Sea urchins are marine invertebrates, recognized by their spherical, spiny bodies. These creatures living on the ocean floor require oxygen for survival. Their adaptations for obtaining this oxygen from their aquatic environment are unique among marine animals.
Main Structures for Breathing
Sea urchins acquire oxygen primarily through specialized external structures. Most sea urchins possess five pairs of thin-walled, gill-like structures, known as peristomial gills, which are located around their mouth on the underside of their body. These delicate projections of the body cavity act as primary sites for gas exchange in species that have them.
Beyond the peristomial gills, the numerous tube feet that cover a sea urchin’s body also play a significant role in respiration. These flexible, hydraulically operated appendages protrude through pores in the urchin’s rigid outer shell, or test. The tube feet have thin walls and a large surface area, making them suitable for the diffusion of gases. In some species, such as heart urchins and sand dollars, which lack gills, the tube feet are the main sites for gas exchange.
The Water Vascular System and Gas Exchange
The water vascular system is integral to how sea urchins obtain oxygen, as it facilitates water circulation and supports the function of the tube feet. This hydraulic system, unique to echinoderms, involves a network of canals that connect to the many tube feet. Water enters this system through a sieve-like structure called the madreporite, located on the urchin’s upper surface.
From the madreporite, water flows through internal canals before reaching radial canals that extend throughout the body. These radial canals connect to the ampullae at the base of each tube foot. When muscles contract, water is forced into the tube feet, causing them to extend, which also helps circulate water for gas exchange.
Gas exchange occurs through diffusion across the thin membranes of both the tube feet and the peristomial gills. Dissolved oxygen from the surrounding seawater passes into the urchin’s coelomic fluid, which fills its body cavity. Simultaneously, carbon dioxide, a waste product, diffuses from the coelomic fluid out into the water. The continuous flow of water maintained by the water vascular system ensures a fresh supply of oxygenated water, sustaining the concentration gradient necessary for efficient diffusion.
An Unusual Breathing Adaptation
In addition to their primary respiratory structures, some sea urchins possess a unique adaptation for oxygen uptake during specific environmental conditions. When exposed to air, such as during low tide, the purple sea urchin, Strongylocentrotus purpuratus, can use its intestine as a facultative lung. This process involves the urchin releasing an “emersion fluid” from its esophagus, allowing air to enter its test and fill the intestine. The intestine, now filled with air, functions as a temporary lung, enabling the absorption of oxygen directly into the coelomic fluid. This adaptation helps maintain oxygen levels in the urchin’s internal fluids during periods of air exposure.