Why do adult cephalochordates bury themselves in sand or gravel?

Adult cephalochordates, also known as lancelets or amphioxi, bury themselves in sand or gravel primarily to facilitate their lifestyle as suspension feeders. By remaining buried, they can position their anterior end—specifically their mouth—above the surface of the substrate.123

This positioning allows them to effectively filter food particles from the surrounding water. While buried, they draw water into their mouth through the beating action of cilia on their gill slits; the pharynx then traps suspended food particles in a sheet of mucus for digestion. Although they are capable of swimming, they spend much of their time in this buried state, emerging to swim near the bottom, particularly at night.2314