The oceanic world is full of captivating organisms, but few have captured public attention quite like the creature commonly known as the “sea pickle.” This translucent, gelatinous tube can range from the size of a finger to the length of a bus. Often mistaken for a singular jellyfish or plastic debris, the sea pickle is actually a complex marine animal with a unique role in the open ocean environment.
Classification and Physical Form
The sea pickle is scientifically known as a pyrosome, a free-floating colonial tunicate. This organism is classified within the phylum Chordata, the same group that includes vertebrates like fish and mammals, due to features it possesses in its larval stage. Pyrosomes belong to the class Thaliacea, a group of pelagic filter feeders that spend their entire lives adrift in the water column.
The tubular structure of the pyrosome colony is what gives it the common name “sea pickle” and is composed of hundreds to thousands of tiny, individual animals called zooids. These zooids are embedded in a common, gelatinous matrix known as a tunic, forming a hollow, cone-shaped cylinder that is open at one end. While most colonies are relatively small, some giant pyrosomes can reach lengths of up to 60 feet, creating a tube wide enough for a human to swim through.
Each zooid, typically only a few millimeters long, is positioned on the wall of the tube with its feeding apparatus oriented to the outside. The exterior surface of the colony is bumpy, with each bump representing an individual organism. Water is drawn in through the zooid’s incurrent siphon and filtered. It is then expelled into the colony’s central cavity, eventually exiting through the large, shared opening at the closed end of the tube.
The Biology of a Colonial Organism
The coordinated activities of the individual zooids are responsible for the colony’s movement and feeding. Locomotion is achieved through a slow process of jet propulsion. This is generated by the collective beating of fine, hair-like structures called cilia within the zooids’ branchial baskets, which continuously pump water for respiration and feeding.
Pyrosomes are efficient filter feeders, using a mucus net within the branchial basket to capture microscopic plankton, such as phytoplankton, and detrital matter. Their feeding efficiency is considered high among zooplankton species, allowing them to graze large volumes of water. This continuous water flow also powers the colony’s slow movement through the water.
The pyrosome lives up to its Greek name, Pyrosoma, meaning “fire body,” through its bioluminescence. Each zooid possesses a pair of light organs that emit a bright, pale blue-green light when stimulated. When one zooid begins to glow, the light is detected by its neighbors, triggering a wave that propagates throughout the colony and even to nearby pyrosomes. This coordinated flashing is thought to serve as a form of non-neural communication among the zooids.
Distribution and Massive Blooms
Pyrosomes are typically organisms of the open ocean, preferring the warmer waters of tropical and subtropical seas. They are pelagic, drifting freely, with their distribution largely dictated by ocean currents and temperatures. Pyrosomes often undertake daily vertical migrations, ascending to surface waters at night to feed and descending to deeper depths during the day.
Despite their usual preference for warm water, pyrosomes have recently demonstrated an unusual capacity to form massive, dense population blooms in regions where they were historically rare. A notable event occurred in the mid-2010s when the warm-water species Pyrosoma atlanticum suddenly appeared in unprecedented numbers along the North Pacific coast. These blooms extended from Northern California up to the Gulf of Alaska, representing a significant range expansion.
The appearance of these aggregations was linked to unusually warm ocean conditions, such as the persistent marine heatwave referred to as “the blob.” As the pyrosome population exploded across the continental shelf, it led to widespread media coverage and scientific concern. The biomass of these gelatinous organisms suggested a temporary shift in the structure of the regional marine ecosystem.
Role in the Marine Ecosystem
Pyrosomes occupy a complex position in the marine food web, acting as both consumers and a food source. By efficiently filtering phytoplankton, they compete directly with other zooplankton, which form the base of many pelagic food webs. Pyrosomes are preyed upon by at least 62 different pelagic organisms, including certain fish, sea lions, and marine turtles. They are also consumed by various benthic species after death.
The gelatinous bodies of pyrosomes play a significant role in the ocean’s biological carbon pump, the process of transporting carbon from the surface to the deep ocean. Pyrosomes have a relatively high carbon content—about 35% of their dry weight—compared to other gelatinous plankton. When the colonies die, they sink rapidly in events known as “jelly-falls,” delivering a substantial pulse of organic carbon to the deep-sea floor.
When blooms occur, the organisms can have negative ecological and economic impacts. The dense aggregations exert intense grazing pressure on phytoplankton, reducing the food supply for other zooplankton and the species that feed on them. Economically, the volume of the colonies has been known to clog and damage fishing nets and other gear, forcing commercial fishers to relocate or sort their catch from the gelatinous mass.