In which oceanic zone do clams and crabs survive by burrowing in the sand?

Mesopelagic zone

Explanation:

The Mesopelagic zone, also referred to as the Twilight zone, is the second oceanic zone from the top, lying just bellow the Epipelagic zone. It is occupying the waters at depths between 200 meters and 1,000 meters. The sunlight is only managing to reach the top few meters of this zone, while the rest is not receiving any sunlight, thus this zone is not supporting the plant species because they need sunlight for the process of photosynthesis. There are lot of animal species that live in this zone though, and they are ll very well adapted for it, often having hard shells or heavily scaled bodies, larger lungs and heart, and very well developed gills. Some of the animals that live in here are the crabs and the clams. They have developed all the needed characteristics to live in this zone. Because they are prey to lot of larger animals, both the crabs and the clams have started to use the soft sediments at the bottom as cover, digging themselves below it, and mostly managing to evade trouble and end up as food for the other species.

a. by using fur or clothes to trap heat energy near the body

not only is it colder, it also becomes darker, less salty, and more dense.