Turritopsis dohrnii is called the immortal jellyfish because it can potentially live forever. Jellyfish start life as larvae before establishing themselves on the seafloor and transforming into polyps. These polyps then produce free-swimming medusas, or jellyfish. Mature T. dohrnii are special in that they can turn back into polyps if they are physically damaged or starving, according to the American Museum of Natural History, and then later return to their jellyfish state.

The jellyfish, which are native to the Mediterranean Sea, can repeat this feat of reversing their life cycle multiple times and therefore may never die of old age under the right conditions, according to the Natural History Museum in London. T. dohrnii are tiny — less than 0.2 inch (4.5 millimeters) across — and are eaten by other animals, such as fish, or may die by other means, thus preventing them from actually achieving immortality.

Sponges are made up of colonies of animals, similar to corals, and can also live for thousands of years. Glass sponges are among the longest-living sponges on Earth. Members of this group are often found in the deep ocean and have skeletons that resemble glass, hence their name, according to NOAA.

A 2012 study published in the journal Chemical Geology estimated that a glass sponge belonging to the species Monorhaphis chuni was about 11,000 years old. Other sponge species may be able to live even longer.

Corals look like colorful, underwater rocks and plants, but they are actually made up of the exoskeletons of invertebrates called polyps. These polyps continually multiply and replace themselves by creating a genetically identical copy, which over time causes the coral exoskeleton structure to grow bigger and bigger. Corals are therefore made up of multiple identical organisms rather than being a single organism, so a coral's life span is more of a team effort.

Deep-water black corals are among the longest-living corals. Black coral specimens found off the coast of Hawaii have been radiocarbon dated to be 4,265 years old, Live Science previously reported.

Ocean quahog clams (Arctica islandica) inhabit the North Atlantic Ocean. This saltwater species can live even longer than the other bivalve on this list, the freshwater pearl mussel. One ocean quahog clam found off the coast of Iceland in 2006 was 507 years old, according to National Museum Wales in the U.K. The ancient clam was nicknamed Ming because it was born in 1499, when the Ming dynasty ruled China (from 1368 to 1644).

"In the colder waters surrounding Iceland the Ocean Quahog has a slower metabolism and so grows slowly and may even live for longer than 507 — scientists just haven't found an older one yet!" Anna Holmes, curator of invertebrate biodiversity (bivalves) at National Museum Wales, wrote on the museum's website in 2020.

Tube worms are invertebrates that live on the ocean floor. Bacteria in their tubes create sugars from chemicals, which they absorb as food, according to the Woods Hole Oceanographic Institution's Dive and Discover website. Some tube worms live around hydrothermal vents, but the longest-living species are found in colder, more stable environments called cold seeps, where chemicals spew from cracks or fissures in the seafloor, according to the Exploring Our Fluid Earth website hosted by the University of Hawaii.

A 2017 study published in the journal The Science of Nature found that Escarpia laminata, a cold-seep species of tube worm in the Gulf of Mexico, regularly lives up to 200 years, and some specimens survive for more than 300 years. Tube worms have a slow metabolism and few natural threats (such as predators), which has helped these creatures evolve such long life spans.

Greenland sharks (Somniosus microcephalus) live deep in the Arctic and North Atlantic oceans. They can grow to be 24 feet (7.3 meters) long and have a diet that includes a variety of other animals, including fish and marine mammals such as seals, according to the St. Lawrence Shark Observatory in Canada.

A 2016 study of Greenland shark eye tissue, published in the journal Science, estimated that these sharks can have a maximum life span of at least 272 years. The biggest shark in that study was estimated to be about 392 years old, and the researchers suggested that the sharks could have been up to 512 years old, Live Science previously reported. The age estimates came with a degree of uncertainty, but even the lowest estimate of 272 years still makes these sharks the longest-living vertebrates on Earth, Julius Nielsen, a researcher at Greenland Institute of Natural Resources, previously told Live Science.

Freshwater pearl mussels (Margaritifera margaritifera) are bivalves that filter particles of food from the water. They live mainly in rivers and streams and can be found in Europe and North America. The oldest known freshwater pearl mussel was 280 years old, according to the World Wildlife Fund for Nature. These invertebrates have long life spans thanks to their low metabolism.

Freshwater pearl mussels are an endangered species. Their population is declining due to a variety of human-related factors, including damage and changes to the river habitats they depend on, according to the International Union for Conservation of Nature.

The rougheye rockfish (Sebastes aleutianus) is one of the longest-living fish, with a maximum life span of at least 205 years, according to the Washington Department of Fish and Wildlife. These pink or brownish fish live in the Pacific Ocean from California to Japan. They grow up to 38 inches (97 centimeters) long and eat other animals, such as shrimp and smaller fish, according to the Committee on the Status of Endangered Wildlife in Canada, an independent advisory panel that assesses the statuses of species threatened with extinction in Canada.

A 2021 study published in the journal Science looked at the genomes of 88 rockfish species, including rougheye rockfish, and found genetic calling cards for longevity, including DNA repair pathways that may help ward off cancers. A longer life span allows the rockfish to grow larger and produce more young.

Bowhead whales (Balaena mysticetus) are the longest-living mammals. The Arctic and sub-Arctic whales' exact life span is unknown, but stone harpoon tips found in some harvested individuals prove that they comfortably live over 100 years and may live more than 200 years, according to the National Oceanic and Atmospheric Administration (NOAA).

The whales have mutations in a gene called ERCC1, which is involved with repairing damaged DNA, that may help protect the whales from cancer, a potential cause of death. Furthermore, another gene, called PCNA, has a section that has been duplicated. This gene is involved in cell growth and repair, and the duplication could slow aging, Live Science previously reported.

Studying these long-lived whales could provide hints about how to prolong human life. "My own view is that different long-lived species use different tricks to evolve long life spans, and there aren't many genes in common," João Pedro de Magalhães, an expert in aging science at the University of Birmingham in the U.K., previously told Live Science. "But you do find some common pathways, so there may be common patterns."