Some animals really do spend most or all of their adult lives attached to one place. These are called sessile animals. Good examples include sea sponges, reef-building corals, adult barnacles, sea squirts, oysters, bryozoans, many hydroids, and tube-building polychaete worms. “Not moving” does not mean nothing in the body moves: many still open tentacles, pump water, retract feeding structures, or contract tissues. In many species, the larval stage can also swim before the adult settles permanently.
Most truly sessile animals live in water because currents can deliver oxygen, food particles, sperm, eggs, and larvae without requiring the adult animal to travel. This lifestyle has evolved independently in several very different animal groups.
8 Animals That Do Not Move at a Glance
| Animal | Adult lifestyle | How it feeds | Can another life stage move? |
|---|---|---|---|
| Sea sponge | Attached to a surface | Filters water through pores | Yes, larvae can swim |
| Reef-building coral | Polyps fixed to a skeleton/substrate | Captures plankton; many also receive energy from symbiotic algae | Yes, planula larvae can swim |
| Barnacle | Permanently cemented after settlement | Extends feathery feeding appendages into the water | Yes, larvae are free-swimming |
| Sea squirt | Adult attached to hard substrate | Filter feeder using siphons | Yes, tadpole-like larvae swim |
| Oyster | Adult cemented to a hard surface | Suspension/filter feeder | Yes, larvae swim before settlement |
| Bryozoan | Usually sessile colonial animal | Zooids extend tentacled lophophores | Yes, larvae disperse |
| Hydroid | Many species form attached colonies | Polyps capture plankton with tentacles | Often; life cycles vary by species |
| Tube-building polychaete | Adult remains in an attached tube | Many filter or suspension feed with a crown of tentacles | Yes, larvae are typically dispersive |
Many animals that appear not to move are sessile adults: they stay attached while feeding structures or internal tissues remain active.
What Does Sessile Mean in Animals?
In zoology, sessile usually means an organism is attached to a substrate and does not use locomotion to travel from place to place during that stage of life.
This is different from being completely motionless. A sessile animal may still:
- move tentacles or feeding appendages
- pump water through the body
- open and close shells or valves
- retract into a tube
- contract tissues
- release eggs, sperm, or larvae
The Smithsonian notes that a large proportion of seafloor life is sessile and includes groups such as sponges, tunicates, bryozoans, and worms. Many sessile marine animals begin life as mobile larvae and become attached only after settlement.
Sea Sponges
Adult sea sponges are among the clearest examples of animals that do not travel from place to place. They attach to rock, reef, shell, or other submerged surfaces and spend adult life filtering water.
Sponges belong to the phylum Porifera. Their bodies contain many small openings through which water enters and larger openings through which it exits. Specialized cells create water currents and capture bacteria, plankton, and organic particles.
Calling a sponge “motionless” is still a simplification. Some sponge tissues can contract, and cells inside the body can change position. What the adult generally lacks is whole-body locomotion across the seafloor.
Sponges also demonstrate an important rule for this entire list: an immobile adult can have a mobile larval stage. Many sponge larvae swim before settling and developing into attached adults.
Reef-Building Corals
Corals are animals, not plants or rocks. Most reef-building corals are made of many tiny animals called polyps. After a coral larva settles, the polyp attaches to the substrate and, in reef-building species, begins producing a calcium-carbonate skeleton.
NOAA describes corals as sessile animals that attach to the seafloor. Their tentacles can move and capture food, but the attached colony does not walk or swim away.
Coral life begins differently. Many species release eggs and sperm into the water, producing small planula larvae that can swim. The larva searches for a suitable place, settles, and changes into the sessile polyp stage.
There are exceptions to the simple “all corals never move” rule. Some solitary corals can be free-living rather than permanently cemented. That is why reef-building corals are a more accurate example than saying every coral is completely immobile.
Adult Barnacles
Barnacles are crustaceans, making them relatives of crabs, lobsters, and shrimp. Their adult lifestyle, however, looks very different.
A young barnacle passes through free-swimming larval stages. When the final larval stage finds a suitable rock, shell, pier, ship hull, or even an animal surface, it settles and permanently attaches itself.
The adult cannot crawl away. Instead, it opens plates in the shell and extends feathery appendages called cirri into the water to collect suspended food.
This makes barnacles an especially useful example of the difference between movement of body parts and locomotion of the entire animal. The feeding appendages move constantly while the adult's base remains fixed.
