Jellyfish are ubiquitous marine organisms found across all the world’s oceans. In recent decades, public attention has been drawn to a dramatic increase in their numbers, often manifesting as massive swarms near coastlines. This surge raises a fundamental question: are these events a sign of invasive species taking hold, or are they simply massive population explosions of native species, known as blooms? Understanding the distinction is necessary to address the underlying changes in the marine environment.
Differentiating True Invasive Species from Population Blooms
A true invasive species is defined as a non-native organism that is introduced to a new environment and subsequently causes ecological or economic harm. Introduction often occurs through human activity, such as the discharge of ship ballast water, which transports organisms across oceans to new regions. The ctenophore Mnemiopsis leidyi, transported from its native Atlantic coast to the Black Sea in the 1980s via ballast water, led to the collapse of local fisheries. Another is the large, stinging Rhopilema nomadica, which entered the Mediterranean Sea through the Suez Canal from the Indo-Pacific and has since established itself as a recurring pest along the Levantine coast.
In contrast, jellyfish blooms are massive, temporary population increases of species that are native to a particular body of water. These blooms are natural demographic events, often seasonal, where the population density of the medusa stage peaks over a short period. The majority of widely reported “jellyfish invasions” are actually these native species blooms that appear more frequently or intensely due to shifts in their local environment. A native bloom is a symptom of ecosystem change, whereas an invasion introduces a new, disruptive element.
Anthropogenic Drivers of Jellyfish Surges
Overfishing and Eutrophication
The increasing frequency and intensity of both native blooms and invasive species explosions are linked to human activities that alter the marine ecosystem. One significant driver is the widespread practice of overfishing, which removes both the predators of jellyfish and the competitors that feed on the same zooplankton. This removal of natural checks and balances opens an “ecological space” that gelatinous species are highly effective at filling. Eutrophication, the enrichment of water by excess nutrients, also favors jellyfish over many fish species. Nutrient runoff fuels plankton blooms, a food source for jellyfish, and the subsequent decomposition of this organic matter can create low-oxygen “dead zones.” Jellyfish are far more tolerant of these hypoxic conditions than most fish, allowing them to thrive in areas where their competitors cannot survive.
Climate Change and Infrastructure
Warming waters accelerate the reproductive cycle of many jellyfish species. The life cycle of many jellyfish includes a sessile polyp stage, which asexually buds off the swimming medusae. Warmer temperatures can increase the budding rate of these polyps, leading to a greater number of medusae being released into the water column. Coastal development and marine infrastructure, such as piers, jetties, and offshore rigs, provide an abundance of artificial hard substrates. These structures offer ideal surfaces for the polyp stage to settle and proliferate, increasing the overall recruitment base for the medusa populations.
Ecological and Economic Consequences of Mass Occurrences
The massive occurrence of jellyfish, whether native blooms or invasive species, initiates a series of cascading effects in the marine ecosystem. Ecologically, large swarms shift the structure of food webs by competing directly with fish for zooplankton and preying on fish eggs and larvae. This predation pressure hinders the recovery of commercial fish stocks, further exacerbating the effects of overfishing. When these large jellyfish populations eventually die off, their decomposition can also alter the biochemical cycling of nutrients like carbon and nitrogen, potentially favoring microbial communities over higher trophic levels.
These mass occurrences create significant economic hardship for coastal communities and industries:
- Commercial fisheries suffer when nets become clogged by the sheer weight of the jellyfish, making them difficult to haul and sometimes damaging the gear.
- Large swarms pose a threat to industrial infrastructure by clogging the seawater intake screens of coastal facilities, including power plants and desalination facilities.
- These clogs can lead to operational shutdowns and costly repairs, as seen in the temporary closure of nuclear power plants in countries like Scotland and Sweden.
- The presence of stinging species deters tourists, leading to reduced coastal recreation, with some affected tourist destinations reporting economic losses in the millions of dollars.