Crabs, which are decapod crustaceans, are often found in rockpools, on beaches and even in restaurants. With over 4500 species found worldwide, they are incredibly diverse creatures, with the smallest being just a few millimetres wide and the largest having a leg span of up to 4 m (13 ft)!
Covered with a tough exoskeleton and typically having a short, projecting tail, these crustaceans are armed for battle with two large claws at the front of their bodies. We have all seen the classic sideways or crab walk, but one question remains: Can crabs breathe underwater?
Can Crabs Breathe Underwater?
It is common knowledge that fish have gills to survive life underwater, just as humans have lungs to breathe on land. But did you know that crabs also have gills?
It is these gills that allow them to breathe underwater. Crab’s gills are located under the carapace, which is just in front of the first set of legs they use for movement. The crab can take in the oxygen it needs to survive through these gills and take it to the crab’s bloodstream.
To breathe underwater, crabs must draw water containing oxygen over their gills. They do this using an appendage called a scaphognathite. The scaphognathite is located on the crab’s underside near where its claws join onto its body.
The water can then pass over the crab’s gills so the gills can extract the oxygen. Blood also passes over the gills and returns carbon dioxide to the water. The carbon dioxide exits the crab’s body near the mouth.
Can Crabs Breathe Out Of The Water?
Unlike many other creatures that can breathe underwater, crabs can also breathe above the water!
Crabs have special plates called Articulating Plates. These articulating plates can seal in any stored moisture, which helps to keep their gills moist when on land. This means the gills can keep extracting oxygen from the water even when the crab is out of water. If you have ever seen a crab blowing bubbles, they do this because it keeps oxygen flowing to their gills.
Crabs can draw in air containing oxygen which then passes over their gills. The air combined with the moisture in the crab’s gills causes the bubbles and allows the crab to extract and use the oxygen. The bubbles are then released near the crab’s mouth.
How Long Can Land Crabs Live Out Of The Water?
Land Crabs can spend their whole lives outside of the water as long as their gills remain moist. Many crab species, such as coconut crabs, are terrestrial and live well outside the water. The difference between land crabs and aquatic crabs is that if you fully submerged a land crab underwater, it would not survive.
How Long Can Aquatic Crabs Live Out Of The Water?
Amazingly, aquatic crabs such as the blue crab can spend 1 to 2 days outside the water! Despite being primarily adapted for aquatic life, these crabs can stay out of the water as long as their gills remain moist.
Famously the European green crab is regarded as being almost indestructible. This is because they can spend over a week outside of the water!
This is not regarded as a positive trait, however, since the European green crab is invading many areas of the United States, causing a lot of competition for food and habitat for native species.
Many species of crab have their natural habitat in intertidal zones. This means that the area is completely submerged underwater during high tide and is fully exposed to the air during low tide. These areas can be especially challenging to live in, so crabs are perfectly adapted to living in this habitat.
The strong exoskeleton of the crab can protect it from any harsh waves, while the ability to breathe in and out of water allows the crab to survive in both high and low tides.
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