Echiurans are exclusively marine and the majority of species live in the Atlantic Ocean. They are mostly infaunal, occupying burrows in the seabed, either in the lower intertidal zone or the shallow subtidal (e.g. the genera Echiurus, Urechis, and Ikeda). A few are found in deep waters including at abyssal depths.[8] They often accumulate in sediments with high concentrations of organic matter.
One species, Thalassema mellita, which lives off the southeastern coast of the US, inhabits the tests (exoskeleton) of dead sand dollars. When the worm is very small, it enters the test and later becomes too large to leave. In the 1970s, the spoon worm Listriolobus pelodes was found on the continental shelf off Los Angeles in numbers of up to 1,500 per square metre (11 square feet) near sewage outlets.[9] The burrowing and feeding activities of these worms churned up and aerated the sediment and promoted a balanced ecosystem with a more diverse fauna than would otherwise have existed in this heavily polluted area