Careful prodding made it easier for the group to see the crabs. Specifically, Clark took the lead to push forward since he knew that Ralph would not be able to easily break the shells of the water tempered crabs with his axes.
He naturally brought out a hammer he used to forge, but Clark still stopped him. It was because he was worried that Ralph would end up breaking his hammers. This was just how tough the shells of the water tempered crabs were.
When they crawled over the rocks and even in to the crushing falls, everyone in the group could spot them moving as if they were just out for a walk. It was odd to see that the water which could break and splinter tougher trees was doing nothing. At most, it just slowed them down and gave them a good wash.
They didn't even move as Clark used his spear to flick one on to the shore from the rocks. He was splashed with the drops of water from the water fall and could tell that they had some force behind them.
Creation is hard, cheer me up!