ENHS Nature Trails December 2004 Article 3

A Salty Thought
by Reida Kimmel

Last August Chuck and I visited the New England Aquarium in Boston Massachusetts. We saw wonderful displays of fish from all over the world, colonies of very noisy but adorable penguins, and an enormous circular tank of warm water Atlantic species, including sharks and huge sea turtles. We saved the much advertised jellyfish exhibit until last, expecting subtly lit tanks of these beautiful luminous creatures gliding somnolently through the water. What greeted us instead was a biology lesson about a fact of global warming of which we had never heard.

 The creatures that we call jellyfish are among the oldest and the simplest multicellular animals. The two phyla of these soft-tissued animals are almost all marine and are all carnivorous. They capture their prey with stinging cells called nematocysts. Food enters the oral cavity, and the digested remains exit the same way because these simple creatures have no anal opening. Instead of three layers of cell types forming the bodily organs, the jellies have only two. Between the ectoderm and the endoderm there is a jelly-like layer of loose cells, the mesoglea. Jellyfish have no central nervous system, only nerve nets. We know the Ctenophora as comb jellies, delightfully luminous. They never sting us. The coelenterates, on the other hand, include the tentacled medusae like the sometimes deadly man o' war and Australia's fearsome box jellies. Cyanea, the sea blubber, is the Goliath of the jellyfish, three and a half meters wide, trailing tentacles thirty meters long.

For all their simplicity and ancient tenure on this earth, however, jellyfish are very sophisticated in their survival strategies, able to reproduce both sexually and asexually, and able to feed greedily on whatever comes their way. And herein lies our scary tale. With the advent of global warming there has been ocean warming. Dr. Barbara Sullivan has been studying the decline of tautog (a smallish fish living near pilings, shipwrecks and rocky reefs) and other fish species in Narragansett Bay, Rhode Island. Before the recent oceanic warming, larvae of fish and copepods* could coexist successfully with jellyfish, because the jellyfish were not active in the months when the water was cold. The small larvae had a several months head start, growing and developing in the colder water to which their early development was attuned. Now the water warms much earlier in the spring and the jellies (especially Mnemiopsis leidi, the sea walnut, a native comb jelly about which we will have more to say later) have more time to feed and reproduce and feed some more.

In the Gulf of Mexico the cannonball jelly, the moon jelly Aurelia aurita (which is native and abundant on our coast too), and two alien species, the Mediterranean big pink jelly Drymonema dalmatinum and the Australian spotted jelly Phyllorihiza punctata, have become so phenomenally numerous that they fill the nets of the shrimp trawlers to overflowing. There are few shrimp to be found in the mounds of gleaming soft pink jellies that the fishermen haul aboard. If warmer water in the temperate latitudes is contributing to the decimation of larval fish and crustacean populations, the introduction of alien jellyfish populations via discharged ballast water from ships' holds is perhaps equally pernicious.

In the Black Sea our native sea walnut, Mnemiopsis leidyi arrived with dumped ballast in the 1980s. It had no predators and its population soared. Previous plankton-rich environments became watery deserts. Without plankton, the food chain is broken. The sea walnut has a predator however, the Beroe comb jelly, Beroe cucumis. This species has recently arrived in the Black Sea and is also thriving. Biologists hope that the Beroe comb jelly will reduce the population of sea walnuts , and that with the establishment of some sort of balance, the Black Sea plankton will recover. Who knows? I find it so scary to think that this ancient, possibly pre-Cambrian life form, might profoundly alter life in the sea as we know and depend upon it. But after all, it's not the jellyfish who are to blame, is it?

* Editor's Note: These crustaceans comprise the largest biomass in the oceans as well as provide the largest source of protein for much of the fish caught commercially--the fish we love to eat. Even whales feed on them. Copepods can live almost anywhere there's water. "Predatory freshwater copepods have been successfully used to control pests like Dengue fever." And, "trillions of little copepod guts produce countless fecal pellets contributing greatly to the marine snow and therefore accelerating the flow of nutrients and minerals from surface waters to the bottom of the seas."
http://www.uni-oldenburg.de/zoomorphology/Biology.html
These guys are considerably more important to us than the top of the food chain.

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