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Dive Dry with Dr. Bill

#591: Keyhole Currency!

Back in the old days (when I was young), I arrived on Catalina and began familiarizing myself with the local marine life. It was certainly different from the boots and old tires we dredged up in Boston Harbor during my Harvard daze. We did do a field trip to the New Hampshire coast back then and saw a few critters that were at least similar to the ones here, but most of my knowledge of marine life came from "book learning" before I arrived on the island. My early years teaching marine biology here were full of new discoveries. Unfortunately classmate Al Gore had yet to fully deploy "his" invention of the Internet so there were few resources available to learn from unless I drove out to the library at USC's Wrigley Marine Science Center. That was a whole day trip in my old Willys M38A1 military jeep!

At the time my tiny private school, the Catalina Island School for Boys, held the permit for archaeological digs here on Catalina. Our archaeologist, Duke Snyder, invited me out on a major dig at the Isthmus to identify the marine critters found in the kitchen middens (trash dumps). The Island Company (SCICo) had given us two weeks to do a salvage dig there in anticipation of the construction of a new hotel at the site. Of course 45 years later I'm still waiting for them to break ground on that hotel! Things moved slower back then...

Outside of tasty abalone, one of the things frequently found in those Native American garbage dumps was a shell that we learned served as a form of money and ornamentation for the early island residents. I took a discarded shell and created a 60s style necklace from it using rawhide. Ah, those carefree days of hippie-dom. The shell resembled those of limpets, which I was familiar with from the East Coast. These other limpets are snails with shells that hardly seem sufficient for protection. They clamp down on the rocks during low tide to prevent the snails from drying out while exposed to the air. Living in the upper tidal zones, they have little to fear from submarine predators although nasty seagulls will occasionally pry them loose ("mine... mine... mine").

However, these shells were much larger than the limpets I was familiar with. I learned that they came from the giant keyhole limpet (Megathura crenulata). This lonely snail is the only member of its genus, and therefore has no close living relatives. Later, as I spent more time underwater, I found and observed these animals alive. One funny thing about them is that the shell is relatively small compared to the snail's body, which usually engulfs all but the central part.

Now I used to think these snails, like many of their distant cousins, were vegans and fed solely on seaweed. However, analysis of the contents of their stomachs by scientists has proven otherwise. Just like me, they are omnivores (although I certainly wouldn't share a meal with them). In addition to red and brown algae, they consume sea grasses, bacteria, hydroids, bryozoa, nematode worms, small bivalves and other snails, crustaceans and tunicates. The dominant stomach contents were red algae and tunicates. It is thought that some of the other invertebrates eaten by them were ingested along with the seaweed they were attached to or lived on. Just goes to show you have to be really careful about who you associate with!

The giant keyhole limpet comes in many flavors. The small one I filmed recently had a black body, but they are also gray, brown or beige and can have a very mottled appearance. Their muscular foot is usually yellowish to orange in color. Maximum size is about 10 inches for the animal itself. These primitive snails are somewhat similar to abalone, but do not have the full shells that those tasty critters use for defense. Like abalone and other snails, they use a scraping mouthpart known as a radula to ingest food. Once processed, the digestive wastes are excreted through the central hole in their shell. This keeps the poop well away from their gills and mouth.

Giant keyhole limpets are found from as far north as Mendocino down into Baja California, although they are most common from southern California south. They are generally seen in relatively shallow water. Although I don't observe them often in our waters, when I do they are usually in rocky reefs and breakwaters such as Casino groyne and the two rock quarries on the island.

Years ago while diving Anacapa Island I filmed a giant keyhole limpet releasing its gametes through the central hole in the shell. Relatively little appears to be known about its reproduction. The sexes are separate and gametes may be present any time of year. One source casually referred to it as a "dribble spawner." I was unaware that a certain by-product of giant keyhole limpet metabolism, hemocyanin, is quite valuable medically in the treatment of bladder cancer. It is also used as a carrier in certain vaccines. Yep, your health may depend on keeping the populations of this snail healthy!

© 2014 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. You can also watch these episodes in iPod format on YouTube through my channel there (drbillbushing). Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

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Giant keyhole limpet "wampum" and living individual with shell largely hidden; young black Megathura
in dive park and yellow-orange foot.

This document maintained by Dr. Bill Bushing.
Material and images © 2014 Star Thrower Educational Multimedia