When the comb jelly, mnemiopsis leidyi, was introduced into the black sea, its population exploded to 500 comb jellies per cubic yard in 1988. the jellies devoured all the zooplankton and fish larvae, drastically reducing the numbers of the native fish populations. this situation demonstrates

The answer is invasive species.

Invasive species are species that are not native to specific ecosystem, but they have a capability to spread widely through the ecosystem, This way they can threaten native species of the ecosystem or cause environmental harm. 

After being introduced into the Black Sea, they greatly affect local ecosystem. In the new area, they have no natural predators and are able to spread. If they spread beyond the area they are introduced, they become invasive species which can overgrow native biotopes and are a great threat to the local biodiversity. This could lead to the extinction of some native species.

the answer is d (apex) good luck : ))