What is the most dangerous sea urchin?

Answer 1

The most dangerous fish in the ocean is the piranha.

The piranha is a Freshwater Fish for Christ's sake!

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There's no such thing as one "most dangerous fish."

In venomous fish, the stonefish stands above all. Its spines pack a poison that can kill a human who steps on it, and the thing looks like a rock so you're likely to.

In predatory fish, there are a lot of contenders but I'll have to go with the shortfin mako shark, which is a seriously predatory animal.

Those are both marine animals. In freshwater fish, I give the nod not to the piranha--a fish not considered the most dangerous even in the Amazon River basin where it lives (people there consider the candiru, a catfish known for lodging itself in people's urethras, the most dangerous fish in the Amazon) but to one of two pangasiid catfish, the Paroon shark (p. sanitwongsei) or the Mekong catfish (p. gigas). These are huge fish--the Paroon shark grows to 10 feet in length and the Mekong catfish is larger--that are renowned for eating dogs and for tearing the guts out of fishermen with the spines in their pectoral fins.

Answer 2

I believe that it's the Moray eel.