Jellyfish populations have increased globally in recent decades, leading many to conclude these gelatinous organisms are aggressively invading new territories. Massive aggregations, often called “blooms,” can appear to be an invasion or sudden takeover of a marine environment. However, the relationship between jellyfish and the label of “invasive species” is biologically complex. Many large blooms involve native species responding to favorable conditions created by human activity, but certain non-native species are indeed transported across oceans with harmful results. Understanding this distinction is important for managing changes in ocean ecosystems.
Distinguishing Invasive Species from Natural Blooms
A true invasive species must meet specific criteria beyond simply having a large population. An organism is considered invasive if it is non-native to an ecosystem and if its introduction causes, or is likely to cause, economic or environmental harm. This term distinguishes harmful newcomers from non-native species that are established without negative impact.
By contrast, a natural bloom is a temporary, rapid increase in the population of a species native to that area. These native blooms are often triggered by environmental changes. Examples include nutrient runoff from agriculture, which fuels the plankton jellyfish eat, or the overfishing of their natural predators or competitors. Therefore, a massive aggregation of jellyfish can be a native population thriving in a human-altered environment, not necessarily an invasion.
Pathways for Global Jellyfish Spread
The primary mechanism for transporting non-native jellyfish across great distances is commercial shipping, which bypasses natural geographic barriers. The most significant vector is ballast water, taken up by ships for stability and then discharged thousands of miles away. This water can contain microscopic, free-swimming juvenile stages of jellyfish, known as ephyrae, or earlier larval stages.
Another major human-mediated pathway is hull fouling, or biofouling, where organisms physically attach to a vessel’s submerged surfaces. The sedentary polyp stage of the jellyfish life cycle can affix itself to the bottom of a ship or oil rigs, effectively hitching a ride. Once the polyp releases the medusa (the adult form) in the new environment, an established population can quickly take hold. The movement of aquaculture stocks also presents a risk, as non-native species can be inadvertently transported along with farmed marine life.
Notable Examples of Invasive Jellyfish
One destructive example of a truly invasive species is the Warty Comb Jelly, Mnemiopsis leidyi, native to the Atlantic coasts of the Americas. This ctenophore was introduced into the Black Sea in the early 1980s, most likely via ship ballast water discharge. Its population grew exponentially in the nutrient-rich waters, reaching high densities by 1989.
The rapid proliferation of Mnemiopsis leidyi contributed to the collapse of the Black Sea’s anchovy fishery by consuming vast quantities of fish eggs and larvae. Following its success, the species spread further into the Caspian Sea in 1999 and was later recorded in the North and Baltic Seas.
Another significant case involves the Australian Spotted Jellyfish, Phyllorhiza punctata, native to the Southwest Pacific. This large species appeared in the Gulf of Mexico around 2000, likely introduced as polyps attached to ship hulls or in ballast water. Phyllorhiza punctata grew to an impressive size in the Gulf, with individuals reaching up to 25 pounds. In the summer of 2000, a massive bloom of millions caused significant disruption to commercial fishing operations.
The Broad Impacts of Invasive Populations
Once established, invasive jellyfish populations exert ecological and economic pressure on their new environments. Ecologically, their primary impact is as voracious predators, consuming vast amounts of zooplankton, fish eggs, and fish larvae (ichthyoplankton). This predation reduces the food base for native planktivorous fish and limits the reproductive success of commercially important fish stocks.
The invaders also compete directly with native fish for zooplankton resources, shifting the marine food web towards a “gelatinous” state. Economically, the sheer biomass of these species creates severe problems for human infrastructure and industries. Large aggregations clog fishing nets, forcing the temporary closure of productive fishing grounds and causing millions of dollars in losses. High-density blooms also pose a threat to coastal power plants and desalination facilities by clogging cooling water intake pipes, leading to shutdowns and costly maintenance.