Echinoderms are a diverse phylum of marine invertebrates, including starfish, sea urchins, sea cucumbers, brittle stars, and sea lilies. Recognized by their unique pentaradial symmetry and specialized internal hydraulic system, they exclusively inhabit ocean environments worldwide. Echinoderms do not possess the complex, gill-like structures found in fish or crustaceans. This absence of traditional respiratory organs means they have evolved several alternative methods to achieve the necessary gas exchange.
How Echinoderms Breathe Without Gills
Most echinoderms, including sea stars and many sea urchins, rely on simple, unspecialized structures for oxygen uptake. These are known as dermal branchiae, or papulae, which are small, finger-like projections extending outward from the body wall. They are lined with a very thin layer of tissue.
The papulae are hollow extensions of the coelom, the main body cavity, and are bathed internally by the coelomic fluid. Gas exchange occurs via simple diffusion across the thin surface, moving dissolved oxygen from the seawater directly into the internal fluid. Carbon dioxide waste is simultaneously released into the water through the same process.
The effectiveness of this simple diffusion mechanism is linked to the animal’s low oxygen demand. Echinoderms possess a relatively low metabolic rate compared to many other marine animals. This low rate means they require less oxygen, making diffusion across the widespread skin surface sufficient for their needs.
For sea stars, these papulae are distributed across the aboral (top) surface of the body, maximizing the surface area available for exchange with the water. This reliance on widespread surface area projections, rather than concentrated organs, is an adaptation to their often sedentary existence. The internal coelomic fluid then distributes the absorbed oxygen throughout the organism’s body.
Variations in Respiratory Organs Across Species
While dermal papulae are the general strategy for many classes, other echinoderms have evolved specific internal respiratory structures to suit their distinct body shapes and lifestyles. Sea cucumbers (Holothuroidea), for example, developed a unique system called the respiratory trees. These are a pair of complex, highly branched tubes located within the body cavity, extending from the cloaca near the anus.
The sea cucumber actively pumps water in and out of the cloaca and into the trees, a process known as ventilation. This active movement ensures a continuous supply of oxygenated water contacts the thin walls of the internal structures. The respiratory trees are connected to the coelomic cavity, allowing for efficient oxygen distribution throughout the tissues.
Brittle stars (Ophiuroidea) utilize specialized invaginations called bursae. These are ten small sacs, typically five pairs, located around the central oral disc. Each bursa opens to the exterior through a slit, and water is continuously circulated within these pockets by microscopic hair-like structures called cilia. This ciliary action, sometimes supplemented by muscle contractions, ensures the water inside the bursa is constantly refreshed, facilitating gas exchange. Brittle stars increase their rate of bursal ventilation when oxygen levels in the surrounding water decrease.
Sea urchins (Echinoidea) are capable of using diffusion across their general body surface, but they also possess five pairs of specialized peristomial gills. These small, sac-like outgrowths are positioned around the mouth opening on the oral (bottom) surface. Fluid can be pumped through the gills’ interiors by muscles associated with the jaw apparatus, but this usually occurs only when the animal is low in oxygen. In species like sand dollars and heart urchins, which lack these gills, the tube feet are the primary sites of gas exchange.
The Dual Function of Tube Feet and the Water Vascular System
Distinct from specialized exchange surfaces is the internal physiological support structure known as the water vascular system (WVS), which plays an indirect role in respiration. The WVS is a network of fluid-filled canals that powers the tube feet, the small, movable appendages used for locomotion and grasping. Because the walls of the tube feet are exceptionally thin and constantly exposed to seawater, they serve as effective secondary sites for gas exchange in all echinoderms.
Oxygen diffuses directly into the fluid within the tube feet, where it is distributed into the internal canals of the WVS. The water vascular system helps to circulate this oxygenated fluid throughout the body, acting as a low-pressure hydraulic system. This circulation ensures that the oxygen absorbed through various structures reaches the deeper tissues.