Pyrosome
Pyrosomes, genus Pyrosoma, are free-floating colonial tunicates that usually live in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical or cone-shaped colonies up to 18 m (60 ft) long,[2] made up of hundreds to thousands of individuals, known as zooids. Colonies range in size from less than one centimeter to several metres in length. They are commonly called "sea pickles".[3][4] Other nicknames include "sea worms", "sea squirts", "fire bodies", and "cockroaches of the sea".[5]
Each zooid is a few millimetres in size, but is embedded in a common gelatinous tunic that joins all of the individuals.[3] Each zooid opens both to the inside and outside of the "tube", drawing in ocean water from the outside to its internal filtering mesh called the branchial basket, extracting the microscopic plant cells on which it feeds, and then expelling the filtered water to the inside of the cylinder of the colony. The colony is bumpy on the outside, each bump representing a single zooid, but nearly smooth, although perforated with holes for each zooid, on the inside.[2][5]
Pyrosomes are planktonic, which means their movements are largely controlled by currents, tides, and waves in the oceans. On a smaller scale, however, each colony can move itself slowly by the process of jet propulsion, created by the coordinated beating of cilia in the branchial baskets of all the zooids, which also create feeding currents.[5]
Pyrosomes are brightly bioluminescent, flashing a pale blue-green light that can be seen for many tens of metres. Pyrosomes are closely related to salps, and are sometimes called "fire salps". Sailors on the ocean occasionally observe calm seas containing many pyrosomes, all luminescing on a dark night.[2][5]
Pyrosomes feed through filtration and they are among the most efficient filter feeders of any zooplankton species.[6]
Although many planktonic organisms are bioluminescent, pyrosome bioluminescence is unusual in its brilliance and sustained light emission,[7] and evoked the following comment when seen by the eminent scientist Thomas Huxley at sea:
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