Survival food harkens from two distinct histories. On the one hand...
Crabs aren't the smartest creatures, but they're certainly tasty. In a survival situation, these little crustaceans can provide a great source of calories and protein to keep you going. They're also plentiful around most bodies of fresh and salt water.
However, crabs are adept at scuttling away quickly and hiding in crevices, so going after them isn't easy. Ideally, you want them to come to you. A crab trap accomplishes this goal, and can be reset daily to generate a sustainable food source. Fortunately, you don't need a purpose-built device — it's easy to make your own crab trap from simple materials.
In the following video from KH Daily, a group of resourceful kids from Cambodia show how to build a dugout crab trap. A round hole is dug into the clay soil near the edge of a flooded paddy field, and the edges of the hole are smoothed to form a slippery funnel.
A cut plastic water jug is then placed into the hole, and its smooth sides prevent crabs from crawling out once they fall in. Scraps of bait and a border of branches also help lead the crabs into the pit. See the full technique in the video below:
Plastic buckets and jugs can be found even in the most remote towns and fishing villages, so this technique should be viable in almost any coastal location where you can find crabs.