Invertebrates

Animals without backbones make up a huge percentage of the ocean’s biodiversity. They aren’t just small (like pteropods) or squishy (like jellyfish); the giant squid and corals that build huge reefs are also invertebrates.

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Marine parasites may be small in size, but they can be present in very high numbers and put together can weigh even more than all the top predators in an estuary or bay ecosystem!...
During the summer of 1998, scientists at the Virginia Institute of Marine...
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The blanket octopus can rip a poisonous tentacle from a Portuguese man-o-war...
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Hyperiid amphipods are small crustaceans related to sand fleas and distantly...

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Census of Marine Life researchers discovered this unusual transparent sea cucumber ( Enypniastes sp.) in the Gulf of Mexico at 2,750 meters depth. It creeps forward on its tentacles pretty slowly, at around 2 centimeters per...
Smaller than the head of a pin, this arrow squid (Doryteuthis plei ) embryo...
What are corals? Corals themselves are animals. But tropical reef-building...

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Jellyfish and comb jellies are gelatinous animals that drift through the ocean's water column around the world. They are...
A mantis shrimp ( Odontodactylus scyllarus ) holds her clutch of eggs in her clubbed claws. Usually these claws are weapons that punch hard-shelled prey at speeds of more than 50 miles an hour...
A crown-of-thorns starfish ( Acanthaster planci ) on a reef in the Marianas Islands. An “outbreak” of these coral-eating starfish can decimate a reef, and they have done great damage on Australia's...
Dr. Clyde Roper recounts the tale of his encounter with a giant squid specimen ( Architeuthis dux) that was found washed up on a Massachusetts beach in this excerpt from "Eyeball to Eyeball," an...
Dead man’s fingers ( Alcyonium digitatum ) are soft corals named for their appendage-like appearance when thrown ashore by storms. The finger-like clumps of coral polyps come in various shades of...
This hyperiid (in the genus Cystisoma) has only one pair of eyes—but they are very big. You can see them here as the entire surface of its head and the convex orange sheet of retinal cells in the...
Seahorses are not just tropical creatures. They can be found in colder waters like those found off New Zealand, Argentina, Eastern Canada, and the UK. This long-snouted seahorse ( Hippocampus...
Portrait of a scallop with black and white striped lips, whose shell is encrusted with a red sponge.
These newly hatched arrow squid larvae ( Doryteuthis plei ) are each tinier than the head a pin. Free from their yolk sac, they will drift with the current out to sea as zooplankton. Many animals eat...
The feathery strands at the back of this nudibranch’s ( Chromodoris willani ) body are no mere adornment: they’re its gills! Nudibranchs, shell-less snails or sea slugs, are named for these tufted...
This male giant squid is on display in the Sant Ocean Hall at the Smithsonian’s National Museum of Natural History . It measures about 2.7 meters (9 feet) long and weighs a little more than 45.5...
Flower-like clusters of polyps make up this coral colony. Their pink color comes from the zooxanthellae living inside. More about coral reef ecosystems can be found in our Coral Reefs featured story .
Found in Arctic waters, this rare deep-water species of larvacean , Oikopleura gorskyi , eats by filtering particles from the seawater it drifts through. Larvaceans build 'houses' around themselves...
In 1874, Reverend Moses Harvey of Newfoundland bought a dead giant squid caught by fishermen. More about the giant squid can be found in our Giant Squid featured story .
Depending on whom you talk to, jellyfish are either fascinating, a nuisance, a toxic menace, or some combination of the above. Jellyfish plop into the media spotlight when their presence causes beach...
This sea potato ( Echinocardium cordatum ) looks similar to its root vegetable namesake, but it's a sea urchin! The spines on this urchin are more hair-like than the spikes seen on some more commonly...
Like many deep sea creatures, this tiny comb jelly ( Bathocyroe fosteri ) has a transparent body, enabling it to blend into the surrounding waters. This ctenophore is very common around the Mid-...
The over 1,000 species of ribbon worms ( Nemertea ) are mostly found in marine environments (like the Hubrechtia found in a mud flat, in the photo). These worms have both a mouth and an anus (unlike...
Small amphipods, the blue bug-like creatures, can be seen here catching a ride on this enormous sea spider as it makes its way across the bottom. Many Antarctic creatures, like the dinner-plate-sized...
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