Invasion of the jellyfish
Image credit: Zando Escultura
In the water world jellyfish are the true poets. Moving in the water column with their shiny transparent bodies they seem to perform a graceful dance. But jellyfish can also injure and even kill humans when they swarm along beaches. It is important to know that these graceful sea creatures swarm in masses because we humans created the ideal conditions for jellyfish to multiply rapidly, by polluting the oceans…
The original Dutch version of the article can be read in Eos-magazine no. 2 (Febr. 2009) or downloaded here. The English version follows below:
Jellyfish and jellyfish like creatures inhabit our oceans since more than 500 million years. Only recently, since they have been known to swarm and thereby interfere with human activities, have these beautiful creatures caught the attention of scientists.
Jellyfish have an important ecological role in marine and some fresh water ecosystems. They are food for seabirds, pink salmon, moon fish, sea turtles and other animals that are immune to their poison. Jellyfish feed on fish eggs, larvae and small invertebrates. Their huge capacity for adaptation explains why they have been around on earth since so long and why their populations explode from time to time. Jellyfish populations can explode out of the blue and colonize whole areas as a result of their fast reproduction being characterized by a combination of sexual and asexual reproduction.
“Jellyfish swarms points towards an ecosystem being out of balance as a result of anthropological environmental disruption.”
People
Swarm forming in jellyfish has been a natural phenomenon since millions of years, but the increased number of always bigger swarms where previously jellyfish were not present, is an increasing cause of concern. Areas of thousands of kilometers are being colonized and fisheries and beach tourism feel threatened in their existence. Over the whole world, from beaches in Hawaii, the Gulf of Mexico, the Mediterranean Sea to Australia, jellyfish take over touristic beaches. Typically for jellyfish are the nettle cells on their tentacles that inject paralyzing poison into prey. Yearly, worldwide 150 million people are injured by jellyfish and some people die. In the Philippines 20 to 40 people die each year after being stinged by a sea wasp. The sea wasp, strictly taken not a true jellyfish but a box jellyfish, is probably the most venomous animal that exists and can kill a human within 3 minutes. Next to their impact on fisheries and tourism jellyfish also have been known to ‘boycot’ industrial installations and ships by obstructing sea water carrying pipelines.
Perfect conquerors
Humans are directly helping jellyfish populations to explode. According to William Hamner of the University of California swarms points towards an ecosystem being out of balance as a result of anthropological environmental disruption. Jellyfish take advantage of different kind of environmental stress present in many marine ecosystems. They are possibly the only group of animals that is able to colonize dead zones, heavily polluted areas on sea with an oxygen deficit. Much of their dispersal happens passively, jellyfish being transported with water currents or travelling on board of ships, either in the ballast water or as a polyp on the outer walls of ships. As a result they can easily invade areas where before they were absent. The global climate change aids jellyfish in their dispersal, since increasing water temperatures enhance reproduction and growth, and increase their geographical distribution. Overfishing is also helping the jellyfish populations, by leading to less competition with fish that feed on the same prey species.
Invisible polyps
Before scientists were never really interested in jellyfish, as a result information on these animals is scarce and the true impact of humans on jellyfish populations is hard to tell. At present however swarming of jellyfish has caught the interest of scientists and especially the polyp stadium of jellyfish gets major attention. Scientists now assume that certain areas in the oceans are overgrown with polyps, but as for now it is unknown where these fields of polyps are located.
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