The predator hiding under the sand turns out to be a large flounder, which becomes apparent when it gobbles up the crab. The easiest way to catch it would be to throw one of the kunai he brought, but Naoki drifts along carefully, barely moving.

When he's right over the fish he grabs it by its side, using a little chakra to hold onto the slippery fish as it kicks up sand and tries to get away.