Crabs possess a respiratory system that allows many species to thrive in both aquatic and terrestrial environments. While their primary breathing mechanism is adapted for water, many crabs have evolved remarkable abilities to take in oxygen from the air.
Gills: Crabs’ Aquatic Breathing System
Crabs primarily breathe using gills, specialized feathery organs located within protective chambers on either side of their carapace, known as branchial chambers. These gills contain a vast network of blood vessels, providing a large surface area for efficient gas exchange. Crabs draw water into these chambers, typically through openings near the base of their legs.
Once inside the branchial chamber, water flows over the gill filaments, allowing dissolved oxygen to diffuse into the crab’s bloodstream. Simultaneously, carbon dioxide, a waste product, diffuses from the blood into the water and is then expelled, usually near the crab’s mouthparts. This constant, unidirectional flow of water is maintained by a specialized appendage called the scaphognathite, which rhythmically beats to circulate the water over the gills.
Air Breathing: Adaptations for Land Survival
Many crab species, especially those that spend time out of water, have developed adaptations to breathe air, although they still rely on their gills. A primary challenge for air-breathing crabs is keeping their gills moist, as dry gills cannot facilitate gas exchange. Some terrestrial crabs have gills that are stiffer and more widely spaced, which helps prevent them from collapsing when out of water.
The branchial chambers of air-breathing crabs are often modified to retain water around the gills, effectively acting as a “portable lung.” Some species, like the coconut crab, have highly vascularized linings within these chambers, referred to as branchiostegal lungs, which can absorb oxygen directly from the air. These modified linings often have convoluted surfaces to increase the area for gas absorption. Crabs also employ behavioral strategies, such as carrying water in their gill chambers, periodically returning to water, or absorbing dew, to ensure their gills remain sufficiently hydrated for respiration.
The Balance of Water and Air Respiration
The respiratory strategies of crabs demonstrate a spectrum of adaptation between aquatic and terrestrial life. Even crabs well-adapted for land, such as the terrestrial ghost crabs, still require some form of moisture for their gills to function, as their respiratory organs are fundamentally designed for water. Aquatic crabs, while capable of surviving out of water for periods, experience a significant reduction in oxygen consumption in air compared to water, and prolonged exposure can lead to gill desiccation and eventual suffocation.
Conversely, highly terrestrial crabs, like the adult coconut crab, have gills so specialized for air breathing that they can drown if submerged in water for too long. Their respiratory control systems also shift, with aquatic species being more sensitive to oxygen levels in water, while terrestrial species become more responsive to carbon dioxide levels in air.