I can understand why crabs would have blindsight in water, as they can notice movement in the water and (apparently) changing salt levels to detect enemies. But it makes no sense for a normal crab to have blindsight out of water.
But from a purely mechanical standpoint, nothing is stopping me from having a crab familiar, and getting 30 feet of blindsight by sharing senses with my familiar. Essentially having a constant "detect invisibility" seems a bit strong, though, for a 1st-level spell.
Is this correct? Do crabs have blindsight out of water?