Reaching about 125 mm (about 5 inches), the Giant Keyhole Limpet, Megathura crenulata G.B. Sowerby I, 1825, is one of the behemoths of the limpet universe. A herbivore gastropod, this unusual mollusk stands out not only because of its size, but also because its mantle can extend out, completely enveloping the shell. The Giant Keyhole Limpet’s blood molecule hemocyanin (a molecule that carries oxygen pretty much like hemoglobin does in vertebrates) has the potential for use as a vaccine-carrier protein in the treatment of cancer and other immunologic diseases. The photo was taken in Santa Cruz Island (off South-Central California) by renowned underwater photographer Scott Johnson in February 2019, and used with permission. On his Facebook page, Scott wrote “Here I thought I was just taking a photo of a large keyhole limpet, Megathura crenulata. Only when going through the photos later did I see that the flash also illuminated an octopus eye peeking back at me from the dark hole behind the limpet." Read more about mollusks in our Southwest Florida Shell Guide.