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Carcinus maenas occurs in two color morphs: green after molting and red after prolonged intermolt. The ability to ion regulate in C. maenas is well documented in green morphs at low salinity, but no studies have focused on fluctuating salinity levels, like that of a tidal environment. We investigated 1) how the ability to ion regulate differ between the two morphs, 2) how oscillating salinity changes affect both morphs, and 3) if these changes vary with sex. Red and green morphs of both sexes were exposed to constantly low salinity (12 ppt) or oscillating salinity, changing from 12 to 32 ppt every 6 h. We assessed hemolymph osmolarity, running endurance on a treadmill, reaction time, and mRNA and protein expression of key ion transporters in the gills. Green morphs are better ion regulators than red morphs, with higher mRNA expression of cytosolic carbonic anhydrase (cCA), Na+K+ATPase (NKX), and Na+H+ antiporter (NHX). Greater mRNA expression of cCA, NHX, and NKX in crabs exposed to constantly low salinity shows that low salinity is more stressful than oscillating salinity. Little difference was noted between sexes. Our data show that the well‐documented ion regulatory capabilities need to be viewed in a color morph specific context. Oscillating salinity being less strenuous than constantly low salinity is in agreement with the conditions in the crabs’ natural habitat as they are rarely exposed to constantly low salinity. Grant Funding Source: NSF‐IOB0640478 to M.F., APS undergraduate summer research fellowship to A.H.