Nomura's jellyfish.

This Jellyfish, living near the coasts of China and Japan, is tied to be the largest species of jellyfish (the other is the Lion's Mane Jellyfish). They can grow up to 2 meters across its diameter and can weigh up to 450 lbs. Crazy.

To my knowledge, they sting but are not known to be deadly stings. Perhaps the threat is the fact that they travel in swarms (see picture 1). Currently, these swarms are presenting a problem to Japanese fishermen, as they are expecting their 3rd large swarm of jellyfish in 4 years. This is a problem not because they are some sort of terrifying predator, but they ruin catches. Their weight and quantity break nets, allowing large catches to escape -- considering these nets can be up to 100 meters wide. So people intentionally try to catch and kill them to lower their population, but here comes their genius defense system:

Like all living things, the biological 'success' is to have offspring -- this jellyfish achieves this by releasing millions of sperm and eggs whenever it is attacked. Since they travel in swarms, that means their sperm and egg will likely meet other individual's sperm and egg. So while it in essence doesn't 'defend' itself, in the grand scheme of things, it does. If one wants to attack it, it will have more to deal with, hence not really being able to defeat the numbers. In the case of the fishermen, when they try to kill off jellyfish, they just form new polyps that will later return in larger numbers.

But the question remains, why are there so many suddenly showing to begin with? Here comes the catch-22 (this works as a pun, since it deals with catches): Overfishing. When people catch too many fish in an area, the ecosystem obviously will change a little bit (at the very least). Combine this with people killing sea turtles, and you have a jellyfish dilemma. The reduced number of fish means fewer fish are eating these jellyfish and their polyps, therefore actually managing the population (the polyps cannot defend themselves by dropping sperm and eggs as they are not fully developed). Turtles also eat the jellyfish farther out in the ocean, meaning that the polyps tend to stay away from the shores. However, since the swarms drift closer to shore since there is nothing killing them, we get large numbers of mature jellyfish and hence larger numbers of offspring.

So, while we don't want people to stop having an income (this is village fishing, not large industrial ships) so they can take care of their family, slowing down would help them maintain a fish population and will be able to catch more fish since fewer swarms of jellyfish will ruin their ventures. But it's difficult to expect struggling people to see this as a long-term issue when those who are doing well are struggling to do the same.

Here is a video from Arkive.org of a turtle feasting on one.