SalpsThaliacea

Nothing multicellular grows faster than a salp. Under good conditions the body swells visibly from one day to the next, which is why a stretch of water that held nothing yesterday can be full of them today.

A salp is a tunicate that never settles down, an open-water relative of the sea squirt shaped like a transparent barrel with hoops of muscle wrapped around it. The muscle squeezes, water is forced in one end and out the other, and that single stroke does two jobs at once: it pushes the animal forward and it drags its food out of the water through an internal net of mucus. Feeding and swimming are the same action, which is a large part of why the growth rate is possible.

Then there is the chain. A solitary salp buds a string of clones off its own body, and they leave home still joined together, sometimes in ropes longer than a diver, all of them pumping in sequence. That generation reproduces sexually and produces solitary animals again, so the species alternates between being one body and being many. The chain is not a school. It is one individual's offspring, physically attached.

The consequence reaches well past the animal. Salps eat phytoplankton and pack what is left into dense pellets that sink fast, and when a swarm dies the bodies sink too. Carbon that would otherwise have stayed circulating in warm surface water drops into the deep instead, and salps are one of the quickest routes that carbon has out of the top of the ocean.

For a diver they are an occasional event rather than a fixture. They show up in blue water, often on a safety stop, as clear jelly barrels or long linked chains that catch the light and are easy to take for drifting plastic. Nothing in the group stings.