Hermit crabs represent a successful transition from an aquatic to a terrestrial life. Although often kept as pets on land, their ancestry is entirely marine, which raises a fundamental question about their biology. Since most crustaceans rely on gills to extract oxygen from water, land-dwelling hermit crabs must have developed a unique method of breathing to survive in the air. This involves a specialized respiratory organ that acts as a bridge between the sea and the shore. This adaptation allows these animals to breathe air while still possessing a structure derived from their aquatic past.
The Respiratory System: Modified Gills and Branchial Chambers
Terrestrial hermit crabs, such as those in the genus Coenobita, possess gills, but these organs are significantly modified for air breathing. Unlike the large, feathery gills of fully aquatic crabs, the gills of land hermit crabs are reduced in size and number. This modification reflects the shift from extracting dissolved oxygen from water to absorbing oxygen directly from the air.
These modified gills are housed within a protective structure called the branchial chamber, located beneath the carapace. The chamber walls have become highly specialized, featuring a network of fine blood vessels. This vascularized lining, sometimes referred to as a branchiostegal lung, functions much like a primitive lung, absorbing oxygen from the air drawn into the chamber.
The combination of the reduced gills and the vascularized chamber walls creates an effective respiratory surface for aerial gas exchange. In some species, this surface is supplemented by a third area known as the abdominal lung. This additional respiratory patch is formed by thin, highly folded tissue on the abdomen, which significantly increases the total area available for oxygen uptake.
The Role of Water in Gas Exchange
Despite their adaptation to breathe air, the gills of hermit crabs, like the respiratory surfaces of most land-dwelling invertebrates, must remain continuously wet to function. Gas exchange relies on diffusion, where oxygen gas must first dissolve into a thin film of water before it can pass through the membrane and into the crab’s bloodstream. Without this moist surface, oxygen transfer halts entirely.
The delicate tissue of the gills is highly susceptible to desiccation when exposed to dry air, which can cause the respiratory structures to collapse. If the gills dry out, the crab suffocates, even though it is surrounded by oxygen-rich air. This physiological necessity is the reason why humidity is important for the survival of terrestrial hermit crabs.
The branchial chamber offers protection by enclosing the gills, which slows the rate of water loss. However, the chamber alone is not sufficient to prevent desiccation over long periods. The crab must actively maintain the moisture level within the chamber to ensure the continuous solubility of oxygen on the respiratory surfaces. This establishes a direct link between the crab’s ability to find and retain water and its ability to breathe.
Behavioral Adaptations for Maintaining Humidity
Hermit crabs have developed specific behaviors and physical tools to manage their internal water supply. The most significant adaptation is the use of the borrowed gastropod shell, which acts as a portable reservoir. A small amount of water, often called “shell water,” is maintained within the deepest part of the shell, surrounding the soft abdomen.
The crab uses this shell water to continually moisten its gills by drawing the liquid up into the branchial chamber. Specialized hairs, or setae, on the walking legs and claws wick water up from the shell reservoir to the gill chamber. To replenish this supply, the crab periodically dips its entire shell into a water source, often submerging completely to ensure the reservoir is full.
When the crab is not actively drinking, it uses its largest claw, the cheliped, to seal the shell opening. This action creates a humid microclimate inside the shell, minimizing the evaporation of the shell water and reducing overall desiccation. Land hermit crabs will instinctively seek out humid environments, such as burrowing beneath the substrate or hiding in damp leaf litter, to limit external water loss and conserve the moisture needed for breathing.