Commonly called the warty comb jelly, Mnemiopsis leidyi is native to the waters found off the east coast of North America.
PHOTOGRAPH BY Erickson Smith An invasive comb jelly, Mnemiopsis leidyi, hitched a ride in the ballast tanks of ships from the east coast of North America, ending up in the Black Sea. Since arriving in 1982, the jellies have turned from harmonious North American dwellers to voracious predators eating away at fish stocks to within a smidge of collapse. What has become of the Black Sea? Through alluring rhythmicity, the comb jelly, M. leidyi, effortlessly beats its bodily combs to glide between the ripples of the deep and shallow oceans with a gentle touch of tranquillity. As it moves, a radiance of bioluminescence permeates far and wide through the comb jellies’ body creating a hypnotic spectacle. But don’t be fooled by their show as for in the Black Sea this species has become to be known as the villain. Small, transparent, and slow, their appearance fabricates a misguided underestimation of their powers in the ecosystem. Their arrival to Southeast Europe was unexpected but in 1982 the comb jellies were discovered in the Black Sea. Although native to the east coast of North and South America, the aquatic species were transported in the hidden ballast tanks of ships. Ballast water is fresh or saltwater that is held within the cargo holds of ships providing stability and manoeuvrability during a voyage. However, it also provided a safe passage for the comb jellies’ insatiable appetite. Before the comb jelly invasion, the Black Sea was already facing the consequences of high salinity and pollution levels, but the arrival of M. leidyi was the fishing industry’s last wish, a final nail in the coffin. The gelatinous comb jellies asserted pure dominance over the sea. In 1989 the species reached suffocating biomass levels of more than 1kg per m2; bear in mind they are only 5 inches in length. Before long, the species population decreased within the Black Sea, instead venturing to new waters. What impact did the comb jellies have on the Black Sea ecosystem and where are they now?
The warty comb jelly is a ravenous carnivore eating up to 10 times its weight a day opportunistically feeding on the Black Sea’s native zooplankton species. These constitute small aquatic microorganisms such as crustaceans, rotifers, copepods, jellyfish, and even other comb jellies. By beating cilia near its mouth, the comb jelly pumps a flowing current of water into its body cavity where hapless prey is stuck to adhesive cells called colloblasts. The victims are then transferred to the mouth to be devoured. As the insatiable comb jellies feast through these various planktonic species, they disrupt the sea’s natural food web. Zooplanktonic species feed upon the endless mass of algae keeping numbers in check, but as their numbers have dropped frequent algal blooms have materialized. Eutrophication has been rife leaving behind a blemished ecosystem with decreased oxygen, increased turbidity, and an overall loss in biodiversity. The most profound losses were felt in the Black Sea fishing industry. Not only do the comb jellies decimate the zooplankton that fish such as the anchovy depend on, but they also indulge upon their eggs and larvae. Consequently, hundreds of local fishermen who depended on this livelihood lost their jobs as profits were selfishly munched away. Yearly catches of 400,000 tonnes rapidly plunged to 40,000 a year as catch rates were slashed by 90%. The economic hits followed. Profits from fisheries dropped from $17 million to $0.3 million per year leaving an industry clawing at scraps.
Local fishermen start their work early in the morning on the shores of the Romanian Black Sea. PHOTOGRAPH BY Anca Milushev The comb jellies’ mastery of the Black Sea is dependent on multiple factors. Since escaping its native coastal waters of New England the comb jelly escaped the most dominant force: predation. Butterfish and other species of comb jellies keep M. leidyi in check stopping their numbers from uncontrollably proliferating. Fortuitously the Black Sea lacked any species with an appetite for M. leidyi leaving them in a protected solace with the native species at their mercy. The very speed of the comb jellies bloom is another secret to its success. They are self-fertilizing hermaphrodites meaning they possess both sexual organs and can reproduce on their own. What’s more, they are seen to reproduce rather successfully even in harmful conditions. M. leidyi can produce eggs for up to 12 days without any food with hatchling success as high as 90%. It’s the perfect strategy for rapid colonization. A more recent recipe for the comb jellies triumph proposed by certain scientists takes a rather sinister twist. Leave an adult M. leidyi without food for some time and put them with their own larvae, before long the comb jellies’ cannibalistic side kicks in. M. leidyi are seen to devour their own offspring. It’s been suggested that this phenomenon is a way for adults to survive the harsh winter conditions. As thousands of larvae are produced each summer, this booming population before long hoovers up all the zooplankton in sight, leaving the winter months gruelling and barren. Careless as it sounds, these unfortunate rejected offspring become a lifeline stored meal come winter for these merciless parents.
The water currents created by M. leidyi to feed and hunt are undetectable by their prey. PHOTOGRAPH BY Shutterstock M. leidyi has devastated the Black Sea. They have eaten their way through essential fish stocks and led to widescale eutrophication and water quality decline, but is there any chance of getting rid of them? It appears the answer may lie with another invasive species. An ideal scenario would be the arrival of a species that could predate upon M. leidyi and in 1997, that’s exactly what happened. Unbeknownst to us, we introduced another species of comb jelly to the Black Sea called Beroe ovata. Off their native American coast, B. ovata hunts the inferior M. leidyi, being responsible for controlling their numbers. Since we accidentally introduced B. ovata to the Black Sea, also supposedly through contaminated ballast water, their impact has been heroic. M. leidyi abundance instantly decreased followed by a dip in B. ovata numbers showing its dependence on feeding on the other species. What also followed was an increase in zooplankton numbers, anchovy catches and anchovy egg densities. This mistake appears as a potential lifeline to revive and recover a suffocated ecosystem.
The Black Sea is a marginal Mediterranean Sea that lies between Eastern Europe and Asia. PHOTOGRAPH BY Zheltukhin Aleksandr The story of the Black Sea is one of hypocrisy and forgetfulness, as the ecosystem heals, other seas are just starting to face the consequences of this comb jellies spread. M. leidyi has not only infested the waters of the Black Sea but has recently slipped silently into the Caspian, Aegean, Baltic, Adriatic, and North seas. The introduction of new invasive species can be relatively invisible to us, and only small establishing populations are required to decimate ecosystems. Could it be that the same devastation in these new seas could occur on the horizon? We are yet to tell and find solutions, maybe another invasive species may slide under our guard and come save the day.
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