Mysterious Shells Offer Insight into Life of Seldom Seen Sea ‘Butterfly’
• Planktonic Pteropod ‘Pseudoconches’ Provide Perplexing Puzzle for Point Dume Beachcombers
BY SUZANNE GULDIMANN
All along the shore at Westward and Point Dume this week transparent objects that look like tiny glass ornaments have washed ashore. Easily mistaken for jellyfish or shards of glass at first glance, these delicate shells are made by several species of pteropod, planktonic sea slugs known as sea butterflies.
Sea butterflies are transparent, ethereal and remarkably beautiful. They have evolved from having a single foot like terrestrial slugs and snails to having thin, wing-like lobes that enable them to “fly” through the water, but they are classed as planktonic animals because they are carried by the currents and cannot swim far on their own.
“Weather conditions, currents, can cause them to be thrown ashore,” Cabrillo Marine Aquarium biologist Mike Schaadt told the Malibu Surfside News.
“They’re at the mercy of the major currents,” Schaadt explained. “They’re easily found in the open ocean. They follow the sun, rising near the surface during the day. They can get caught in a wind shift or brought in by a current.
“By the time the pteropod gets to the waves it breaks apart, it’s sort bodied,and just the shell washes up on the shore.”
Schaadt, who is the director of the CMA and whose background includes plankton research, told The News that the triangular pteropod shells probably belong to the genus Clio and that the comma-shaped shells are most likely a species of Corolla.
“They’re fairly common but not known by a lot of people,” Schaadt said. “Corolla is an active swimmer. Its foot has evolved into wings. They’re really beautiful.”
Corolla has what’s called a pseudoconch, a flexible, rubbery shell that can grow to about three inches. Its “wings,” which it flaps to propel itself through the water, can reach a length of six inches.
Clio is smaller and has a harder shell. Both types of sea butterfly feed exclusively on plankton. They are eaten, in turn, by fish—including tuna, and by baleen whales, including the gray whale, according to Schaadt.
Schaadt added that pteropods may be at increased risk of extinction from ocean acidification. He explained that their shells are made of a fragile gelatinous substance. “The acid may dissolve their shells,” he said.
The original study, published in the journal Nature in 2005, concluded that “When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution.
“Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries, as suggested previously.”
While the living animals are seldom seen, near-invisibility protects them from predators and observers, beachgoers may continue to spot the shells of sea butterflies washed in with the tide. Although they resemble jellyfish, pteropods do not sting and are worth a closer look.
Several beachcombers at Westward Beach were examining the pseudoconches with surprise. “I’ve never seen these before,” one beachgoer told The News. “They’re amazing.”
“That's what I like about marine science,” Schaadt said. “There are still surprises, things you’ve never seen.”
Extremely low afternoon tides on Thursday, Friday and Saturday offer an excellent opportunity for naturalists of all ages to explore an ordinarily unseen world.
More information on the historic Cabrillo Aquarium, located in San Pedro, is available online at http://www.cabrillomarineaquarium.org