Sea Squirts
Sea squirts, or ascidians, are tunicates in the phylum Chordata—the same phylum that includes vertebrates. Their life cycle contains one of the most dramatic changes from mobile to sessile life.
The larva resembles a tiny tadpole and swims using a tail containing a notochord. After finding a suitable surface, it attaches and undergoes metamorphosis. The tail and notochord are lost, and the animal becomes a stationary filter-feeding adult.
Adult sea squirts draw water through one siphon, remove suspended food particles, and expel water through another. Smithsonian Ocean describes adult sea squirts as sessile animals attached to the bottom or other hard surfaces.
Some are solitary, while others form colonies of many individual zooids.
Oysters
Oysters are bivalve mollusks. Like several animals on this list, they begin life in the plankton but become attached after settlement.
Smithsonian Ocean explains that young oysters search for a suitable place to settle and then cement themselves to hard material such as rock, existing shells, or other firm surfaces. Once established, an adult oyster does not crawl around like many mobile mollusks.
The animal continues to perform plenty of internal and small-scale movement: it opens and closes its shell and moves water across its gills while feeding. But its overall position remains fixed.
Oysters are a better example of a truly attached adult than mussels. Mussels attach using byssal threads and can sometimes adjust their position, while oysters cement one valve directly to the substrate.
Bryozoans
Bryozoans are tiny colonial animals sometimes called “moss animals.” Most marine bryozoans live as attached colonies on rocks, shells, docks, seaweed, and other hard surfaces.
A colony may look like a crust, branching plant, lace, or gelatinous patch, but it is made from many microscopic individuals called zooids.
Each feeding zooid can extend a crown of tentacles called a lophophore to capture suspended particles. The individual animals therefore move their feeding structures while the colony itself remains attached.
The Smithsonian Tropical Research Institute describes bryozoans as small, sessile filter-feeding invertebrates, most of which are marine and colonial. Their larvae provide the mobile stage that allows new colonies to spread to other surfaces.
Hydroids
Hydroids are members of the cnidarian class Hydrozoa. Many species form branching or encrusting colonies of tiny polyps fixed to rocks, shells, docks, algae, or other underwater surfaces.
The attached polyps use tentacles equipped with stinging cells to capture plankton and other small prey. A colony may sway with water movement, but it does not travel across the habitat like a fish or crab.
Hydrozoan life cycles are diverse. Some species also produce a free-swimming medusa stage, while others have reduced or absent medusae. For that reason, it is more accurate to say that many hydroid polyps are sessile rather than claiming that every hydrozoan is a permanently stationary animal.
Tube-Building Polychaete Worms
Many marine polychaete worms build or occupy tubes attached to hard substrate. Familiar examples include feather duster worms and calcareous tube worms.
The adult animal may remain inside the same tube while extending a crown of tentacles or radioles to collect suspended food and exchange gases. If disturbed, it can rapidly retract the feeding crown into the tube.
So these worms are not motionless internally—their bodies and feeding structures move—but the adult does not normally leave the tube and travel around the seafloor.
Smithsonian fouling-community surveys list tube-building polychaetes among the sessile animals commonly growing on submerged surfaces.
Why Do So Many Non-Moving Animals Live in the Ocean?
Water makes sessile life practical in ways that air usually does not. Currents transport materials directly past an attached animal.
A stationary marine animal may receive:
- oxygen dissolved in water
- plankton and suspended organic particles
- nutrients
- sperm released by other individuals
- larvae and other organisms that become food
Water also carries away wastes and disperses eggs or larvae. An adult therefore does not always need to travel in order to feed, breathe, or reproduce successfully.
Are Sessile Animals Primitive?
No. Sessile should not be confused with “primitive,” “inferior,” or “less evolved.” Every living animal lineage has been evolving for the same amount of geological time since its ancestors diverged from other lineages.
The old version of this page suggested that sessile animals belong to ancient lineages with less complex bodies and linked that directly to evolutionary simplicity. That is too broad. Sponges, corals, barnacles, oysters, tunicates, bryozoans, hydroids, and polychaete worms belong to very different branches of the animal tree.
Sessility is an ecological strategy that evolved in different forms, not a single evolutionary stage that animals “move beyond.”
Do Sessile Animals Have Low Metabolism?
Not necessarily. A stationary lifestyle can reduce the energy required for locomotion, but there is no rule that every sessile animal has a universally low metabolic rate.
Metabolism varies with species, temperature, body size, feeding strategy, oxygen availability, reproduction, and many other factors. Reef-building corals, filter-feeding bivalves, sponges, and colonial bryozoans have very different physiologies.
