Most starfish do poop, though the process looks nothing like what you would expect from a more familiar animal. Starfish belong to the class Asteroidea, and the majority of species possess a complete digestive tract that ends in a small anus on the top (aboral) surface of their central disc. Solid waste exits through that opening after food has been processed by a surprisingly complex two-stomach system. But not all starfish species even have a functional anus, and the way they handle both solid and dissolved waste involves some biological tricks that set them apart from nearly every other animal group.
How Starfish Digest Their Food
To understand what comes out, it helps to know what goes in and how it gets processed. Starfish are predators and scavengers. Many species feed on bivalves like mussels and clams, and they do it in a way that tends to surprise people: they push one of their two stomachs out of their mouth, insert it into the prey’s shell, and begin digesting the soft tissue externally. This everted organ is called the cardiac stomach, and it essentially turns the prey’s shell into a temporary dining room.
Once the cardiac stomach has liquefied enough of the prey, partially digested material is pulled back inside and passed into the second stomach, called the pyloric stomach. From there, nutrients are distributed into branching digestive glands (pyloric caeca) that extend into each arm. These glands absorb the usable nutrients. Whatever remains, the indigestible bits of shell, sediment, and other debris, continues through a short intestine and eventually reaches the anus on the upper surface of the disc. That is where solid waste leaves the body. The whole process is slow compared to vertebrate digestion, and starfish produce relatively small amounts of fecal matter because so much of the digestion happens externally before food even enters the body.
The External Stomach and Why It Matters for Waste
The fact that starfish digest prey outside their own bodies has a direct consequence for how much waste they actually produce. When a starfish feeds on a mussel, the cardiac stomach secretes enzymes that break down soft tissue while the food is still inside the mussel’s shell. The starfish absorbs a nutrient-rich soup and leaves behind much of the hard, indigestible material in the shell itself, rather than swallowing it. This means their digestive tract handles a relatively clean, pre-processed meal compared to an animal that swallows prey whole.
As a result, starfish fecal output is modest. The solid waste that does pass through tends to be fine particles of calcium carbonate from shell fragments, bits of sediment, and other material that made it past the external digestion stage. You would not notice a starfish’s droppings on the seafloor unless you were looking very carefully, and even then, identifying them requires magnification. The waste emerges from the anus as a small, mucus-bound pellet or strand.
Starfish Species That Lack a Functional Anus
Here is where it gets interesting. Not every starfish species has a working anus. Several groups within the order Paxillosida, including sand stars in the genus Astropecten, have either a vestigial anus or none at all. A morphological study of Astropecten indicus, for instance, found no anus but instead an “involuted cone like structure at disc centre” that does not serve as a waste exit.1Longdom Publishing. New Occurrence of Abnormal Sea Star, Astropecten indicus from Mudasalodai, South East Coast of India For these species, all solid waste must be expelled back out through the mouth. They essentially eat and defecate through the same opening.
This might sound inefficient, but it works well enough for the sand stars’ lifestyle. Astropecten species tend to swallow small prey whole, including tiny snails and bivalves, digest what they can, and then regurgitate the shells and other indigestible remains. The mouth serves double duty as both entrance and exit. It is a fundamentally different waste strategy from the through-gut approach used by most other starfish, and it means that for these species, the answer to “do they poop?” is technically no, at least not from the end you would expect. They vomit their waste instead.
Dissolved Waste and How It Leaves the Body
Solid waste is only part of the picture. Like all animals, starfish also produce dissolved metabolic waste, primarily ammonia, as a byproduct of breaking down proteins. Getting rid of ammonia is crucial because it is toxic at even low concentrations. Starfish do not have kidneys or any dedicated excretory organ. Instead, dissolved waste leaves the body by diffusing across thin-walled surfaces that are in constant contact with seawater.
