How do sea stars breathe？
How do sea stars breathe？
On their dorsal surface, sea stars have fleshy, tube-shaped protrusions, which are called papulae. Papulae appear between bony plates. Some sea star species have a single papula. In some species, the papulae appear in clusters, while in other species the papulae are branched. The main functions of papulae are respiration and gas exchange. They are connected to the water vascular system.
【Extended Reading: How do sea stars carry out gas exchange and excretion of waste?】
Sea stars do not possess lungs, gills or other respiratory organs. Gas exchange and excretion of nitrogen waste are carried out directly through the body surface. Sea stars excrete ammonia (NH3) from areas of the body wall that are relatively thin. Excretion is mostly through the process of diffusion via the papulae and tube feet. Phagocytic cells in the body cavity called coelomocytes engulf metabolic waste. When these cells become filled with waste they make use of the body fluid for transportation to the papulae. There they accumulate at the tips of the papulae and tube feet. Once the papulae contract, the coelomocytes excrete the metabolic waste outside the body. Other coelomocytes containing metabolic waste can excrete this waste through the suckers, tube feet and epithelial cells. Metabolic waste that is produced near the digestive tract can be directly excreted from the digestive tract along with undigested food.
Papulae and tube feet are the important sites for gas exchange. The oscillation of cilia in cells that line the body cavity allows for flow of body fluid. In addition, the cilia on the external epithelium guide water flow to increase the speed of gas exchange. In sea stars that inhabit sandy surfaces, such as Luidia quinaria and the maple starfish, the papulae are branched to increase the surface area for gas exchange. The papulae are protected by small columns. Sea water flows through the cracks between these columns.