To avoid being heard by sharks, marine iguanas can stop their own hearts for up to 45 minutes.
Wait, so sharks can hear heartbeats?
Their senses are insane. They have an organ that can detect electric fields created by muscle movement. It’s so sensitive that it can even detect the electric fields from paralyzed prey. Something like 1 millionth of a volt.
Fun fact: you know that smell you get after a bit of light rain, that really earthy smell? It is called petrichor and one of the components is a product given off by a bacteria that dies when it gets wet. That is called geosmin. Humans can detect geosmin at concentrations 200,000 times more dilute than a shark can sense blood in the water (1 part in a million for sharks detecting blood, 1 in 200 trillion for humans detecting geosmin)