The two main sites for this are the tube feet and the papulae. Tube feet are the small, flexible projections on the underside of the arms that starfish use for locomotion and gripping surfaces. Papulae (sometimes called skin gills or dermal branchiae) are tiny, finger-like extensions on the upper body surface. Both structures have very thin walls, and because seawater continuously washes over them, ammonia and carbon dioxide diffuse out while oxygen diffuses in.2ScienceDirect (Elsevier / Journal of Experimental Marine Biology and Ecology). Physiological responses of the intertidal starfish Pisaster ochraceus, (Brandt, 1835) to emersion at different temperatures The water vascular system, which powers the tube feet, also circulates internal fluid that carries waste products to these exchange surfaces.
This means starfish are constantly “excreting” in the chemical sense, leaking ammonia and other dissolved waste into the water around them. It is a passive, ongoing process rather than a discrete event. You will never see a starfish excrete in the way you would notice a fish urinating, because the waste leaves molecule by molecule across the entire body surface.
Cellular Housekeeping Inside the Body
Starfish also deal with internal waste at the cellular level. Their body cavities are filled with coelomic fluid, a liquid that bathes the internal organs and serves as a kind of circulatory medium. Floating in that fluid are specialized cells called coelomocytes. These cells function a bit like a primitive immune system combined with a waste disposal crew. They engulf and digest bacteria, dead cells, and other particulate debris through phagocytosis.
Research on the ochre sea star (Pisaster ochraceus) showed that coelomocytes readily phagocytose bacteria in culture, and that their activity increases when stimulated by signaling molecules similar to the immune-signaling proteins found in mammals.3PubMed Central. Stimulation of starfish coelomocytes by interleukin-1 In practical terms, coelomocytes act as roving garbage collectors. They pick up cellular debris, package it, and either digest it internally or transport it to the body surface, where it can be shed. Some coelomocytes migrate through the papulae and are released directly into the water, taking their waste payload with them.
This cellular waste-handling system is important because starfish lack the kind of enclosed blood vessels and filtering organs that vertebrates rely on. Coelomocytes fill the gap. They keep the internal environment clean without the need for a liver or kidneys, which is part of how starfish get by with such a simple body plan.
What Starfish Waste Contributes to the Ecosystem
Starfish waste is not just biological trivia. The ammonia and phosphate that starfish release into surrounding water are inorganic nutrients that other organisms can use. On coral reefs, this matters. A study of common echinoderms on Philippine coral reefs found that the nutrients excreted by echinoderms represent a meaningful pathway for recycling inorganic nitrogen and phosphorus back into the reef community.4Journal of Experimental Marine Biology and Ecology. Ammonium and phosphate excretion in three common echinoderms from Philippine coral reefs The amount varies by species, nutritional status, and even time of day, but the basic dynamic is consistent: starfish and their relatives are quietly fertilizing their environment through metabolic waste.
The solid waste matters too, though in subtler ways. Starfish that feed on mussels, barnacles, and other encrusting organisms help break down calcium carbonate structures. The fine shell fragments that pass through their digestive systems and are excreted (or regurgitated) become part of the sediment budget on the seafloor. In areas where starfish are abundant predators, their feeding and waste production contribute to the turnover of hard substrate, creating space for new organisms to settle.
Crown-of-thorns starfish (Acanthaster planci), which feed on living coral, are a dramatic example. When populations of these large starfish boom on a reef, they consume enormous amounts of coral tissue and produce calcium carbonate waste from the coral skeletons. Their fecal output during outbreaks contributes measurably to reef sediment production. The ecological footprint of starfish waste scales with how many of them are present and what they are eating.
How Brittle Stars Compare
Brittle stars are close relatives of starfish, belonging to the same phylum (Echinodermata), and they are often confused with true sea stars. But their waste management is fundamentally different. Brittle stars have no intestine and no anus at all. Their digestive system consists of a mouth, esophagus, and a large stomach that occupies most of the central disc, and it simply ends there.5bioRxiv. Three-dimensional morphological analysis of the dynamic digestive system in the green brittle star Every bit of solid waste has to come back out through the mouth.
