How to Distinguish from Similar Species: In Puncturella and Fissurella species the dorsal hole is an elongated slit. Megathura crenulata (the giant keyhole limpet) lives farther south, grows much larger, and the mantle covers much to all of the shell when the animal is alive.
Geographical Range: Afognak Island, Alaska to Camalu, Baja California.
Depth Range: Low intertidal to subtidal
Habitat: Common in rocky areas all along the coast
Biology/Natural History: Species is omnivorius but prefers encrusting bryozoans. Extends its mantle extremely when it encounters a seastar predator (such as Pycnopidia, Leptasterias, Pisaster, or Orthasterias) so that the shell is largely covered and the seastar has no place to grab onto the shell. Often contains a symbiotic polychaete worm Arctonoe vittata (picture) in its mantle cavity which may bite the seastar as well. Diodora's blood contains hemocyanin, has a low pH (7.1) and no Bohr effect.
|This Diodora aspera in the two photos above was found at Kalaloch Beach #4 in July 2019. It has an unusually flat shell with a partly concave profile. It appears that the shell is so flat that the animal cannot fit inside its shell and instead extends far below it even when disturbed. What caused it to grow this way is unclear. Photo by Kayla Nash, July 2019|
This keyhole limpet, found intertidally at Cape Flattery, has its shell thickly overgrown by a sponge so the animal cannot be seen at all except from underneath. The limpet's foot and the edge of the shell are exposed on the underside. I did not check to see whether all these holes in the sponge were oscula or whether there was still a passage leading down to the limpet's vent. If not, how does this limpet excrete wastes? I am not certain whether it has a passageway down to the head region. Photo by Dave Cowles, July 2019