Final answer:
The introduction of the comb jelly Mnemiopsis leidyi into the Black Sea had a negative impact on the ecosystem, causing a decline in native species and the collapse of fisheries.
Step-by-step explanation:
When the comb jelly, Mnemiopsis leidyi, was accidentally introduced into the Black Sea, it had a significant impact on the ecosystem. Comb jellies are voracious predators that feed on plankton, which disrupted the food chain and led to a decline in native species. The introduction of Mnemiopsis leidyi caused a collapse of the Black Sea's fisheries, as important fish species drastically decreased in numbers.