How do sea cucumbers breathe using respiratory trees?

Sea cucumbers (class Holothuroidea) utilize specialized structures called respiratory trees to facilitate gas exchange. These trees consist of a pair of branched, hollow outpouchings that extend from the cloaca (hindgut) into the coelomic cavity.12

The breathing process functions through the following mechanism:

• Ventilation: The muscular cloaca acts as a pump, rhythmically drawing seawater into and out of the respiratory tree system.12
• Gas Exchange: As water circulates through the branched tubules, oxygen exchange occurs. In the left respiratory tree, the branches are closely associated with the intestinal hemal sinuses, allowing for the oxygenation of blood via the coelomic fluid. The right respiratory tree remains free within the coelomic fluid and does not have a direct association with the hemal system.2

While sea cucumbers also perform some respiration through their oral tentacles, the respiratory trees are considered the primary site for gas exchange in most members of this class.12