Crab traps resemble square or rectangular boxes and are made of wire or chain-link mesh. The smallest and cheapest crab trap is meant for recreational, sport crabbing, and it functions as a metal substitute for a crab net. When the trap is lowered into the water and hits the bottom, the release in tension causes two or more of the sides to open and admit crabs. The crabber sits above the trap on a pier or in a boat, and when he feels a tug on the line or has waited 10 to 15 minutes, he pulls up the trap to see if it has any crabs. Pulling in the trap draws the sides back up, locking crabs inside.
Commercial traps are also square or rectangular boxes, but they operate in a slightly different fashion. This is because they are dropped off by a fishing boat, left on site, and retrieved later. These traps have an opening that is the right size to admit the desired type of crab, but the opening is shaped in a way that won't let the crabs back out through the same opening. Smaller crabs might also get into the trap, but they can escape if they want to. Some traps allow for these openings to be refitted for different types of crabs. The smallest examples might be made out of heavy duty plastic instead of steel mesh, and these are sometimes used by recreational crabbers. At the other end of the spectrum, traps used by Alaskan commercial crab fishermen are so large that they can weight over 800 pounds each when completely outfitted.
Crab fishermen in the Chesapeake Bay or Gulf of Mexico will often use fish heads or chicken necks to bait their traps. Alaskan crab fishermen will use chopped up fish such as herring.
A boat captain will travel to crab-habitat area and drop the traps over the side. Each trap will have a line tied to it, and a marker float tied to the line. This allows the fishermen to later find and retrieve their traps. The traps are left on the sea floor for anywhere from one day to a few days, depending on the crab species sought. It's important not to leave traps on the bottom for too long. Hungry crabs are known to attack and eat one another.
Big traps of the kind used in Alaska often have a bio-degradable section in their mesh walls. This is so that if a trap is lost at sea, it does not ensnare crabs to no useful purpose, damaging the overall crab population. However, many smaller crab traps have no such feature. Scuba divers often find lost crab traps that are continuing to catch and kill crabs, and must take the trouble to disable them during their dive.