This is part two of a series about mnemiopsis jellies. Read part one first to understand how these jellies are indiscriminate feeding machines.
Leidy’s comb jelly feeding efficiency has been on display in the Black Sea since 1982, thousands of miles from their native ecosystem. That's after an unknown ship picked up ballast water containing Leidy’s comb jellies in the western Atlantic Ocean. In order to maintain stability during transit in the open ocean, large ships often carry millions of gallons of ballast water. Upon reaching its destination in the Black Sea the ship dumped this water, therefore unintentionally introducing Leidy’s comb jellies to an area that was not endemic to this species.
|Comb jellies at the New England Aquarium | Photo: S. Cheng|
In their native waters of the western Atlantic Ocean Leidy’s comb jelly populations are held in check by a variety of predators, including fish, turtles, sea jellies and other ctenophores. Without natural predators to balance their population in the Black Sea, their numbers soared. As a consequence, there was a massive decline in the anchovy (Engraulis encrasicolus) population in the region, which was widely blamed on the newcomers.
|A shoal of European anchovies (Engraulis encrasicolus) | Photo: Etrusko25 via Wikimedia commons|
Indeed, the comb jellies’ ability to capture and eat a wide variety of prey launched a twofold attack on the Black Sea anchovy population. First, the comb jellies competed with both adult and larval anchovies for food. Anchovies feed predominantly upon zooplankton, which is also on the menu for comb jellies. Secondly, the comb jellies were able to consume the anchovies’ eggs and larva, driving the population into further distress. As a consequence of this foreign invasion, the anchovy population declined, and the commercial catch plummeted; the Black Sea anchovy catch dropped from over 300,000 tons in 1982, to under 100,000 tons by 1990.
|This ctenophore, likely Beroe ovata, resembles a swimming sac. They lack adhesive cells for prey capture, and instead use their forward directed mouth to engulf prey whole (video). They feed voraciously on other comb jellies.|
The story of these comb jellies continues as a tale of bio-escalation. In 1997, the predatory ctenophore Beroe ovata found its way to the Black Sea. Like Leidy’s comb jelly, B. ovata naturally inhabit the western Atlantic Ocean, and it is likely they were also accidentally introduced through a ship’s ballast water. While Leidy’s comb jelly can feed upon a wide range of prey species, B. ovata are a rare case of very narrow food specialization; their primary food source is Leidy’s comb jelly.
Due largely in part to the introduction of B. ovata, Leidy’s comb jelly populations are now reduced in the Black Sea, though they still remain a dominant entity in the pelagic community. Fortunately, Black Sea anchovies have an enormous reproductive potential, and the population is showing signs of recovery.
|The lines within this Mnemiopsis leidyi are the wormlike larva of the lined anemone (Edwardsia lineata). Their mouths extend into the comb jelly's digestive cavity, robbing it of energy.|
Being engulfed by predatory B. ovata is not the only concern for Leidy’s comb jellies; in their native waters they are also subject to parasitism. The lined anemone, Edwardsia lineata, resembles most any other anemone in their adult phase. Anemones typically spawn by releasing eggs and sperm that combine to form free-living planulae. These planulae eventually settle onto the ocean floor or other hard surface and metamorphose into adult anemones. In the case of lined anemones, their planulae seek out Leidy’s comb jellys and parasitize them. The planulae dig into the ctenophore and reside in its body wall. Once safely within the ctenophore, they develop long wormlike bodies, and extend their mouths into the comb jelly’s digestive cavity, where they feed off of the food that the ctenophore collects. While not fatal to the ctenophore, the parasites restrict the food intake and energy available to the comb jelly. The larval lined anemones will eventually leave the ctenophore, settling to the ocean floor to become adults.
Read previous posts in Peter's Local Species guide: