Crabs can breathe outside of water, a capability that varies significantly among species and comes with limitations. While they primarily respire using gills, adaptations allow them to extract oxygen from the air.
How Crabs Utilize Air for Respiration
Crabs breathe through gills located within protective chambers under their carapace. For these gills to function, they must remain moist, as oxygen diffuses from the surrounding medium into the crab’s bloodstream across their delicate surfaces. When out of water, a crab maintains this essential moisture within its branchial chambers.
The carapace creates a “lung-like” environment, enclosing the gill chambers and trapping moisture. Some crabs have specialized plates that seal gill openings, preventing desiccation. Additionally, certain terrestrial species have evolved a modified branchial chamber lining that functions as an accessory lung, increasing surface area for air gas exchange.
Some crabs employ behavioral strategies, such as blowing bubbles, to keep oxygen flowing over their moist gills when exposed to air. Oxygen from the air dissolves into the retained moisture, allowing the gills to continue extracting it. These anatomical and behavioral adaptations enable crabs to respire effectively even when not fully submerged.
Different Crab Lifestyles and Air Breathing
The extent to which crabs breathe air is determined by their habitat and lifestyle. Fully aquatic crabs are primarily adapted to water breathing, though they can survive out of water for limited periods if their gills stay moist. Blue crabs, for instance, can endure over 24 hours out of water under moist conditions.
Semi-terrestrial crabs, like fiddler and ghost crabs, regularly spend significant time out of water for foraging, mating, or escaping predators. These species have pronounced adaptations for air breathing, including tougher exoskeletons and mechanisms to retain water in their gill chambers. Fiddler crabs, for example, can survive for several days out of water as long as their gills remain moist.
Truly terrestrial crabs, such as coconut and some land hermit crabs, are specialized for life on land and breathe air almost exclusively. Their gills are adapted for air breathing and may even drown if submerged too long, as they are not efficient at extracting oxygen from water. These crabs rely on environmental humidity or stored water to keep their gills moist, sometimes absorbing water from dew or food.
The Dangers of Prolonged Exposure
Despite adaptations, crabs face risks when exposed to air for extended periods. The primary danger is desiccation, which directly impacts their ability to breathe. As gills dry, their delicate structures can collapse, significantly reducing the surface area for oxygen absorption. This impairment leads to suffocation.
Even for species well-adapted to air breathing, there is a finite limit to how long they can survive out of water. Lack of sufficient moisture means their gills cannot efficiently extract oxygen from the air. Aquatic crabs will quickly deplete oxygen in any trapped water if not aerated, leading to suffocation within hours if left in a small, uncirculated container. For most aquatic crabs, survival out of water is limited to 24-48 hours, even under ideal moist and cool conditions.