Within UNDERWATER HIGHWAY's SEACOW FARM, OMORI and friends can talk to FARMER JIM inside of the SEACOW BARN. From there, FARMER JIM will ask the group to defeat all the SQUIZZARDS on his farm.
For each SQUIZZARD vanquished after talking to him for the first time, FARMER JIM rewards the party with 500 CLAMS. This quest is finished after defeating one SQUIZZARD, but you can repeat it as many times as you like for multiple CLAMS.
If the player has defeated a total number of 100 SQUIZZARDS, then the player will be rewarded with the "Squizzard Exterminator"achievement.