A tidepool curiosity is the giant keyhole limpet

By Kelly Stewart

As you look down on the rocky shoreline that leads to the water’s edge, it looks like an impossible habitat wherein life could survive. But upon further investigation, you will find there are many creatures living in between the cracks and under rocks clinging on for dear life.

Here in La Jolla, there were extreme low and high tides beginning just before Christmas due to the full moon that coincided with the winter solstice. Many curious visitors and locals alike took advantage of the low tides to poke around the tidepools finding hermit crabs, mussels, small fish and other creatures.

Perhaps a few people were lucky enough to spot a giant keyhole limpet (

Megathura crenulata

). Closely related to the abalone, giant keyhole limpets are a bit different in that their mantle (fleshy and muscular inside parts) completely covers the outside of their shell, with only the “keyhole” in the top of the shell visible.

The mantle may be a variety of colors; some have stripes or other patterns. Giant keyhole limpets hide in cracks and crevices, clinging tightly to the rocks. When they emerge from hiding places, they creep along using their muscular foot. To breathe, they draw water in underneath the shell, pass it over their gills and then expel it through the keyhole. To eat, they use their file-like rasping tongue (the radula) to scrape algae off rocks.

Sometimes called whale’s eyes (see photo), giant keyhole limpets are collected and eaten in some cultures. There are also reports of the shells being used as currency in the early California days.