Giant isopod

The giant isopod is a sea creature you catch while swimming in the sea (dive where you see bubbles). Look for a medium shadow. In the northern hemisphere look for it in Jul – Oct, from 9 AM – 4 PM, 9 PM – 4 AM. It can only appear after you have caught 80 sea creatures. Nook's Cranny pays 12,000 Bells.

A Medium shadow that darts off very fast, around midday and late at night, after 80 catches. Dive right on top of it and tap A to swim faster; it sells for 12,000 Bells.