Since crab are aquatic animals and thus when they are placed in freshwater, they do not get sufficient salt concentration to make themselves survive. The cells of the crab body (which was hyper tonic in salt water) will now become hypo tonic. Thus the osmo-regulation process will lower down thereby reducing the osmotic pressure as compared to the fluid outside. Also the distilled water lacks basic nutrients required for the crab and its family to survive.
The crabs are salt water organisms, so they'll have a higher concentration gradient than the distilled water. In other words, they're hypertonic to the distilled water. This will cause them to take on water and expand; their cells bursting.
