What Is the World’s Longest Animal?

Nothing can compete with a blue whale in terms of sheer size, but the gigantic marine mammal seems short when compared with a siphonophore discovered in underwater canyons near Western Australia in 2020. The siphonophore -- a worm-like entity composed of a colony of cloned individuals -- measured approximately 150 feet (46 m), which researchers say makes it the longest living creature ever recorded.

Siphonophores might look like long, harmless strings, but they are actually predators that feed by catching prey with stinging tentacles that stretch out in the water and catch fish or small crustaceans.

The researchers who came upon the siphonophore also discovered several other species that they believe have never been recorded, including a long-tailed sea cucumber, glass sponges, and an octopus squid. But the siphonophore was the main event, according to researcher Nerida Wilson. "It was just amazing to see this huge organism spread out like a spiral UFO, hovering in the water column," she said. "We couldn’t believe what we were seeing."

For the record, the longest blue whale on record measured 111 feet (34 m).

Some long stories:

The title of the world's longest animal goes to the bootlace worm (Lineus longissimus), which can reach lengths of up to 55 meters (180 feet), according to the Guinness World Records. These marine nemertean worms are found in the North Sea and have slender, ribbon-like bodies that allow them to stretch to incredible lengths.

While blue whales are the largest animals by mass, bootlace worms surpass them in length. Blue whales typically grow to about 30 meters (98 feet) in length, making them shorter than the maximum recorded length of a bootlace worm. The worm's slender body allows it to extend much longer than the more massive whale.

Other notably long animals include the lion's mane jellyfish, whose tentacles can reach lengths of up to 36.6 meters (120 feet), and the giant squid, with tentacles that can extend to about 13 meters (43 feet). However, both fall short of the bootlace worm's record length.

Bootlace worms are primarily found in the North Sea, along the coasts of Britain and Norway. They inhabit shallow waters and are often buried in sand or hidden among rocks on the seafloor. Their habitat allows them to blend in and avoid predators while searching for small prey.

Bootlace worms are carnivorous and feed on small crustaceans and other marine invertebrates. They catch their prey using a proboscis—a long, sticky appendage that can be rapidly everted to ensnare or entangle their food. The proboscis is also equipped with a toxin that helps subdue their catch.

Measuring extremely long animals like the bootlace worm involves careful observation and sometimes a bit of estimation. Scientists may measure specimens that are washed ashore or captured, using meter tapes for smaller lengths and more complex methods, such as marked ropes or laser measurement tools, for larger specimens. They ensure accuracy by taking multiple measurements and using the average length.