Sea scorpions had strong defences - spines, claws and armour plating. They walked on six legs, the back two of which were flattened into paddles. Out of the water they were cumbersome but could swim a little underwater.

They were normally sea-floor dwellers but they could also live in freshwater and on land. Megalograptus browsed the seafloor looking for fish, trilobites and other animals in the sand and mud. It also ate its own kind.

Megalograptus could only grow in size by shedding its hard shell and growing a new one. While it was naked, it gathered with others in shallow water for safety in numbers. During moulting, it took advantage of its briefly soft body and abundant nearby neighbours to mate.