By Henry Workman
Marine Science Today Writer
Species of jellyfish provide greater competition for more complex organisms than previously thought. Recent research published in Science reveals that in areas where plankton-feeding fish populations have dropped off, mostly due to overfishing, jellyfish that occupy a similar ecological niche have taken their places. The jellyfish populations have established themselves in these areas with greater efficiency and prevalence than would have been expected, given their lack of key evolutionary advantages such as eyesight. Researchers from Spain and Roger Williams University suggest this tendency may be a sign of greater jellyfish numbers in the ocean’s future, which has potential negative implications for many human activities.
Jellyfish were among the first multicellular organisms to appear on Earth and have existed for over 500 million years. Their means of survival have remained mostly constant in the course of the ocean’s changing conditions. They maintain a simple motion that draws water towards them as they propel their bodies through the water. Prey, usually plankton, that come into contact with their tentacles are killed by poisons and digested in a process that may take months, enabling the jellyfish to survive for long periods without food. Jellyfish with larger bodies have increased contact with prey, at the cost of moving more slowly to conserve energy.
On the whole, fish that have similar diets to jellyfish have a survival advantage in their habitat. Traits such as eyesight as well as small, easily maneuverable bodies make it possible for them to hunt food sources, whereas jellyfish must contact their prey by chance. However, the jellyfish is able to compete or take over when fish populations are down thanks to some of its unique qualities. Jellyfish require a low caloric intake, in part because they lack energy-intensive internal structures such as a central nervous system. For many species, the sting of their tentacles is enough to deter most predators, and because of the vast diversity of jellyfish and poisons, immunity is never a guarantee. In addition, jellyfish can be over 96% water by volume, making them much less profitable prey than fish in terms of carbon content.
There has been increasing evidence recently that human activities such as overfishing and pollution could be contributing to a rise in jellyfish populations. Significant surges have been observed in coastal waters that were once rich in profitable species, such as those off of Japan, southwest Africa, the Mediterranean, the Northeastern United States, and elsewhere. After species at the top of the food chain grow too few to sustain fishermen, they often continue to make a living by turning to smaller species, such as sardines and anchovies. These plankton-feeders, in turn, also become depleted in time.
The fishing industries suffer when once abundant species are slower to recover against the growing amount of competition. Beach activities slow down as well, since certain jellyfish species pose risks to humans. There have even been cases of nuclear power plants experiencing costly problems stemming from water filters clogged with jellyfish.
Copyright © 2011 by Marine Science Today, a publication of OceanLines LLC.