Sea urchins are marine invertebrates belonging to the phylum Echinodermata, which also includes sea stars and sea cucumbers. These globe-shaped animals are encased in a hard, spiny shell called a test. Unlike fish, sea urchins lack complex respiratory organs such as gills or lungs. Their respiration relies on a system of simple, distributed structures that interact directly with the surrounding seawater.
Specialized Structures for Oxygen Intake
The primary surfaces for gas exchange are the hundreds of tube feet, or podia, which extend through pores in the calcareous test. These thin-walled, cylindrical projections are distributed across the entire body surface. They function in locomotion, feeding, and respiration, creating an enormous surface area necessary for efficient oxygen uptake.
The podia are continuously extended and waved between the spines, ensuring fresh, oxygenated water constantly flows over their delicate membranes. This movement maximizes the concentration difference between the dissolved oxygen in the seawater and the internal fluids. Five pairs of branched, sac-like structures called peristomial gills are located around the mouth on the underside of the body. While these gills are not the main respiratory organs, they play a secondary role in gas exchange.
The Passive Process of Gas Exchange
The physical mechanism driving oxygen acquisition is simple diffusion, a passive process that does not require the sea urchin to expend energy. Diffusion occurs when molecules move naturally from an area of higher concentration to an area of lower concentration. Dissolved oxygen moves from the surrounding seawater, where the concentration is high, across the thin membrane of the tube feet into the animal’s internal fluid.
This simple transport method is sufficient because sea urchins have a relatively low metabolic rate. The thin epidermis and large surface area of the tube feet minimize the distance oxygen must travel to enter the body cavity. Constant water flow over the respiratory surfaces is necessary to continually replenish the oxygen supply and carry away the waste product, carbon dioxide, thereby maintaining the necessary concentration gradient for diffusion to occur.
Internal Oxygen Circulation and Delivery
Once oxygen diffuses into the body, it must be distributed to all internal tissues and organs without a centralized heart or complex blood vessels. The primary transport medium is the coelomic fluid, which fills the large internal body cavity (coelom). This clear fluid bathes the internal organs and carries dissolved oxygen molecules throughout the organism.
Suspended within the coelomic fluid are mobile cells called coelomocytes, including red spherule cells. These cells contain the dark-red pigment echinochrome A. While this pigment is often mistaken for a respiratory carrier, its primary function is as a potent antioxidant and a component of the immune system, not as an oxygen-binding molecule like hemoglobin. Therefore, most oxygen is simply dissolved in the coelomic fluid for transport.
The water vascular system, which operates the tube feet, plays an indirect role in circulating the oxygenated coelomic fluid. Fluid moves from the specialized ampullae at the base of the tube feet, where gas exchange occurs, into the main body cavity. The rhythmic contractions of the body wall muscles and the movement of the feeding apparatus (Aristotle’s lantern) also help to stir and circulate the coelomic fluid. These internal movements ensure that oxygenated fluid reaches distant tissues and that carbon dioxide-rich fluid returns to the exchange surfaces for expulsion.