Flatworm

The flatworm is a sea creature you catch while swimming in the sea (dive where you see bubbles). Look for a tiny shadow. In the northern hemisphere look for it in Aug – Sep, from 4 PM – 9 AM. Nook's Cranny pays 700 Bells.

A tiny, very slow shadow from late afternoon until morning. It barely moves, so it is an easy catch during its short run.