Invasion of the Jellies: Unwelcome Visitors to the Black Sea
by Keith Bayha, Ph.D. Student, University of Delaware
Jellyfish and their distant relatives, comb jellies, are fascinating to watch in an aquarium. The beautiful pulsing bell, fringed with delicate tentacles, gracefully sweeps the jellyfish along. Lengthwise bands, cilia-covered “combs” that shimmer in the sunlight, slowly propel the comb jelly through its watery home. Captivating. However, when these same invertebrate animals multiply with abandon in a natural system, the results are far less enchanting.
Jellyfish of the Black Sea
During the last 50 years, the Black Sea ecosystem has changed dramatically. And human beings living around this nearly land-locked sea have played no small role. When it comes to the explosion of jellyfish and comb-jelly populations, over-fishing and uncontrolled agricultural runoff are the apparent culprits. As commercial fishermen took an ever-increasing number of carnivorous fish at the top of the food chain, their prey species, including jellyfish, were able to multiply with little resistance. In the late l960s and l970s, mackerel, a major predator, were eliminated from the Black Sea ecosystem, which may be why severe outbreaks occurred of a formerly uncommon jellyfish, Rhizostoma pulmo. The same animal seen gliding through a tranquil pool of water is not so attractive when heaped in decaying piles along favorite bathing beaches.
The second stimulus for jellyfish (scyphozoans) and comb jelly (ctenophores) outbreaks in coastal areas of the Black Sea has been the release of excessive nutrients into its waters through agricultural runoff. Both fertilizers and animal wastes, when carried by rain into creeks, rivers, and ultimately, the Black Sea, provide an abundant source of nutrients — compounds containing nitrogen or phosphorus, which promote the growth of algae and other aquatic plants. When excessive plant growth occurs in response to uncontrolled nutrients entering a body of water, the process is known as eutrophication. Eutrophication, when severe, as it has been in the Black Sea, results in the depletion (and even absence) of oxygen in the water. Low-oxygen aquatic environments are less likely to sustain large and diverse fish populations.
Though there has been some recent improvement in the health of the Black Sea ecosystem, a major trend has been the replacement of several large, economically important fisheries with a system dominated by the gelatinous zooplankton (drifting animals) we call jellyfish and comb jellies. Both ctenophores and scyphozoans thrive on the same planktonic food that has supported the sea’s larval and adult fish populations. Apparently, the jellies compete very well for that resource.
Jellyfish of the Black Sea
Two members of the scyphozoan phylum that have made dramatic appearances in the Black Sea are Rhizostoma pulmo and Aurelia aurita. R. pulmo, mentioned earlier, is native to the Mediterranean, North Sea, Black Sea, and Sea of Azov. Feeding on zooplankton plus the eggs and larvae of fish and shellfish, this jellyfish was not formerly considered a big player in the Black Sea’s animal community. In fact, in some parts of its range, this jellyfish was considered beneficial as it offered protection to certain fish larvae and served as food for sea turtles. However, in the late l960s and 1970s, conditions in the Black Sea enabled its numbers to swell to such an extent that it proved a great nuisance on area beaches.
A. aurita, commonly called the “moon jelly” for its pale, disk-like appearance (as shown in the illustration above), occurs worldwide. In the 1970s, moon-jelly populations surged so powerfully in the Black Sea that scientists claimed this species of jellyfish, alone, was consuming 62% of all zooplankton produced in that body of water — food that previously supported important fisheries. Today, moon jellies are still extremely common over the Black Sea’s open waters.
Comb Jellies of the Black Sea
Though the jelly-like comb jellies are classified in a different animal phylum than jellyfish (which signifies a distant relationship), they also share the jellyfish's fondness for zooplankton, along with the eggs and larvae of fish and invertebrate animals. Ctenophores commonly found in the Black Sea are Pleurobrachia pileus, Mnemiopsis (different species), and Beroe ovata.
P. pileus, which uses two tentacles to grab food, occurs mostly in deeper, low-oxygen water, where it enjoys large quantities of Calanus, a tiny, shrimp-like creature known commonly as a copepod.
Mnemiopsis is a comb jelly native to the Atlantic coastal region between Massachusetts and southern Argentina. In its natural range, it has been known to heavily impact ecosystems by consuming vast quantities of zooplankton, eggs, and the larvae of fish and invertebrates that would otherwise support populations of more desirable species. Mnemiopsis was first observed in the Black Sea in the early 1980s, where it is thought to have been transported and dropped by the ballast water of freighters arriving from distant shores in the western Atlantic.
By the late 1980s, populations of Mnemiopsis soared, populations of its favored zooplankton food plummeted, and, coincidentally, commercial catches of the zooplankton-eating anchovy (Engraulis encrasicolus) were drastically reduced. A few years ago, Mnemiopsis was accidentally introduced into the Caspian Sea, where a similar impact is beginning to be seen.
Beroe ovata also is native to the same general region as Mnemiopsis. However, unlike most ctenophores, Beroe shuns crustacean zooplankton, but prefers dining on other ctenophores. Interestingly, the primary food for Beroe ovata is Mnemiopsis. Considering that Beroe was first detected in the Black Sea in l997, this relationship offers one explanation for why Mnemiopsis populations have declined in recent years. It is now thought that purposely introducing Beroe may be a potential solution to the problem of Mnemiopsis in the Caspian Sea.
For a thorough review of the trends in jellyfish blooms in anthropogenically
impacted regions, please see Claudia Mills' paper at http://faculty.washington.edu/cemills/jellyblooms2001.pdf.
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