This gives brittle stars more in common with the anus-less sand stars like Astropecten than with most true starfish. In brittle stars, the stomach acts as both the primary digestive organ and the waste-holding chamber. After nutrients are absorbed, indigestible remains are compacted and expelled orally. It is a dead-end gut, and it works because brittle stars tend to eat small food particles, detritus, and tiny organisms that do not produce large amounts of indigestible residue.
Sea urchins and sea cucumbers, the other major echinoderm groups, both have complete through-guts with a mouth at one end and an anus at the other. Sea cucumbers in particular are famous for their prolific waste output. They process huge volumes of sediment, digesting organic matter and excreting clean sand. In that company, starfish fall somewhere in the middle: most species have a complete gut, but some have independently lost the functional anus, and all of them produce relatively modest solid waste because of how thoroughly they pre-digest their food externally.
Microplastics and What Starfish Cannot Digest
One modern concern about starfish digestion involves material that their systems were never designed to encounter: microplastics. Starfish that feed on filter-feeding prey like mussels are now exposed to the plastic particles that accumulate in those prey animals. A recent study examining microplastic transfer from blue mussels to common starfish (Asterias rubens) documented a trophic transfer coefficient of 0.36, meaning that a meaningful fraction of the microplastics present in the mussels ended up in the starfish that ate them.6PubMed Central. From prey to predator: an in-situ observation of microplastic trophic transfer from Mytilus edulis to Asterias rubens
Microplastics are a problem for any animal’s waste system, but they may pose particular challenges for starfish. External digestion does not filter out plastic particles the way it filters out large shell fragments. Tiny plastics suspended in the partially digested slurry get drawn back into the starfish’s body along with everything else. Once inside, some particles may pass through the gut and be excreted normally, but others could accumulate in the pyloric caeca or other tissues. The long-term effects on starfish health are not yet well understood, but the pathway is clear: microplastics enter through contaminated prey and become part of what the starfish’s waste system has to deal with.
This is not unique to starfish, virtually every marine predator now faces some version of this problem, but it adds a new dimension to the question of what starfish expel. Their waste now contains materials that did not exist in the ocean a century ago, and those materials persist in the environment long after they leave the starfish’s body. Whether microplastics alter the ecological role of starfish waste, for instance by changing the nutrient content of their excretions or the composition of their fecal pellets, is a question researchers are only beginning to explore.
When Starfish Are Stranded or Stressed
Intertidal starfish like the ochre sea star (Pisaster ochraceus) face a recurring challenge that directly affects their ability to manage waste: they get stranded by the tide. When the water recedes, a starfish sitting on an exposed rock can no longer rely on seawater flowing over its papulae and tube feet to carry away dissolved ammonia and COâ‚‚. Gas exchange and waste diffusion slow or stop.2ScienceDirect (Elsevier / Journal of Experimental Marine Biology and Ecology). Physiological responses of the intertidal starfish Pisaster ochraceus, (Brandt, 1835) to emersion at different temperatures
During these emersion periods, starfish essentially hold their metabolic waste in. Ammonia builds up in the coelomic fluid. The longer the exposure and the warmer the air temperature, the more stressful this becomes. Starfish can tolerate short periods out of water, but extended or repeated emersion, especially on hot days, pushes their physiology. This is one reason intertidal starfish tend to cluster in shaded crevices and tide pools during low tide: they are not just avoiding desiccation, they are staying near enough water to keep their waste-removal systems functioning.
Temperature stress compounds the problem. Warmer conditions increase a starfish’s metabolic rate, which means more ammonia is produced at the same time that the ability to get rid of it is compromised. As ocean temperatures and air temperatures rise, intertidal starfish may face increasingly frequent periods where waste management becomes a real physiological bottleneck, not because the gut stops working, but because dissolved waste has nowhere to go.