Misadventures with comb jellies
Looking through the mask, it appears as if an alien lifeform is trying to communicate with the divers, flashing its light signals from eight rows of eyelashes. Indeed, an encounter with a comb jelly (also called a ctenophore) is an unforgettable one, though the species has caused problems in the past.
Although comb jellies behave delicately, they are decisive predators when on the hunt for small organisms like plankton. Alternatively, they may decide to catch a small fish, worm or larva. In such cases, the prey is swallowed directly or captured by the tentacles, which have special adhesive cells called colloblasts that hold on to the prey. Once consumed, the prey is then digested by enzymes in the stomach.
There are more than 100 species of ctenophores in a wide range of sizes (up to 1 metre) and forms. Some colonise the seabed, like the subspecies Tentaculata (found in the western Pacific and resemble an anemone).
In the early 1900s, the first comb jelly (Mnemiopsis leidy) made its first appearance in the Black Sea, after probably as stowaways in the ballast tanks of a ship from America. At first, the comb jelly invasion was unnoticed, so they were allowed to flourish without any control. It had the ability to double its weight within 24 hours; hence, by 1990, the total biomass of these animals in the Black Sea had increased to about one million tons.
Other estimates put the biomass at one billion tonnes. The effects of this invasion was dramatic. Having up to 240 jellyfish per cubic metre of water had major effects on the ecosystem and fisheries. The jellyfish clogged up the fishing nets and ate anchovy larvae. As a result, within a decade, the anchovy stocks plummeted, with the catches dropping to below ten percent. The fishermen in the Black Sea were in dire straits.
In the 1990s, relief came in the form of another comb jelly that had possibly arrived in those parts as a stowaway. The Beroe ovata attacked the first invader and reduced its population dramatically. This allowed the anchovy stocks to recover. Since then, there have been two new types of comb jellies living in a delicate balance in the Black Sea — with yet unknown consequences.
This situation is comparable to what had taken place in the Caspian Sea when the comb jelly had been probably introduced via the Volga-Don Canal. Perhaps the Baltic could flourish again in the coming years with the help of the introduced species from North Atlantic.