Terrestrial hermit crabs (Coenobita species) have successfully transitioned from the sea to land. They are often observed interacting closely with damp materials like sponges, moss, or saturated substrate, appearing to drink from them. The mechanism by which these crustaceans hydrate using a porous material, rather than an open pool of water, involves specialized anatomy and the physics of water movement. Understanding this process requires examining the crab’s biological needs, the properties of the material it uses, and the specific physical actions it employs to draw water.
Why Hermit Crabs Need a Unique Hydration Strategy
The primary reason land hermit crabs require a constant, accessible source of moisture is their respiratory system. Unlike terrestrial animals that developed lungs, Coenobita species retain modified gills for breathing, a vestige of their aquatic ancestry. These gills are housed within a branchial chamber that must be kept perpetually moist for gas exchange to occur. If the humidity in the chamber drops, the gills dry out, and the crab essentially suffocates, making desiccation a severe threat.
The crab’s shell serves as a mobile reservoir that helps combat this challenge. By carrying a small amount of water, often called “shell water,” within the empty cavity of their adopted gastropod shell, the crab creates a miniature, high-humidity microclimate around its delicate gill structures. This shell water also plays a role in osmoregulation and replenishing fluids lost through evaporation and excretion. Because they continuously lose this stored water, the crabs must regularly seek out and replenish their internal supply, which drives their unique drinking behavior.
How Capillary Action Delivers Water
The materials terrestrial hermit crabs often use, such as natural sponges, sphagnum moss, or fine-grained sand, are effective water sources due to a physical phenomenon called capillary action. Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. This process is driven by two main forces: cohesion (attraction of water molecules to each other) and adhesion (attraction of water molecules to the surface of the material).
Within a sponge or moss, the structure is filled with countless microscopic channels and pores, which act as tiny capillary tubes. The strong adhesive force between the water and the walls of these pores pulls the liquid up and into the material. This upward pull, combined with the cohesive force, allows the material to hold a substantial amount of water without creating a standing pool. The result is a highly saturated surface where the water is held securely in place, providing the crab with a stable, controllable source of moisture.
The Crabs Specialized Drinking Method
The hermit crab’s method for harvesting this capillary-held water involves a set of highly specialized appendages and behaviors. When a crab needs to hydrate, it employs its third pair of maxillipeds, which are small, specialized mouthparts. These maxillipeds are covered in dense, hair-like structures called setae, which function as wicking channels. The crab presses these setal brushes against the saturated material, drawing water up the grooves via capillary action directly into the mouth area for immediate consumption.
Shell Water Replenishment
Beyond direct consumption, the crab also uses a distinct process to replenish the water stored inside its shell. For this, it utilizes the setae located on its two claws, known as chelipeds, in addition to the maxillipeds. The crab uses these appendages to scoop or brush water from the moist surface, transferring the fluid backward toward the entrance of the shell. Once the water is at the shell opening, the crab further manipulates the fluid into the mantle cavity using the inverted beating activity of the scaphognathite, a gill bailer structure. This dual approach allows the terrestrial hermit crab to efficiently manage its internal hydration and maintain the moisture required for its modified gills.