The problem of adjusting to constantly changing water levels is not unique to sea urchins. All intertidal organisms face similar challenges. Most of these organisms function best when submerged, so the main threat they face is dessication when the water recedes. They prevent it by trapping as much water as possible, either in burrows or within their bodies, and sealing off all possible avenues for water loss. Urchins are at an advantage in having a hard, relatively impermeable exoskeleton that retards water loss. Most sea urchins occur in pools in the infratidal zone and are able to remain submerged even at low tides.