As a senior fisheries biologist with the National Oceanic and Atmospheric Administration, Richard Brodeur conducted decades of seaside research. However, in 2014, he encountered a peculiar-looking planktonic organism for the first time in Oregon, known as pyrosomes. These organisms appear like single-ended tubes, or like translucent cucumbers. Local fishermen colloquially call them “sea cucumbers” or “gummy bear worms,” and although Brodeur had been familiar with them off the coast of California, this was his first encounter with them in Oregon.
Since then, Brodeur occasionally encountered pyrosomes again, but by 2017, the situation became out of control—with pyrosomes’ numbers exploding. During a research survey, as many as 60,000 pyrosomes could be caught in a net within just five minutes. Researchers mounted GoPro cameras to the trawls to record the dense clusters of pyrosomes underwater, even observing them in waters 100 meters deep—not because that is the extent of pyrosomes’ habitat, but because that’s as deep as the equipment could go. Jennifer Fisher, a research assistant at Oregon State University’s Cooperative Institute for Marine Resources Studies, was astounded by the sight of pyrosomes and nothing else during a survey in May 2017. That experience convinced Fisher that, should the apocalypse come, pyrosomes could survive alongside cockroaches.
Studying pyrosomes themselves reveals them to be quite remarkable creatures. Strictly speaking, what we refer to as a “pyrosome” is actually a colony made up of countless tiny individuals, or zooids, linked together. Each tiny individual is only a few millimeters in length, has a tubular structure, and possesses a filtering ability. As seawater flows in from one end, various small particles are caught in the filter and become food for the pyrosomes. If necessary, pyrosomes can expel water from the opening of their colony to propel the entire group. Although they are made up of countless individuals, all these zooids come from a single embryo clone, sharing identical genetic information. Thus, pyrosomes are considered one of the largest known planktonic organisms. The largest species—the spindle-shaped pyrosome—can reach a staggering 30 meters in length, enough for humans to swim inside their tubular structure. The species encountered by Brodeur in Oregon, the Atlantic pyrosome, can grow up to 60 centimeters in length.
According to theory, the Atlantic pyrosome should only be found along the southern California coast, south of 34°N latitude. But since 2014, pyrosomes have migrated north to the Oregon coast, above 42°N latitude. By 2017, pyrosomes had even been sighted along the coast of Alaska.
Emerging alongside these pyrosomes was a severe and long-lasting marine heatwave. Starting in 2013, water temperatures in the northeastern Pacific rose by about 3 degrees Celsius in certain areas, a phenomenon known as “The Blob.” Warm water then rapidly spread south along the Pacific coast of North America, covering the entire western coast. In 2019, another heatwave known as “The Blob 2.0” formed. These consecutive marine heatwaves have resulted in the northeastern Pacific remaining unusually warm for the past decade, leading to a significant reduction in Pacific cod numbers, northward migration of tuna, and mass starvation and death among seabirds.
One of the beneficiaries of the ocean heatwave appears to be the pyrosome. It is still uncertain whether the mass appearance of pyrosomes is a direct consequence of the ocean heatwave, but these organisms seem to prefer warm seawater. Meanwhile, the surge in pyrosome numbers coincided with the change in temperatures.
However, the unusual increase in pyrosome numbers has changed the way energy is transferred through the food web. A study published in the journal Nature Communications by Dylan Gomes of Oregon State University and his colleagues points out that the dramatic increase in the number of pyrosomes leads to significant changes in the middle and lower trophic levels of the entire food web. The rapidly growing pyrosomes consume too much energy, leading to a substantial reduction in food for other lower-level trophic organisms such as pteropods, small jellyfish, krill, sardines, etc. This could not only reduce the numbers of these species but also decrease the food sources for higher-level predators such as carnivorous jellyfish, thereby affecting the overall balance of the ecosystem.
The fundamental issue with the widespread impact of pyrosomes is that the energy they consume is almost no longer passed on to higher trophic levels. Research models show that over 98% of the pyrosomes ultimately sink to the seabed and are not consumed by other living organisms.
“Most of the energy and nutrients within their bodies end up being trapped on the seafloor,” Gomes points out, “From an ecosystem perspective, they are an ecological dead end—no other organism in the ecosystem can gain energy from the pyrosomes.” Currently, scientists are not entirely sure why the organisms in the ocean do not choose pyrosomes as food. One speculation is that the energy content of pyrosomes may be very low, so preying on them may not be worth the effort.
Even though there are predators that usually choose to eat jellyfish with a similar texture, some researchers suspect that compared to jellyfish, pyrosomes may have a tougher texture, making them a less popular food choice. Additionally, it may also be because pyrosomes are a relatively new species to the local biota, and predators have not had time to add them to their diet.
Whatever the reason, today pyrosomes continue to absorb energy within the ocean’s food web. Data from the National Oceanic and Atmospheric Administration (NOAA) show that since the appearance of the ocean heatwave, the catch of Chinook salmon and cod in the studied area has decreased by 67%, showing the impact of this phenomenon.
Although it is not yet certain whether the ocean heatwave is the only cause for the significant decline in fish numbers, as fluctuations in fish populations may be influenced by a variety of factors, including natural cycles, this phenomenon has undoubtedly attracted the attention of the scientific community. Gomes also warned that considering factors such as the decline in oxygen content in seawater due to warming, existing models may still be inadequate.
Nonetheless, researchers are making their best efforts to understand how the ocean heatwave is affecting the ecosystem of the Northeast Pacific and trying to reveal the mechanisms involved. “This is the first attempt to understand how the ocean heatwave is changing the ecosystem of the Northeast Pacific,” added Gomes.
