Animals that live in water include far more than fish. The list runs from sponges with no organs and sea slugs that run on sunlight to the largest animal ever to exist. Marine invertebrates alone account for about 95% of all species in the oceans, according to Private Scuba’s invertebrate guide.
Aquatic animals live in fresh or salt water for most of their lives. Estimates put the number of species in aquatic habitats around one million, though many remain undiscovered, as EBSCO Research Starters explains. That diversity explains why naturalists, from Aristotle to Victorian aquarium keepers, spent centuries arguing over what counts as a plant, an animal, or a monster.
This list follows a rough path through evolutionary history. It starts with drifting invertebrates and the earliest relatives of backboned animals, then moves through fish and finishes with marine mammals. Each entry includes a bit of history and, where possible, a place in US waters where the animal turns up.
Key Insight: Living in water does not mean breathing water. Aquatic mammals have lungs rather than gills, so they cannot breathe underwater, a point Study.com’s aquatic animals lesson makes clearly.
1. Eastern Emerald Sea Slug
The eastern emerald elysia (Elysia chlorotica) is a leaf-green sea slug about an inch long. It lives in salt marshes and tidal pools along the US East Coast, from Nova Scotia down to Florida. American naturalist Augustus Addison Gould described the species in 1870.
Its fame comes from a trick called kleptoplasty. Young slugs eat the alga Vaucheria litorea, then keep its chloroplasts alive inside their own cells. Fed on sunlight, an adult slug can go for months without another meal.
- Habitat: shallow, brackish marshes and tide pools
- Color: bright green from stolen chloroplasts, with a leaf-like body
- Oddity: it is one of the few animals known to use stolen photosynthetic machinery
2. Portuguese Man O’ War
Despite the look, the Portuguese man o’ war (Physalia physalis) is not a single animal. It is a siphonophore, a floating colony of specialized individuals that cannot survive apart. Carl Linnaeus named it in 1758, and sailors had already linked its gas-filled sail to a Portuguese warship under full canvas.
The float rides the wind while tentacles trail well beyond 30 feet below the surface, stunning small fish with venom. Strong onshore winds push these animals onto Florida and Gulf Coast beaches, where lifeguards often respond with purple warning flags.
Pro Tip: A man o’ war washed onto the sand can still sting. Beachgoers should keep children and dogs away and report the animal to a lifeguard.
3. Buoy Barnacle
The buoy barnacle (Dosima fascicularis) is a crustacean that spends its life adrift in the open ocean. It builds a spongy, gas-filled float from its own secretions and hangs from it in clusters. Those floats let colonies ride currents across entire ocean basins and sometimes land on Pacific and Atlantic beaches after storms.
Barnacles have an odd place in medieval folklore. Europeans of that era saw goose barnacles on driftwood and decided that barnacle geese hatched from them. The belief was convenient, because it let some people classify the birds as fish and eat them during religious fasts.
4. Jellyfish
The moon jelly (Aurelia aurita) is the jellyfish most American beachgoers meet. Its clear, saucer-shaped bell shows four pale horseshoe rings, and it drifts in coastal water along both US coasts. It is a member of the cnidarians, a group whose members have simple, symmetrical bodies with stinging tentacles around the mouth, including jellyfish, coral, and sea anemones, according to Kvarøy Arctic’s ocean grouping.
Jellyfish are among the oldest animals still around, with fossil traces reaching back more than 500 million years. They manage without much of what other animals rely on, since they have no heart, brain, or bones, Vedantu’s aquatic animals page notes.
| Feature | True Jellyfish | Portuguese Man O’ War |
|---|---|---|
| Body Plan | One animal | Colony of linked individuals |
| Movement | Pulses its bell to swim | Wind pushes its sail-like float |
| Typical US Encounter | Bays and estuaries | Florida and Gulf beaches after onshore winds |
5. Salp
Salps (Salpa maxima) look like clear, barrel-shaped jellies, but they are closer to us than to jellyfish. They belong to the tunicates, and tunicates are non-vertebrate members of the phylum Chordata, according to Wikipedia’s marine invertebrates entry. Salps pump water through their hollow bodies, which both propels them and traps tiny algae.
