A sea cucumber uses respiratory trees to extract oxygen from water pumped through its anus.
Sea cucumbers are echinoderms, relatives of sea stars and sea urchins. At the rear of the body, water is drawn into a branching pair of internal respiratory trees. Oxygen diffuses across their thin tissues, and the oxygenated water is then expelled. This unusual arrangement also serves as part of the animal’s water-vascular physiology.
Most sea cucumbers live on or in the seafloor, where they consume sediment and organic particles. By processing sediment, they recycle nutrients and help alter the chemistry and structure of seabed habitats. Some species are important commercially, especially in parts of the Indo-Pacific.
Their respiratory system is often described casually as breathing through the anus, but the respiratory trees—not the opening itself—perform the gas exchange. Sea cucumbers can also defend themselves by releasing sticky tubes or, in some species, internal organs that may later regenerate.