Periodically, and usually shortly after intense storm events, beach visitors may observe an odd sight along the wrack line: strange-looking objects that look somewhat like a football cut in half. These robust, leathery clumps are most likely gumboot chitons (Cryptochiton stelleri), the largest chiton species in the world. When alive, chitons are usually found on subtidal rocky reefs, where these large herbivores graze on seaweed growing attached to the rocky reef.
During intense storm events, when wave action and surge are particularly strong, these mild-mannered invertebrates are swept off the reef, tumble along the seafloor, and are deposited upon the beach. In some cases they are alive, and can survive if they can return to the ocean, but in many cases they are either beached already dead, or succumb to their injuries shortly after being stranded in the wrack line, and unable to survive exposed to air.
These deposition events occur infrequently, but can cause quite a site to those walking the beach. Eventually the carcasses either desiccate into a curved semi-sphere or decompose, leaving the 8 white, calcareous plates behind, each looking like the folded wings of a moth or butterfly.
Gumboot stranding events in the past have been recorded on Jellywatch.
The SIMoN Species Database has more information about the gumboot chiton.
Stranding events: Around 20 gumboot chitons washed ashore at Twin Lakes State Beach on January 27, 2023, all of which were around 6 by 4 inches.