They often link into chains several feet long, then break apart and repeat the cycle. In spring, blooms can turn patches of the open ocean into soup. Their dense fecal pellets sink quickly, which carries carbon toward the sea floor.
Salp is one of many water animals whose names start with the same letter. Readers who like alphabetical browsing can compare it with the full list of animals that start with S.
6. Copepod
A copepod (Calanus finmarchicus) is a crustacean the size of a grain of rice, and it may be the most abundant multicellular animal on Earth. These animals are zooplankton, which is the term for animal plankton. Oceans, lakes, and puddles all host them.
In the Gulf of Maine, Calanus finmarchicus is a staple food for North Atlantic right whales. Copepods store oil-rich fat as they feed in spring, and a whale that filters enough of them gets a rich meal from a tiny animal. Freshwater copepods share ponds with larvae of mosquitoes, dragonflies, and other insects that live in water.
7. Sponge
The giant barrel sponge (Xestospongia muta) grows on Caribbean and Florida Keys reefs, and large specimens can stand taller than a diver. Sponges are in the phylum Porifera. They have no brain, muscles, or organs, and they feed by pulling water through thousands of pores. Fossil chemistry points to sponge-like animals more than 600 million years ago, which makes them contenders for the oldest animal lineage.
People have used natural sponges for bathing and cleaning since Greek and Roman times. Greek immigrants helped build a sponge-diving industry at Tarpon Springs, Florida, in the early 1900s, and the town still celebrates that heritage. As Study.com notes, real sea sponges do not look like square kitchen sponges, but have a more rounded shape.
8. Sea Anemone
The aggregating anemone (Anthopleura elegantissima) carpets rocks in Pacific Coast tide pools from Alaska to Baja California. It is a polyp with a sticky, flower-like crown of tentacles armed with stinging cells. Bits of sand and shell cling to its column, which helps the animal shade itself at low tide.
Single anemones in a patch are often clones, and neighboring clone groups fight with special tentacles along clear borders. Victorian naturalists loved anemones because they could be kept alive in glass tanks. Writer Philip Henry Gosse helped popularize the home aquarium in the 1850s partly through his books on them.
9. Coral
Elkhorn coral (Acropora palmata) builds thick, branching thickets on shallow reefs off Florida and the Caribbean. Each branch is a colony of tiny polyps that secrete limestone skeletons, and corals fall in the same cnidarian group as jellyfish. They also house algae that supply much of their food.
For centuries, scholars assumed corals were plants. French physician Jean-André Peyssonnel argued in the 1720s that they were animals, but the idea took time to gain acceptance. In 2006, federal regulators listed elkhorn and staghorn coral as threatened under the Endangered Species Act, the first corals to receive that protection.
10. Sea Pen
The orange sea pen (Ptilosarcus gurneyi) lives on soft Pacific Coast bottoms, where it anchors in sand with a fleshy base and fans out feathery branches. A sea pen is a colony of polyps with different jobs, and the shape looks just like an old quill pen, which gave the group its name. If disturbed, the colony can deflate and sink back into the sediment.
Several species show up in aquarium and dive references. Monterey Bay Aquarium’s deep-sea list includes the droopy sea pen, and the orange sea pen appears under sea pens in this marine invertebrates directory.
11. Sea Lily
The Caribbean sea lily (Neocrinus decorus) looks like a plant on a stalk but is an echinoderm, a relative of sea stars and urchins. Echinoderms have five-point radial symmetry and live only in marine environments, according to MarineBio’s invertebrate overview. A sea lily’s feathery arms filter drifting food from deep water.
Crinoids, the group that includes sea lilies, have a fossil record starting about 480 million years ago. Their disc-shaped stalk pieces are common in Midwestern limestone, and Iowa named the crinoid its state fossil. Many fossil crinoids are older than the first dinosaurs by hundreds of millions of years.
12. Sea Squirt
The sea vase (Ciona intestinalis) is a soft, translucent sac that sits attached to docks, hulls, and rocks. Water enters through one siphon and exits through another, and the animal squirts a jet when squeezed, which gave sea squirts their name. As adults they look like simple filters, yet they have a hidden heritage.
In the 1860s, Russian zoologist Alexander Kovalevsky saw that a sea squirt larva has a tail with a notochord, the stiff rod that marks the chordates. Darwin took notice, and the finding tied tunicates to the lineage that led to vertebrates. Some invasive tunicates now foul oyster farms and docks along the New England coast.
