Sea Cucumbers Breathe Through Their Butt

While most creatures have a dedicated system for respiration, such as gills or lungs, the humble sea cucumber has evolved a more multipurpose anatomy. Its rear opening, or cloaca, does more than just excrete waste; it's the gateway to a pair of complex organs known as respiratory trees. Through muscular contractions, the sea cucumber actively pumps oxygen-rich seawater in and out of its body cavity. This water washes over the intricate, branching surfaces of the respiratory trees, allowing for the gas exchange necessary to sustain life, making its anus a functional equivalent to a lung.

This unique respiratory system makes the sea cucumber's posterior a surprisingly vital and active part of its body. So active, in fact, that some species of pearlfish have adapted to use the sea cucumber's cloaca as a safe home. But this orifice also serves a more dramatic purpose. When threatened by a predator (Review), some sea cucumbers can perform a startling act of self-defense called evisceration, violently expelling their respiratory trees and other internal organs through their rear to entangle and confuse the attacker. Incredibly, they possess the regenerative ability to regrow these lost organs over time.