Sea cucumbers have bypassed the standard evolutionary route for respiration by developing a highly specialized respiratory tree that forces them to physically breathe through their own anuses.

A closer look

Sea cucumbers have bypassed the standard evolutionary route for respiration by developing a highly specialized respiratory tree that forces them to physically breathe through their own anuses. They achieve gas exchange by repeatedly drawing water into their cloaca and expelling it, effectively absorbing oxygen from the seawater through the delicate membranes of their hindquarters. Because an open, water-pumping orifice in the ocean is practically an invitation for trouble, certain species of parasitic pearlfish actually use the sea cucumber's breathing hole as a hiding place, swimming right up the animal's digestive tract to seek shelter. Living as a squishy cylinder on the ocean floor is indignity enough, but having to literally inhale through your backside while fending off opportunistic fish trying to treat your rectum like a cheap motel is an evolutionary punishment that borders on the downright cruel