Here's how they do it:
1. Gills: Crabs primarily use gills for respiration when underwater. Gills are feathery structures located on the sides of the body, near the base of their legs. The gills are covered by a protective gill cover or carapace.
- When underwater, crabs take in oxygen dissolved in water. Water is drawn into the gill chambers, and the gills extract oxygen from it.
- The oxygen is then transported throughout the crab's body through its circulatory system.
2. Modified Respiratory Organs: Some crabs have evolved modified respiratory organs that enable them to breathe air directly. These modifications allow them to survive in semi-aquatic or terrestrial environments.
- For example, fiddler crabs have a specialized structure called a respiratory chamber or "gill chamber." This chamber contains modified gills that can trap air and allow the crab to breathe oxygen from the atmosphere.
- Land crabs, such as the ghost crab, possess similar respiratory adaptations that enable them to live and breathe on land.
3. Aerial Respiration: Crabs that live in intertidal zones or semi-terrestrial habitats have adaptations that allow them to respire atmospheric oxygen. They can breathe air directly through modified structures, such as:
- Spiracles: Some crabs have spiracles, which are small openings on their body that allow air to enter the respiratory system.
- Pseudotracheae: These are specialized structures found in some crabs that function like tiny air tubes, facilitating air exchange between the atmosphere and the crab's respiratory system.
4. Aquatic Adaptations: Crabs that primarily live underwater often have efficient gill systems and rely heavily on dissolved oxygen in the water. Their gills are designed to maximize oxygen uptake from their aquatic environment.
It's important to note that different crab species may have variations in their respiratory mechanisms, and some crabs may employ combinations of these adaptations to survive in diverse environments.