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The Collaborative International Dictionary
Pyrosome

Pyrosome \Pyr"o*some\, n. [Pyro- + -some body.] (Zo["o]l.) Any compound ascidian of the genus Pyrosoma. The pyrosomes form large hollow cylinders, sometimes two or three feet long, which swim at the surface of the sea and are very phosphorescent.

Wiktionary
pyrosome

n. Any of several pelagic, colonial tunicates, of the genus (taxlink Pyrosoma genus noshow=1), found in tropical waters

Wikipedia
Pyrosome

Pyrosomes, genusPyrosoma, are free-floating colonial tunicates that live usually in the upper layers of the open ocean in warm seas, although some may be found at greater depths. Pyrosomes are cylindrical- or conical-shaped colonies 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.

Each zooid is only a few millimetres in size, but is embedded in a common gelatinous tunic that joins all of the individuals. 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, though perforated with holes for each zooid, on the inside.

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.

Pyrosomes are brightly bioluminescent, flashing a pale blue-green light that can be seen for many tens of metres. The name Pyrosoma comes from the Greek (pyro = "fire", soma = "body"). Pyrosomes are closely related to salps, and are sometimes called "fire salps". Sailors on the ocean are occasionally treated to calm seas containing many pyrosomes, all luminescing on a dark night.