Neil Cramery, of Room 5 at Thorrington School, asks :-
How do crabs breathe both in and out of water?
Harry Taylor, a Zoologist at the University of Canterbury, responded.
Crabs are primarily water-living animals and always possess gills to obtain oxygen from the water. Some also have lungs to breath air. The gills are attached near the bases of the legs and are enclosed by gill chambers on each side. Water is pumped into the gill chambers between the legs. It passes over the gills and leaves through openings near the mouth. Under the microscope, the gills are seen to consist of stacks of thin plates providing a large surface for oxygen uptake.
Gills do not work well in air because the moist plates stick together, preventing air getting to the gill surface and truely aquatic crabs like paddle crabs will eventually suffocate out of water. Nevertheless, by keeping very still they use little oxygen and may survive for several days. "Big-handed crabs", found on rocky shores around Kaikoura, similarly survive at low tide by being very inactive. Other shore crabs, like the "hairy-handed crab" and the "tunnelling mud crab" common in the Avon-Heathcote Estuary, move around and feed actively out of water. These crabs are still able to use their gills because they recirculate water through their gill chambers. Exhaled water, runs down the outside of the crab, picking up more oxygen, and re-enters between the legs.
Crabs with both lungs and gills, like the agile "purple rock crab" seen on cliffs around Punakaiki, breath equally well in air or water. Lungs are formed from the inner lining of the gill chambers and in Australian "ghost crabs" which rarely enter water, the lining is highly folded to increase their area, as in our own lungs. Probably the best developed lungs are those of Pacific Island "coconut crabs". These true land animals have very small gills and actually drown if submerged in water.