From Nookipedia, the Animal Crossing wiki
The Wharf Roach (フナムシ, Funamushi) is a new insect in Animal Crossing: New Leaf. They can be found on the beach where it quickly scurries away when the player comes close and vanishes if chased too far.
Donating to the Museum
Upon donation to the museum, the Wharf Roach can be found scurrying near the pond.
The wharf roach is an isopod that lives along coasts and shorelines. It is generally dark gray with brown appendages. They can grow up to 4 centimeters with the males being bigger than the females. It lives in crevices located in rocky coasts just above the splashing water. Its diet is mainly plant matter including diatoms and scavenging plant remains such as detritus.
More information on this topic is available at Wikipedia.
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