Pyrosoma Atlanticum: A Glowing Deep Sea Colony

Pyrosoma atlanticum is a marine organism known for its unique appearance and behaviors in the deep ocean. This gelatinous, cylindrical creature, sometimes called a “sea pickle,” floats through the water. First described in 1804 by French naturalist François Péron, this species continues to be a subject of scientific fascination.

What is Pyrosoma Atlanticum?

Pyrosoma atlanticum is a colonial tunicate, a collection of many individual, tiny organisms called zooids. These zooids are genetically identical and are physically linked, forming a larger, unified structure. The entire colony typically takes on a hollow, translucent cylindrical or cone shape, with one end closed and the other open. Colonies vary greatly in size, often reaching up to 60 centimeters in length, though individual zooids are only a few millimeters long. This organism is classified within the phylum Chordata, and more specifically, the subphylum Tunicata.

Each zooid within the colony is embedded in a shared gelatinous outer layer, known as the tunic. The zooids are arranged around the central hollow tube of the colony. Their small bodies, up to 8.5 millimeters long, possess a broad, rounded branchial sac equipped with gill slits. A structure called the endostyle runs along the side of the branchial sac, producing mucus filters used in feeding.

Life in the Deep Ocean

Pyrosoma atlanticum inhabits the open ocean, found in temperate waters across all the world’s oceans, typically between 50°N and 50°S latitudes. While they can be found at various depths, they are most abundant below 250 meters, extending into the mesopelagic and bathypelagic zones. These pelagic colonies drift in the water column and undertake daily vertical migrations. They ascend towards the surface during the evening to feed and descend around dawn, with large colonies moving up to 760 meters vertically each day.

Movement for the colony is achieved through a form of jet propulsion. Individual zooids draw in ocean water from the outside of the colony, filter out microscopic food particles, and then expel the filtered water into the hollow interior of the cylinder. This coordinated expulsion of water generates a force that propels the entire colony slowly forward. As filter feeders, Pyrosoma atlanticum consumes microscopic plankton, including centric diatoms, silicoflagellates, and even fragments of small crustaceans and coccolithophores. This feeding activity contributes to the marine food web, and when colonies die, their sinking carcasses can transport carbon to the ocean floor, playing a role in carbon cycling.

The Bioluminescent Glow

A key characteristic of Pyrosoma atlanticum is its ability to produce light, known as bioluminescence. Each individual zooid within the colony possesses a pair of luminescent organs located on either side of its inlet siphon. These organs are capable of generating a bright blue-green light. The light emission can be triggered by various external stimuli, such as mechanical disturbances or even light itself.

When one zooid is stimulated and emits light, this light can, in turn, trigger nearby zooids to also luminesce, creating a wave of light that propagates across the entire colony. This coordinated display of light can be intense, visible for many meters in clear water. The precise mechanism for this light production is still being studied, but research suggests a possible role for symbiotic bacteria, specifically Photobacterium, within the pyrosome’s microbiome. However, there is also evidence for an endogenous luciferase, an enzyme that produces light, similar to that found in sea pansies. The bioluminescence likely serves several functions, including defense against predators by startling or confusing them, or potentially as a form of communication within the colony.

Reproduction and Colony Formation

The life cycle of Pyrosoma atlanticum involves both sexual and asexual reproduction, forming its unique colonial structure. The process begins with a single, sexually produced larva that develops into an “oozooid.” This initial oozooid then undergoes asexual budding, a process where it produces new, genetically identical zooids. These newly budded zooids remain attached to the parent oozooid, gradually forming the initial cylindrical colony. As more zooids are budded off, the colony elongates and grows.

Once a colony is established and grows, sexual reproduction can occur. Individual zooids within the same colony or between different colonies can participate in sexual reproduction. This process results in the production of eggs, which then develop into new oozooids, restarting the cycle. This combination of asexual budding for colony growth and sexual reproduction allows Pyrosoma atlanticum to propagate and form the large, free-floating colonies.

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