It is a common and humorous question whether a starfish, or sea star, actually “pees” like a mammal. The answer is complex because these marine invertebrates, belonging to the phylum Echinodermata, manage waste in a way fundamentally different from vertebrates. Starfish possess a unique biology that relies on their aquatic environment to handle the byproducts of their metabolism. This distinct approach to waste management is a direct result of their radial body plan and their evolutionary history.
The Simple Anatomy of Starfish Waste Management
Starfish do not possess the specialized excretory organs found in more complex animals, such as kidneys, bladders, or a complex urinary tract. Their primitive body organization, characterized by radial symmetry, never developed a centralized system for filtering blood and concentrating waste. This absence of dedicated organs means that the function of “peeing” must be accomplished through a decentralized, passive process.
The coelomic fluid that circulates within their body cavity serves a transport function similar to blood in other animals, but it lacks a dedicated filtration system. Instead of concentrating waste into a liquid form like urine, the starfish’s strategy is to simply remove the waste as soon as it is produced. This direct exchange with the surrounding water eliminates the need for sophisticated storage systems.
Excretion Through Diffusion: The Nitrogenous Waste Solution
The metabolic byproduct that forms urine in vertebrates is nitrogenous waste, and in starfish, this substance is primarily ammonia. Ammonia is highly toxic, but it is also exceptionally soluble in water, which is a major advantage for a marine animal. Starfish utilize a process called diffusion, which is the passive movement of a substance from an area of high concentration to an area of low concentration.
The ammonia rapidly diffuses from the starfish’s internal coelomic fluid directly into the surrounding seawater because the ammonia concentration is lower outside the body. This exchange is maximized by specialized, thin-walled structures on the starfish’s surface. These structures include the tiny, finger-like projections called papulae, sometimes referred to as dermal branchiae or skin gills.
These papulae increase the surface area available for gas exchange and waste elimination, acting as the primary exit points for the ammonia. The thousands of tube feet, which are used for locomotion and feeding, also have thin walls that facilitate this same diffusion process. The body also contains phagocytic cells called coelomocytes, which engulf other waste materials and may migrate to the papulae tips to be ultimately ejected.
Distinguishing Digestive Waste
The process of eliminating metabolic waste, or the equivalent of “peeing,” is distinct from the removal of solid, undigested food waste. Digestive waste, which would be the equivalent of feces, is handled through a separate anatomical pathway. Starfish have a two-part stomach, consisting of a cardiac stomach and a smaller pyloric stomach.
Once digestion is complete, the remaining solid, undigested material travels to a short intestine and rectum. Many starfish species possess a small, often inconspicuous anus located on the aboral (upper) surface of their central disk, which is used to expel this solid waste. However, in some primitive species, the anus is vestigial or entirely absent. In these cases, any remaining undigested material is simply expelled back out through the mouth opening on the underside.