Simple creatures, the translucent lobate ctenophores are small, walnut shaped sacs of jelly with only a couple of specialized organs. These organs are used to detect chemical traces in water to help locate food and a statocyst to help the "sea walnut" determine its altitude in the water column. The eight rows of small comb like cilia and the sting free tentacles, the beautiful iridescent highlights and natural bioluminescence make these animals quite photogenic.
Although more complex than sponges, long thought to be the first branch on the animal family tree, recent statistical DNA research shows that the more complex comb jelly branched first. Perhaps the jellies developed their complexity independently after the split or maybe the sponges were already farther along in the development of tissues and such, but devolved at a later period after the comb jellies had branched off.
The comb jellies are also invasive, voracious predators with high rates of feeding, growth and maturation. These self-fertilizing hermaphrodites have a tremendous rate of reproduction, laying 3,000 to 7,000 eggs per day.
Due to factors of geography and water flow, the Black Sea is naturally anoxic, a "dead zone", creating the perfect environment for algae growth. In 1982 the comb jelly, Mnemiopsis leidyi, was accidentally introduced to the Black Sea from its native habitat around the east coast of the United States. Zooplankton eat algae and "sea walnuts" eat zooplankton. Having no natural predators in this environment the jellies were able to out-compete the local species and become ninety percent of the Black Sea biomass by the early 2,000's, causing massive negative impact throughout the marine food chain.
The decrease in zooplankton caused a cascading collapse of populations of plankton eating fish and the larger predatory fish as well as a complete disappearance of dolphins. Anchovy fishing in the Azov Sea, like most fisheries world wide, was already weakened by over fishing and pollution when the comb jellies invaded and commercial fishing ended completely. Since the invasion the financial losses are in the area of $350 million dollars.
Autumn of 2006 found these same invaders in the south Baltic Sea and by 2007 they had populated the northern parts. These prolific comb jellies had increased their mass by another fifty percent as of late 2008.
Not all the news is bad though. In 1997 another comb jelly, Beroe ovata, was accidentally imported to the Black Sea from the same area of the east coast of the United States that the invading Mnemiopsis leidyi occurred naturally. The newcomers only food source at home was M. leidyi and its introduction to the concentrated population of its favorite snack allowed it to thrive. Signs indicate that the Black Sea ecology is beginning to rebalance and the numbers of M. leidyi are on a steady decline. Plans to introduce B. ovata to the Azov Sea with hopes of similar results are in the works.
Simple though they are the comb jellies have an amazing regenerative ability. These creatures are able to effect minor repairs, without scarring, in minutes to hours depending on the amount of damage. Even massive trauma where fifty percent of tissue is lost can be regenerated. This ability makes comb jellies of special interest for research into regeneration as well as investigations into bioluminescence.
A hardy and adaptive organism, the "sea walnut", thrives in the three percent salinity of the Azov Sea as well as the thirty nine percent saline of the eastern Mediterranean Sea. Comb jellies have been found everywhere from the surface of the ocean to the recently discovered kite-like ctenophore anchored at 7,200 meters deep in the Ryukyu Trench.