A Colony of Clones
A single, gelatinous creature is a free-floating colony of hundreds or thousands of individual animals called zooids. These zooids are clones, genetically identical and embedded in a shared, translucent tunic that forms the tube-like structure. The entire organism belongs to the phylum Chordata, making it a distant relative of vertebrates, including humans. The name Pyrosoma comes from the Greek words for "fire" (pyro) and "body" (soma), a reference to its bright bioluminescence. The most common species is Pyrosoma atlanticum, first described by French naturalist François Péron in 1804.
Each individual zooid is only a few millimeters long. They are arranged within the gelatinous wall of the cylinder, which can be up to 6 cm wide. Every zooid pumps water from the outside of the tube into its own internal filtering mesh, called a branchial basket, to capture phytoplankton and other small particles. The filtered water is then expelled into the hollow interior of the colony. This collective action of all zooids creates a gentle jet propulsion, allowing the entire colony to move slowly through the water. Colonies range from less than a centimeter to over 18 meters (about 60 feet) in length. The colony grows as zooids reproduce asexually, budding off new clones to expand its size.
A Chain Reaction of Light
The pyrosome's most distinct feature is its sustained bioluminescence. Each zooid contains a pair of light organs that harbor symbiotic, light-producing bacteria. Unlike many bioluminescent creatures that flash briefly, a pyrosome can produce a steady blue-green glow that can be seen for many meters. The light emission is unique because it can be triggered by an external light source or by mechanical stimulation. When one zooid lights up, its neighbors detect the light and begin to glow in response. This creates a wave of light that travels along the entire length of the colony, a phenomenon that allows separate pyrosome colonies to communicate with each other through light signals.
Pyrosomes are highly efficient filter feeders, among the most effective of any zooplankton species. They perform a daily vertical migration, rising toward the surface at night to feed and descending to depths of 760 meters (2,500 feet) during the day. In recent years, massive pyrosome "blooms" have occurred in the Pacific Ocean, particularly off the coast of North America from Oregon to Alaska. These events, possibly linked to warming ocean waters, have seen unprecedented numbers of pyrosomes, sometimes with 60,000 individuals collected in a single five-minute net trawl. When these massive blooms die, the colonies sink to the seafloor, providing a significant source of carbon for deep-sea organisms and contributing to the marine carbon cycle.