They be Multiplyin' (Jellyfish)

A Comb Jelly is capable in theory of starting its own population because it can release 8,000 eggs into the water per day. The Comb Jelly is known for invading the Black Sea, the most polluted ocean in Europe with no natural predators of the Comb Jelly, and suffering from overfishing. Jellyfish can multiply quite quickly. ¹²

What makes them Grow Grow Grow?:
A variety of factors make make jellyfish reproduce much quicker and more efficient compared to the conditions before.

Food Supply:
An abundant food supply can allow a jellyfish to double its weight daily, allowing it to reproduce shortly after being born.²

Reproduce Quickly:
Female jellyfish can release tens of thousands of eggs per day. In addition, the fertilization of an egg and sperm produces a bottom dwelling polyp that is capable of reproducing by budding, creating even more jellyfish. ²

Able to Live in Low Oxygen Areas:
Jellyfish are one of the few sea creatures that are capable of living in Dead Zones. The Dead Zones have very low oxygen levels, but jellyfish can still thrive. They can dissolve oxygen in their tissues from other areas, bringing their own oxygen supply into a Dead Zone. Being made of 95% water, a jellyfish does not need as much material to survive as other sea creatures. In addition to not requiring as much oxygen, jellyfish are also  capable of surviving for weeks without any food.²

Dead Zones:
Jellyfish also lack predators and competitors in Dead Zones that would control their population. Jellyfish don't survive in Dead Zones, they thrive. There are over 400 Dead Zones in existence, many inhabited largely by jellyfish.²


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Resources: 
¹ Black Sea by the National Science Foundation
² Bloomin' Magic by the National Science Foundation