I was going to try that next, but luckily, when I went to check things, one of my neighbors had enough "extra" shrimp to share to complete what I needed. But it's good to know for future reference. Thanks.
The quest is called "Poem For The Soul" and it is step three. I just thought it was odd you got the option to buy shrimp, but nowhere to plant it. I harvested lighthouse, but I'm not sure if it gave me anything.