"A fish trap baited with organic material allows you to harvest the oceans bounty."
The Shellfish Trap allows the player to catch Any Shellfish without having to interact with the object.
The Shellfish Trap takes the Purity of the used Fibre Netting.
Placing the Shellfish trap in deep water, so it's submerged, and filling it with bait will yield shellfish over time. The Shellfish Trap must be built on dry land first and then carried and dropped into deep water.
Materials that can be added to a Shellfish trap for bait include Any Plant, Wood Choppings, Pumpkin Flesh, many cooked Foods, Worms, Bone, Meat Shreds, Any Raw Steak, Any Raw Meat Cut, Dead Wartbite Cricket, and Any Fish.
Both the location and the bait seem to matter in how many and which fish you catch.