[4] Pyrosomes form cylindrical or cone-shaped colonies up to 18 m (60 ft) long,[5] made up of hundreds to thousands of individuals, known as zooids.

Although many planktonic organisms are bioluminescent, pyrosome bioluminescence is unusual in its brilliance and sustained light emission,[10] and evoked the following comment when seen by the eminent scientist Thomas Huxley at sea:"I have just watched the moon set in all her glory, and looked at those lesser moons, the beautiful Pyrosoma, shining like white-hot cylinders in the water" (T.H.

Each zooid contains a pair of light organs located near the outside surface of the tunic, which are packed with luminescent organelles that may be intracellular bioluminescent bacteria.

The causes remain unknown, but one hypothesis is that this bloom may have resulted in part from unusually warm water along the coast over several preceding years.

Scientists were concerned that should there be a massive die-off of the pyrosomes, it could create a huge dead zone as the decomposition of their bodies could consume much of the oxygen dissolved in the surrounding seawater.