It is therefore better to explain how each animal obtains food while stationary than to describe all non-moving animals as “low metabolism.”
Do Sessile Animals Reproduce Only by External Fertilization?
No. This is another oversimplification.
Some sessile animals broadcast eggs and sperm into the water, but others brood embryos, reproduce by budding, divide asexually, or combine sexual and asexual reproduction. Bryozoan colonies can grow by budding, many corals reproduce both sexually and asexually, and different tunicates have different reproductive strategies.
The shared challenge is not a single fertilization method. It is how a stationary adult disperses genes or offspring—and mobile larvae are one common solution.
What About Sea Anemones?
Sea anemones are often described as sessile because they usually remain attached by a pedal disc. However, many species are capable of slowly creeping, detaching, floating, or otherwise changing position.
For that reason, anemones are excellent examples of mostly stationary animals, but they are less suitable than oysters or adult barnacles for a list focused on animals that truly do not relocate during normal adult life.
What About Mussels?
Mussels may look permanently fixed, but many attach using byssal threads rather than cementing the shell directly to the substrate. Some can release and remake those threads to adjust their position.
Oysters are therefore the clearer example for this list.
What About Sloths?
Sloths move slowly, but they absolutely do move. They climb, feed, change trees, mate, swim, and descend to the ground. A slow animal is not the same thing as a sessile animal.
The same applies to animals that sleep or rest for long periods. “Animals that do not move” in zoology is most accurately understood as animals whose adult bodies are attached to a substrate rather than simply animals that are inactive.
Do Animals That Do Not Move Feel or Respond to Their Environment?
Yes. Sessile animals can respond to light, touch, water flow, chemicals, predators, sediment, temperature, and other environmental changes.
Barnacles withdraw their feeding appendages when disturbed. Tube worms retract into tubes. Coral polyps can contract. Bivalves close their shells. Sponges can regulate water flow and contract parts of their bodies.
Remaining in one place does not mean being unresponsive.
Frequently Asked Questions
What animals do not move?
Examples of adult sessile animals include sea sponges, many reef-building corals, adult barnacles, sea squirts, oysters, most bryozoans, many hydroids, and many tube-building polychaete worms.
Is there an animal that never moves?
Some adult animals remain permanently attached to a substrate and do not use whole-body locomotion. However, their cells, tissues, tentacles, shells, or feeding structures can still move, and many have mobile larval stages.
What is a sessile animal?
A sessile animal is attached to a surface and remains in one location during a particular life stage, usually adulthood.
Are sponges animals even though they cannot walk or swim?
Yes. Sponges are multicellular animals in the phylum Porifera. Adult sponges are attached filter feeders.
Are corals animals that do not move?
Most reef-building coral polyps become sessile after larval settlement and remain fixed to the reef structure. The living polyps can still move their tentacles and contract.
Can barnacles move?
Barnacle larvae can swim, but after settlement the adult becomes permanently attached. Its feeding appendages continue to move while the body stays fixed.
Can oysters move?
Oyster larvae swim before settlement. Adult oysters usually cement themselves to hard substrate and remain there.
Are sea anemones completely immobile?
No. Although they often remain attached for long periods, many sea anemones can slowly change position or detach under certain conditions.
Why can sessile animals survive without moving?
Water currents bring food and oxygen, remove wastes, and help transport reproductive cells or larvae. Many sessile animals also have specialized filter-feeding or tentacle-based feeding systems.
Official and Scientific Sources
- Smithsonian National Museum of Natural History — Sessile Seafloor Communities
- NOAA — Corals Are Sessile Animals
- NOAA Coral Disease & Health Consortium — Coral Larvae and Settlement
- Smithsonian Ocean — Tunicates and Sea Squirts
- Smithsonian Ocean — Oyster, Mussel and Clam Settlement
- Smithsonian Tropical Research Institute — Bryozoans
- Smithsonian Environmental Research Center — Sessile Fouling Communities
- Smithsonian Q?rius — Sponge Feeding
Final Takeaway
The best examples of animals that do not move are sessile adults such as sponges, reef-building corals, barnacles, sea squirts, oysters, bryozoans, hydroids, and tube-building worms. They are not literally frozen in place: feeding structures, shells, tentacles, tissues, and internal water currents still move. The key difference is that the adult animal remains attached to one location, while water currents and mobile larvae solve many of the problems that moving animals solve by traveling.
