In 300 B.C.E., Aristotle proposed that the brittle white shells found on female argonaut octopuses functioned as a kind of boat, allowing the creatures to sail on the water surface. More recently, scientists have suggested that the shell is primarily used to incubate the argonaut's eggs, and also to trap air for buoyancy: Unlike most octopuses, argonauts live near the sea surface rather than the seabed, and they need to control their depth to avoid being crushed by waves or eaten by seabirds. (The diminutive males, which are up to 8 times smaller and 600 times lighter than the females, don't need this buoyancy aid.) To test this idea, scientists captured three female argonauts and scuba dived with them after releasing them into a nearby harbor. In a study published online tomorrow in the Proceedings of the Royal Society B, the researchers report that when they inverted an argonaut underwater to allow the air to bubble out from its shell, the creature became negatively buoyant--i.e. it sank if it didn't swim. However, when the argonaut then swam to the surface, it took air into its shell and reclaimed its control of buoyancy as it dived underwater.
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