pyrosomes

Pyrosomes are pelagic, colonial tunicates within the phylum Chordata. Unlike solitary sea squirts, a pyrosome colony is a single, hollow tube formed by many genetically identical zooids that function together. The tube is built from a shared outer tunic and can vary in length as the colony grows. Colonies are often transparent or translucent, and many species are bioluminescent, emitting blue-green light when stimulated.

Each pyrosome comprises numerous individuals linked into a continuous siphon system. Feeding and circulation are organized within the colony, with specialized zooids arranged along the tube. When disturbed, the colony can glow, a feature that has given rise to the name “pyrosome,” from the Greek for fire and body.

Distribution and habitat are worldwide in the world’s oceans, typically in open marine waters. Pyrosomes are planktonic and most often found near the surface, especially at night when zooplankton activity is higher. They can form seasonal blooms that are noticeable to researchers and divers, though they are generally transient components of the plankton.

Feeding is by filter feeding: the colony produces a mucous net that traps phytoplankton and other small particles, which are then ingested by the zooids. Reproduction occurs through both asexual budding, which enlarges the colony, and sexual reproduction, where gametes are released into the water to form new colonies. The life cycle yields free-swimming larvae that settle to begin new pyrosome colonies.

Notable species include Pyrosoma atlanticum in the Atlantic and Pyrosoma pacificum in the Pacific. Pyrosomes play a role in marine food webs and nutrient cycling and are of interest in studies of zooplankton dynamics and bioluminescence.