The pyrosome is one of the ocean’s most perplexing organisms, often resembling a large, hollow, gelatinous tube or a colossal sea pickle. It is frequently mistaken for a jellyfish or floating plastic debris, yet it is biologically complex. The name Pyrosoma translates from the Greek words for “fire” (pyro) and “body” (soma), referencing the spectacular light show it produces in the dark depths. These creatures drift through the open ocean, presenting a unique form of life that is both massive in scale and intricate in structure.
Classification and Colonial Structure
The pyrosome is a free-swimming organism classified as a tunicate (class Thaliacea), making it more closely related to vertebrates than to jellyfish. It is a colonial organism; the large, visible structure is composed of thousands of individual animals called zooids. These zooids are embedded in a common, transparent, gelatinous matrix known as the tunic, which forms the outer wall of the colony.
The collective structure is a hollow cylinder, or cone, closed at one end and open at the other. Each zooid is positioned in the tunic wall to support the colony’s functions. The zooid’s branchial (incurrent) siphon faces outward, drawing water in for feeding and respiration. The atrial (excurrent) siphon directs the filtered water into the central, hollow cavity.
Shared tissue connects the zooids throughout the tunic, allowing them to coordinate actions and function as a single unit. New zooids are produced asexually through budding, enabling the colony to grow rapidly and repair damage.
Ecology and Bioluminescence
Pyrosomes are pelagic organisms, inhabiting the open ocean water column, typically preferring tropical and subtropical waters. They perform extensive daily migrations, sinking up to 700 meters during the day to avoid predators and rising toward the surface at night. This vertical migration connects nutrient-rich surface waters with deeper ocean layers.
The pyrosome’s primary ecological role is as a filter feeder. Zooids collectively pump massive volumes of water through their internal filtering mesh to capture microscopic plankton, particularly large eukaryotic phytoplankton. High grazing efficiency means that population booms can significantly alter microbial communities. Dead colonies also contribute to the marine carbon cycle by sinking rapidly to the seafloor, providing food for deep-sea scavengers.
The pyrosome’s most famous characteristic is its brilliant, blue-green bioluminescence, which gives the organism its name. Light production is complex and often attributed to symbiotic bacteria living near the zooid’s incurrent siphon. However, research suggests a host-symbiont collaboration, as the pyrosome produces a unique bioluminescent enzyme (luciferase) that reacts with a chemical compound (coelenterazine) acquired from its diet.
The light from an individual zooid can be triggered by mechanical disturbance or light from another source. When one zooid flashes, it triggers a wave of light that propagates across the entire colony, creating a sustained glow. The function of this coordinated display is unknown, but theories include using the sudden flash as a defense to startle predators or as a form of communication.
Size and Locomotion
Pyrosome colony size varies dramatically by species and age. While many common species are only a few centimeters long (earning them the nickname “sea pickles”), others are colossal. The giant pyrosome, such as Pyrostremma spinosum, can reach lengths up to 20 meters. These massive structures are hollow, often with an internal diameter large enough for a person to swim through.
Pyrosomes are generally considered planktonic, meaning ocean currents dictate most of their movement. However, the colony achieves slow, directional movement through collective jet propulsion generated by the coordinated pumping of water by the zooids.
As each zooid filters water, it expels the processed water into the colony’s central cavity. The only exit for this collective stream is the open end of the cylinder, and the resulting outflow creates a gentle thrust. This continuous propulsion allows the pyrosome to slowly move forward through the water column.