brown crescent below the hole in the back. This is because for much of the time, the giant keyhole limpet extends its fleshy mantle out over the shell. Among animals known as mollusks, which includes snails and limpets, the mantle is body wall on the back of the animal and protects the internal organs. In many mollusks the mantle also plays a role in secreting the shell material.
The giant keyhole limpet's mantle is extremely smooth, and there is some thought that this may be a defense mechanism. Predatory sea stars found on the same reefs as these limpets are voracious, and not particularly selective in what they eat. It may be that the mantle is so smooth that sea stars can't effectively grab it with their suction cup-like tube feet. I'm not aware that this has ever been shown to be the case in a scientific publication, so it may be nothing more than an apocryphal story.
But there is one feature of the giant keyhole limpet the importance of which is firmly grounded in scientific research. Hemocyanin from this species is used as a carrier for vacines and drugs used to treat certain kinds of cancers. Hemocyanin is analogous to the hemoglobin found in our blood, and the blood of all mammals. Hemocyanin is a copper-based molecule, while hemoglobin is iron-based. The main purpose of each molecule is to carry oxygen from the gills or lungs to the cells of the animal.
While giant keyhole limpet hemocyanin is of great importance to human beings that makes them a target for harvest by us. There is some evidence that the removal of these animals from the ocean has increased to the point that there has been a noticeable decline of the species in many areas.
This photo was taken with a Canon EF100 mm f/4 lens on a Canon EOS 10D in an Ikelite underwater housing. Lighting was provided by two Ikelite DS-125 Substrobes in eTTL mode. The exposure was set to 1/60 sec at f/4.5 and ISO 100.
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