Starfish, correctly known as sea stars, are fascinating marine invertebrates belonging to the phylum Echinodermata. These organisms are obligate marine animals, meaning their entire life cycle relies on the surrounding seawater environment. The question of whether a sea star can breathe out of water arises frequently because they are sometimes seen temporarily exposed on beaches or tide pools. They do not possess the necessary organs to process oxygen directly from the air. Their unique method of gas exchange is entirely dependent on being fully submerged, which places strict limits on their survival when exposed to the atmosphere.
Aquatic Respiration: How Starfish “Breathe” Underwater
Sea stars do not have lungs or gills like fish, but rely on diffusion for gas exchange. Their primary respiratory structures are extensions called dermal branchiae, or papulae, which protrude through the body wall on their upper surface. These thin-walled, hollow projections are bathed in seawater and contain extensions of the sea star’s internal fluid-filled cavity. Oxygen dissolved in the water diffuses directly across the papulae walls into the internal fluid, while carbon dioxide waste diffuses outward.
The movement of water over the surface of the papulae is maintained by cilia, tiny hair-like structures that constantly sweep the water. This continuous flow ensures a fresh supply of oxygenated water and prevents a buildup of carbon dioxide. Additionally, the thousands of tube feet located on the sea star’s underside contribute a measurable amount of gas exchange. These tube feet, also used for locomotion and feeding, have thin walls that allow for supplemental oxygen uptake via diffusion.
Starfish and Air Exposure: The Survival Window
While sea stars cannot breathe air, many species found in intertidal zones can survive a limited period of air exposure between high and low tides. This survival window is highly variable, depending on the species, size, and local environmental conditions. A small, delicate sea star might only withstand being out of water for a few minutes before suffering serious damage.
Robust intertidal species, such as the Ochre Sea Star (Pisaster ochraceus), have been observed to survive out of the water for up to 28 hours under optimal conditions. These resilient individuals typically use behavioral adaptations, such as seeking shade or attaching firmly to rocks to minimize surface area exposure. Even for these hardier species, exposure to air is stressful and can lead to severe harm if prolonged. The primary factors reducing their survival time are high temperatures, direct sunlight, and low humidity, which accelerate water loss.
The Critical Threat: Desiccation and Oxygen Deprivation
The two main physiological threats that compromise a sea star exposed to air are desiccation and the failure of oxygen transfer. The dermal branchiae are not built to withstand a terrestrial environment. Their thin, moist walls, maximized for aquatic diffusion, rapidly lose water when exposed to air. This water loss quickly leads to desiccation, causing the delicate tissues to dry out and cease functioning.
The respiratory structures collapse without the buoyancy and support of the surrounding water. This structural failure prevents surface area contact for effective gas exchange. Consequently, the sea star cannot absorb atmospheric oxygen, leading to internal oxygen deprivation (hypoxia) and a buildup of carbon dioxide. This dual mechanism of drying out and respiratory collapse ultimately causes the death of a sea star removed from its aquatic home.