So what can marine animals actually hear? Seals are among the first to have their ears tested. They have developed different hearing mechanisms for land and sea and hear well in both environments. For example, seals have erectile tissue in their inner ear, which swells up with blood when they are underwater. “It’s like the penis of a man,” says Ron Kastelein at the Sea Mammal Research Company in Harderwijk, the Netherlands, who did the hearing tests. The blood in the engorged tissue helps conduct sound waves to the inner ear, allowing seals to hear a slightly greater range of frequencies in water than on land.
From New Scientist, 11 April 2015