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.
sea urchin
The classification of a Sea Urchin is Echinoidea
sea urchin
A sea urchin does move, but not very frequently.
because the crab needs the sea urchin for protection and the sea urchin needs the crab for food
Ummm...it's the thing from which a sea urchin hatches?
I think a sea anenome and a sea urchin can live together because i have a little aquarium and there is a sea urchin and and a sea anenome (if that's how you spell it) living in there and they were perfectly fine. BUT if you have a sea urchin do not have any crabs in there, because my sea urchin killed one, and almost killed another one by taking its claw off. :(
a blue tuxedo sea urchin
Spines of the sea urchin can cause injuries of the skin.
It urges the sea to adapt to IT! The litle urchin urger.
A (street) urchin is a child who lives on the street, surviving by engaging in petty crimes. A (sea) urchin is a spiny creature (exoskeleton), with a soft interior. I welcome improvements to this answer.
kelp crab is stronger