13. Acorn Worm
The Atlantic acorn worm (Saccoglossus kowalevskii) burrows in mud and sand along the US East Coast. Its body has three parts, and the front one, a short proboscis, resembles an acorn. Acorn worms are hemichordates, a small group related to both echinoderms and chordates.
The larva puzzled early researchers. In the mid-1800s, German physiologist Johannes Müller found one and took it for a sea star larva. Later work revealed gill slits and a nerve cord running along the back, and today biologists at marine labs such as Woods Hole use acorn worms to study how animal body plans evolved.
14. Lancelet
The Florida lancelet (Branchiostoma floridae) is a skinny, fish-shaped animal about two inches long that lives half-buried in clean sand in shallow Gulf and southeastern Atlantic waters. It has no skull, jaws, or backbone. Instead, it keeps a notochord throughout its life, so lancelets are classed with the non-vertebrate chordates, as Wikipedia explains.
Peter Simon Pallas described a lancelet in 1774 and mistook it for a slug. Kovalevsky’s later embryology work moved it close to vertebrates. Today, scientists treat lancelets as a living stand-in for the animal that may have preceded fish, which is why they sequence their genomes.
15. Bobbit Worm
The bobbit worm (Eunice aphroditois) is a marine bristle worm that buries itself in the sea floor with only its antennae showing. When prey passes, it lunges out and snaps it with sharp jaws, fast enough to cut a small fish in half. Bobbit worms can grow several feet long, and live in warm parts of the Atlantic, Pacific, and Indian oceans.
Pallas named the species in 1788. The common name came from a 1993 Virginia court case that dominated US news, which a diver later used as a grim joke about the worm’s bite. Bobbit worms sometimes arrive unnoticed in saltwater aquariums by hiding in live rock.
16. Octopus
The giant Pacific octopus (Enteroctopus dofleini) is the largest octopus species, and it lives in cold waters from California to Alaska, including Puget Sound. It has a soft body, eight arms, and no shell. It can fit through any opening larger than its beak, which is the one hard part of its body.
Octopuses are among the most capable invertebrates. Squid, cuttlefish, and octopuses have relatively large brains and move by jet propulsion, MarineBio says. Their lives are short, though, and their lifespan usually does not exceed five years, according to WorldAtlas. Octopuses appear on Minoan pottery from the Bronze Age Mediterranean, so people have admired them for more than 3,000 years.
17. Sardine
The Pacific sardine (Sardinops sagax) is a silvery schooling fish that feeds on plankton off the US West Coast. Its schools can contain thousands of fish, moving as one to confuse predators like sea lions and seabirds. Sardines are a food source for larger fish, whales, and people.
Sardines built one of California’s best-known industries. Cannery Row in Monterey was crowded with canneries through the 1940s, and John Steinbeck wrote about it in his 1945 novel. The catch then dropped sharply, and fishing limits on this species have been tightened several times since, because sardine numbers swing with ocean conditions.
18. Salmon
Chinook salmon (Oncorhynchus tshawytscha) is the largest Pacific salmon, and it is a cultural anchor for Native peoples of the Pacific Northwest, who have held first-salmon ceremonies for generations. Atlantic salmon (Salmo salar) once ran up many New England rivers, and the last wild US population survives mostly in Maine. Salmon are anadromous, which means they hatch in fresh water, grow up in the sea, and return to spawn.
- Eggs are laid in gravel nests in cold streams.
- Young fish, called alevin and then fry, grow in the stream.
- Smolts head downstream and enter the ocean.
- Adults feed at sea for one to several years.
- They return to the stream where they hatched to spawn.
Most Pacific salmon die after spawning, while some Atlantic salmon survive to spawn again. Salmon also live in the Midwest, since Michigan began stocking Pacific salmon in Lake Michigan in the 1960s. That is why Chicago anglers can chase them, and they appear among the wild animals that live in Chicago.
19. European Eel
The European eel (Anguilla anguilla) is a snake-shaped fish that lives in rivers, lakes, and coastal waters across Europe and North Africa. It is catadromous, the reverse of salmon: adults grow in fresh water, then swim thousands of miles to spawn in the Sargasso Sea. Their see-through, leaf-shaped larvae drift back on ocean currents.
