Atlantic Fire Ascidian, Pyrosoma atlanticum

Pyrosoma atlanticum is a species of colonial tunicate found in temperate waters worldwide, usually between 50°N and 50°S. It is most plentiful at depths below 800 feet. It is found in colonies that are pelagic and move throughout the water column. In the evening the colony will move closer to the surface and descend back by dawn. Large colonies can rise and descend more than 2,500 feet in a single day.

A colony of this species is cylindrical and can grow up to 2 feet long and 2.5 inches wide. The constituent zooids form a rigid tube in which may be pale pink, yellowish or bluish. One end of the tube is narrower and closed while the other is open and has a strong diaphragm. The outer surface is gelatinous and dimpled with backward pointing, blunt processes.

Individuals zooids are up to 0.3 inches long and have a broad, rounded branchial sac will gill slits. Along the side of the branchial sac runs the endostyle which produces mucus filters. Water is moved through the gill slits into the center of the cylinder by cilia pulsating rhythmically. Plankton and other organisms are caught in the mucus filters in the process and the colony is propelled through the water at the same time. This species is bioluminescent and can emit a brilliant blue-green light when stimulated.

In a study comparing different zooplankton organisms, it was found that colonies of this species were highly efficient grazers of particles greater than 10 micrometers in diameter. This implies that P. atlanticum uses high biomass intake as a strategy rather than investing in energy conservation mechanisms.

Growth of the colony occurs when rings of zooids bud off around the edge of the colony. There are a pair of luminescent organs on either side of the inlet siphon of each zooid. There is no neural pathway between the zooids but each responds to the light produced by other individuals and even by light from other nearby colonies.

Predators of this species include various bony fish such as the spiky oreo, big-eyed cardinal fish and the pelagic butterfish. Dolphins and whales also consume these creatures, often in large amounts.

The  species was first described in 1804 by French naturalist Francois Peron.

Image Caption: Pyrosoma atlanticum. Credit: Show_ryu/Wikipedia(CC BY-SA 3.0)