For crabs to do well on land instead of in water, they need to change a number of physical and morphological traits, including the ability to reproduce without being able to get to open water. Some species have changed their gills to help them exchange gases, but many species also have extra gas exchange organs, usually lungs. People who are good at breathing through their noses and mouths make their lungs bigger and more vascularized. Pulmonary vessels carry oxygenated blood to the heart. The high amount of oxygen in the air leads to low ventilation, which creates an internal hypercapnia that pushes CO(2) out of the body. Land crabs can get blood through either their lungs or their gills, and the way they switch between the two can depend on their respiratory system or how much they are exercising. During their breeding migration on Christmas Island, Gecarcoidea natalis kept their arterial Po(2) steady by uptaking O(2) through branches, but their pulmonary O(2) pressure went down. This was partly because exercise increased the flow of blood through their gills by two times. Related species depend on high levels of haemocyanin and an affinity for oxygen to help them take in nutrients, but this makes it harder for the tissues to unload and lowers their aerobic range. Water crabs use their gills to exchange salt and ammonia with water, but land crabs can’t do this. Birgus latro has adopted uricotelism but other species excrete ammonia in either the urine or as gas. Land crabs minimise urinary salt loss using a filtration-reabsorption system analogous to the kidney. Urine is redirected across the gills where salt reabsorption occurs in systems under hormonal control, although in G. natalis this is stimulatory and in B. latro inhibitory. Different species of crabs live in different types of habitats, from water to land. However, their bodies are not all the same. Each species has its own unique set of physical traits that help it survive in its own environment.
Crabs inhabit diverse aquatic and terrestrial environments, from the ocean depths to tropical forests This raises the question – with such a range of habitats, how do crabs breathe underwater and on land? As it turns out, most crab species do possess specialized respiratory structures called gills that allow them to extract oxygen from water or moisture Let’s take a closer look at how crab gills function and allow these ten-legged crustaceans to thrive across the globe.
Anatomy of Crab Gills
The gills of crabs are delicate feather-like structures located inside a chamber in the upper carapace (shell). This chamber, called the branchial chamber, houses 8-9 pairs of gills and connects to the exterior via openings near the base of the legs.
Oxygenated water enters the branchial chamber through these openings, flowing over the gills so oxygen can diffuse into the bloodstream. At the same time, carbon dioxide passes from the blood out into the water. After passing over the gills, water exits the branchial chamber through excurrent openings.
How Do Crab Gills Work Underwater?
For crabs living underwater, such as blue crabs and Dungeness crabs, their gills rely on a steady flow of oxygen-rich water. To drive this flow, crabs utilize appendages called scaphognathites or gill bailers.
Located near the base of the mouthparts, these flap continuously to pump water through the branchial chamber. As water passes over the feathery gills, dissolved oxygen moves into the bloodstream while carbon dioxide exits. This constant motion ensures crab gills receive ample oxygen even when the crab itself is motionless.
Some crabs, like mud crabs, also have leg paddles for stirring up currents when buried, exposing their gills to fresher, oxygenated water. So whether it’s relying on mouthparts or limbs, crabs have evolved ways to keep their gills flushed with water to breathe.
How Do Crab Gills Work on Land?
Certain crab species, including coconut crabs, land hermit crabs, and red land crabs, live primarily terrestrial lives. But they still possess gills and face the challenge of keeping them moist and functional out of water.
When on land, these crabs use articulating plates to seal off their branchial chamber, trapping moisture against the gills. Specialized appendages nearby can also manipulate moisture, keeping the gills from drying out. Some crabs may blow bubbles, drawing in air that passes over the moist gills before being expelled.
Land crabs seek out damp habitats like forests, burrows, and under vegetation. Here they find the moisture essential for their gills to extract oxygen from the air, just as they would from water.
How Long Can Crabs Stay Out of Water?
Thanks to adaptations like articulating plates, terrestrial crabs can spend their entire lives on land. But for primarily aquatic species, how long can they survive out of their marine habitat?
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Blue crabs – 1-2 days maximum
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Green crabs – Over 1 week
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Fiddler crabs – Several days
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Red king crabs – 10-12 hours
Eventually desiccation threatens even the most resilient intertidal crabs. But keeping gills moist with sealed-in water enables impressive oxygen uptake even in air for many species.
Other Unique Respiratory Adaptations
Beyond standard crab gills, some species have modified respiratory structures:
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Air sacs – Land hermit crabs have evolved cervical air sacs for retaining moisture to breathe on land.
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Modified gill structure – Mud crabs and mangrove crabs have gills specially adapted to tolerate low oxygen conditions.
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Lung-like organs – Birgus latro, the coconut crab, possesses unique structures for aerial gas exchange.
These adaptations expand crabs’ ability to respire in demanding environments, from muddy burrows to coconut palm canopies.
Do All Crabs Have Gills?
The vast majority of the roughly 6,700 crab species have gills. The rare exceptions are:
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Lithodidae family: King crabs like red and blue king crab lack true gills and instead have feather-like papulae for gas exchange.
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Coenobitidae family: Hermit crabs utilize gills as juveniles but replace them with a modified lung-like system as adults.
So while most indeed have gills, a few derived groups have evolved alternate anatomy for respiration.
Unique Respiration Allows Crustaceans to Thrive
The incredible diversity of crabs traces back in part to their flexible respiratory systems. Gills that extract oxygen from both air and water, coupled with adaptations like articulating plates, allow crabs to inhabit tropical forests, frigid seabeds, and everywhere in between.
Whether buried in mud or climbing coconut palms, crabs continue their ancient success story thanks to the superlative design of their gills and adaptations. Respiration clearly contributes to the enduring prosperity and spread of these fascinating crustaceans.
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How do Crabs Breathe In AND Out of the Water?
Where are the gills of a crab located?
The gills of crabs are located on the legs and are responsible for respiration. Compared to the respiratory system of terrestrial animals, the respiratory system of crabs is quite different. Terrestrial animals have lungs that extract oxygen from air, while aquatic animals like crabs rely on gills to extract oxygen from water.
Do crabs have gills?
Crabs have gills on both sides of their bodies, which they use to breathe. Gills are featherlike structures which allow gas to be passed from the outside world into the crab. The gills have a very thin layer which allows dissolved oxygen to enter the crab and carbon dioxide to leave.
How many gills does a crab have?
Crabs have four pairs of gills on each side. Crabs breathe through their gills like fish. Their gills are designated in this way so that crabs can survive out of water for a longer time. Gills should be able to carry oxygen in the crab’s bloodstream. The gills of crabs are present under the carapace near the first pair of walking legs.
How do crabs breathe through gills?
Crabs breathe through gills. For gills to work, they must be able to take in oxygen and transport it into the animal’s bloodstream. The gills of crabs are located under the carapace near the first pair of walking legs. The oxygen that crabs need is taken into the gills either through water or moisture in the air.