Eels baffled scientists for more than 2,000 years. Aristotle suggested they sprang from mud, and a young Sigmund Freud dissected hundreds of eels in 1876 while searching for their reproductive organs. Danish biologist Johannes Schmidt finally traced the tiniest larvae to the Sargasso Sea in the early 1900s.
The International Union for Conservation of Nature lists the species as critically endangered. Its American cousin, Anguilla rostrata, still climbs Atlantic Coast rivers, including streams on the list of wild animals that live in Connecticut.
20. Whale
The blue whale (Balaenoptera musculus) is the largest animal ever known. It is about as long as two school buses, according to Kvarøy Arctic. Like all whales it is a mammal, so it is warm-blooded, nurses its young, and breathes air through a blowhole.
It is also a filter feeder. Krill form one of the largest animal biomasses on Earth and are a major part of humpback whale diets, per EBSCO. Blue whales feed on krill off California each summer and fall, and they can often be spotted from whale-watching boats out of Monterey and Santa Barbara.
American history is tied to whales. Nineteenth-century ports like New Bedford, Massachusetts, and Nantucket grew rich on whale oil, and Herman Melville’s Moby-Dick came from that world. Congress passed the Marine Mammal Protection Act in 1972, and an international commercial whaling moratorium followed in the 1980s.
21. Dolphin
The common bottlenose dolphin (Tursiops truncatus) is the dolphin most Americans see, from Atlantic beaches to Gulf Coast harbors. Dolphins are social, with pods that cooperate in hunting and care for injured members. Kidzee’s aquatic animals list notes that dolphins use echolocation to communicate and find food.
Aristotle recognized more than 2,300 years ago that dolphins breathe air and nurse their young, which put him ahead of many later writers who lumped them in with fish. The US Navy has trained bottlenose dolphins for mine detection since the early 1960s. They are trained, not domesticated, which is a distinction worth knowing when comparing animals that can’t be domesticated.
Dolphins also rest in an unusual way, shutting down one brain hemisphere at a time so they can keep surfacing to breathe. The pattern is one reason they appear among animals that sleep a lot, even though their sleep looks nothing like a human’s.
22. Porpoise
The harbor porpoise (Phocoena phocoena) is a small, shy cetacean that lives in cool coastal waters of the Gulf of Maine, the Pacific Northwest, and Alaska. Cetaceans include whales, dolphins, and porpoises, but porpoises are a separate family from dolphins. The word comes from medieval Latin for “pig fish.”
| Trait | Dolphin | Porpoise |
|---|---|---|
| Snout | Long beak in most species | Short, blunt face |
| Teeth | Cone-shaped | Spade-shaped |
| Dorsal Fin | Usually curved | Small and triangular |
| Behavior | Playful, often rides bow waves | Shy, rarely approaches boats |
Fishing gear causes more trouble for porpoises than predators do, since they get tangled in nets. Their close relative the vaquita, found only in Mexico’s northern Gulf of California, has been reduced to a tiny remnant by illegal gillnetting for another fish.
23. Dugong
The dugong (Dugong dugon) is a gray, rounded marine mammal that grazes on seagrass in warm coastal waters of the Indian and western Pacific oceans. It belongs to the sirenians, a group that includes manatees and dugongs. The dugong’s forked, whale-like tail separates it from the paddle-tailed manatees found in Florida.
| Feature | Dugong | West Indian Manatee |
|---|---|---|
| Tail | Fluke, like a whale’s | Rounded paddle |
| Range | Indian and western Pacific oceans | Florida, Caribbean, and Gulf of Mexico |
| Water Type | Salt water only | Fresh and salt water |
Sailors who saw sirenians from far away may have helped inspire mermaid legends. Christopher Columbus reported seeing mermaids near Hispaniola in 1493, and they were probably manatees. Another sirenian, Steller’s sea cow, was a cold-water giant found in 1741 and hunted to extinction by 1768.
Dugongs spend their days cropping seagrass, much like animals that eat leaves on land. From drifting sea slugs to grazing dugongs, the animals that live in water show how many different ways life has solved the problem of staying alive in the sea.