From Nookipedia, the Animal Crossing wiki
The Spider Crab is a rare deep-sea creature found at the bottom of the sea in Animal Crossing: New Leaf. It is indicated by a circular shadow, and diving is the only way of capturing it.
Donating to the Museum in New Leaf
The information board in the aquarium will list information about this deep-sea creature:
"Known for their long legs, adult male spider crabs can have leg spans of over three yards. This actually makes them the largest species of crab in the world. They are thought to have lived in ocean bottoms for tens of millions of years without changing shape. Their vivid-orange bodies feature white spots, making their coloration truly beautiful."