Are crabs stressed for water on a hot sand flat? Water loss and field water state of two species of intertidal fiddler crabs

• We studied dehydration stress of fiddler crabs using a simple field measure.
• A high intertidal species was more resistant to dehydration than a lower species.
• In the field, both species showed no evidence of dehydration.
• This suggests that behavioral compensation allows sufficient water uptake.
• Larger males may succeed in the high intertidal because of their proportionately lower water loss than smaller males while on the surface in the sun.

Field measures of stress are needed to understand the role of behavior in stress and how behavior trades off with overall fitness. Along Atlantic shores, displaying and reproductive burrow-occupying males of the fiddler crab Uca pugilator live in high intertidal dry sandy sediments, whereas Uca pugnax live in lower intertidal wetter muddy substrata. Water loss was examined in the lab for both species. Water loss of U. pugilator males was approximately linear with time and crabs regained water completely after 2 h of rehydration. Percent water loss was inversely correlated with body mass, irrespective of sex. Males of the higher intertidal U. pugilator lost proportionally less water over time than U. pugnax, which occupies muddier and wetter sediments. A field test of water state on a hot day demonstrated that males of either species seen at the surface next to mating burrows were not stressed for water, suggesting that behavior compensated adequately for water loss when displaying in the sun on the surface. Behavior can therefore compensate for what appears to be a stressful environment. However, such compensation likely comes at the cost of reproductive success, which depends upon remaining on the sediment surface in open stressful conditions. Large-sized males may be more successful in breeding territories owing to their proportionately lower water loss.