Fig. 1. (A) Forces acting on the body of a crab locomoting through a fluid environment. The crab in the diagram is locomoting with its left side leading and is moving upstream against an ambient water current. The fluid motion relative to the crab is the vector sum of the flow due to the ambient current and the flow due to the motion of the crab. Buoyancy counteracts the crab’s weight. Lift, which acts perpendicular to the relative fluid motion, counteracts the weight (positive lift, acting away from the substratum) or augments the weight (negative lift, acting toward the substratum). Drag acts in the direction of relative fluid motion, resisting locomotion and tending to push the crab downstream. Acceleration reaction resists changes in velocity, augmenting drag as a crab accelerates relative to the fluid and counteracting drag as the crab decelerates. (B) A crab overturns, pivoting about its downstream leg, when the overturning moment about its center of mass exceeds the stabilizing moment. The overturning moment is the net horizontal force times the height of the center of mass. The stabilizing moment is the net vertical force times the distance from the center of mass to the trailing leg. (C) A crab that does not actively grasp the substratum washes away when the net horizontal force on its body exceeds the frictional force resisting dislodgment.
