This animal is not the most agile swimmer in the sea. It’s called Nautilus, and it is closely related to cuttlefish and snails. But it swims backwards and often bumps into things.
Thankfully, it has a thick shell, and can retreat into it to avoid predators. And it also dives down deep into the sea during the day when it’s not feeding. The longer it spends inside its shell, though, the harder it is to get oxygen, and levels can get dangerously low if it has to wait for a long time without any new water flowing by. While we might try to hold our breath at those depths, Nautilus holds its heart. Nautilus slows its metabolism down, and it can hold its blood in its enlarged vena cava, spacing out its heart beat to once every one or two minutes.
Sea turtles, also fantastic divers, have a similar mechanism of energy conservation. The deeper they go, and colder the waters get around them, the slower their heart beats, going down to two or three times a minute and slowing their energy use to one tenth of what they would normally use on the shore.
Photographs graciously provided by Adrian Reich of the Wessel lab at Brown University. Thanks to Brad Seibel, our favorite mollusc exercize physiologist, for his help fact-checking. More about Nautilus metabolism can be found here.