Sea butterflies’ shells determine how the snails swim

Sea butterflies flit through the ocean on gossamer wings, each species with a style of its own.

These tiny marine snails, or thecosomes, migrate up to surface waters at night to feed and sink to deeper waters during the day to hide from predators. But exactly how they move through the water has remained a mystery, as these delicate creatures survive only a couple days in captivity.

New videos of sea butterflies in an aquarium reveal that snail species swim and sink at different angles and speeds, depending on the sizes and shapes of their shells, researchers report online September 7 in Frontiers in Marine Science. The finding could help biologists better understand marine snail migration, and inspire new designs for underwater robots (SN: 1/3/18).

Mechanical engineer David Murphy of the University of South Florida in Tampa and colleagues videotaped seven species of sea butterflies collected off Bermuda. The catch included two tiny species with coiled shells about 1 millimeter across, four midsize species with long, conical or urn-shaped shells of about 7 to 11 millimeters, and one species with a flat shell up to 14 millimeters across.

“It’s pretty remarkable how graceful the motion of their wings is,” Murphy says. “Just really beautiful to watch.”

Sea butterflies are marine snails that propel themselves through water with a pair of winglike appendages. “They come in this dazzling array of sizes and shapes,” says David Murphy, a mechanical engineer at the University of South Florida in Tampa. New videos taken by Murphy and colleagues reveal the unique swimming styles of sea butterflies with different shell shapes.

All the sea butterfly species propelled themselves along zigzagging paths as they flapped their wings. Those wingbeats also caused bodies of the tiny, coiled shell species and the midsize, long shell snails to rock back and forth as they swam.

Snails with coiled or elongated shells tended to swim straight up, and to sink straight down whenever they stopped flapping, their shells hanging like pendulums beneath their wings. But the species with a wide, flat shell, Diacria trispinosa, climbed at a shallower angle and drifted sideways as it sank, potentially due to lift generated by its shell.

Bigger, stronger snails also swam faster — with D. trispinosa cruising at an average 84 millimeters per second, while the tiniest coiled shell snails, Heliconoides inflatus and Limacina bulimoides, averaged about 27 millimeters per second. But for coiled shell critters only about a millimeter long, that’s still a pretty impressive pace.

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