The amphibious crab is seen both on land and in the water. You might even notice bubbles or foam coming from their mouths when on land.
This is not a sign that they are struggling to breathe but is rather part of their process of obtaining oxygen.
Crabs breathe through gills, obtaining oxygen from the water, dew, or moisture. Many species of crabs are equipped with a form of bronchial chambers and storage capacity for water. Crabs also have moving plates that can cover gills to keep moisture in. Morphologically (structurally) crabs are the same but are slightly physiologically (functionally) different to breathe in their species-specific habitats.
Breathing on both land and in water is a unique function not seen in many animals. The crab’s ability to do this is quite a wonder.
Read on to learn more about how they do this.
Crabs are crustaceans, like lobsters and shrimp, with segmented bodies covered with an outer shell (exoskeleton), jointed limbs, and antennae.
4,500 species of crabs are found primarily in water, but also adjacently on land.
Crabs that spend a majority of their time on land also utilize water in early life stages and for food sources.
Morphologically, crabs have the same structures.
The carapace of a crab’s exoskeleton covers the head and thorax. The crab’s gills are found under the carapace, close to its first pair of walking legs.
Crabs may also have a lung-like branchial chamber, which houses the gills. This assists crabs in utilizing oxygen from the water or moisture from the air to breathe while on land.
Physiologically, crabs vary slightly depending on their species and habitat.
Marine crabs have more gills than land crabs. The land crab’s branchial chambers have a more elaborate vasculature with a double portal system for prolonged time on land.
Varying species have a respiratory process for physiologically obtaining oxygen to survive in their habitats. Generally, oxygen moves through osmosis, moving through a semipermeable membrane in the gills.
The oxygen moves into capillary blood vessels to transport oxygen throughout the body. Carbon dioxide is released as waste gas.
Crabs are primarily found living in intertidal zones, an ecosystem on the shorelines.
As the water level changes due to high and low tides, crabs may be out of the water for several hours each day.
Some species cannot spend too much time directly in water, since their gills and respiratory process are not equipped like marine crabs.
This means that their gills are equipped to better access oxygen from moisture rather than from water submersion. They could potentially drown if submerged too long.
Crabs access pools of water left behind by the changing tides as well as from dew and moisture.
Conversely, some species of marine crabs can spend time out of the water, some for multiple days.
However, the crab must be kept moist, because it relies primarily on water flowing over its gills for oxygen. Again, they can access water from pools of water and moisture in the air.
Regardless, all crabs have gills which are kept moist to obtain oxygen.
When underwater, crabs draw water over their gills with a scaphognathite.
This leaflike appendage is located on the underside of the crab, pumping water through the gill cavity. The gills pass oxygen into the bloodstream and carbon dioxide is released near the crab’s mouth.
Breathing On Land
When on land, crabs use articulating (movable) plates that help to seal in moisture over their gills, storing water in the gill chamber.
This reduces the surface area that is exposed and keeps the gills from drying out.
Crabs can also store water in their blood, bladder, and pockets in their bodies, using the water as needed to keep the gills moist.
Land crabs have access to water from dew, damp hiding places, and rocky crevices near tidepools.
Bubbles or foaming at the mouth can be seen when crabs are on land. This is part of the oxygen exchange process as air passes over moist gills and carbon dioxide is released.
This video shows crabs blowing bubbles:
Crabs use gills to breathe oxygen both on land and in water.
Underwater, oxygen is processed from water that passes through the crab’s gills.
When on land, the crabs use plates to cover the gills, store water internally, or access water from dew, moisture, and other damp places.
Structurally, crabs are built the same, however, some have psychological adaptations for survival. This could be branchial, lung-like, chambers, or a reduction in gills depending on their species and habitat.