Re: How does a crab's physiology prevent water loss when it is in the open air?

Hi Bridget,

A few months ago I answered a question about respiration in land crabs, which may be of interest to you. Check out this answer in the MadSci archives.

Crabs respire over gills, which must be kept moist for effective gas exchange. The gills are housed in a branchial chamber that protects the gills from damage and provides a conduit for oxygen-carrying water to pass over them. The crab draws a current of water through the branchial chamber by vibrating certain mouthparts.

Most crabs are aquatic, and the relative amount of desiccation that a given species can tolerate can be inferred by its habitat. The branchial chamber provides a semi-enclosed space that protects the gills from desiccation. It has allowed some crabs to live in the intertidal and terrestrial habitats. These animals can tolerate desiccation better than their always-submerged relatives. Even crabs that rarely encounter dry air in their everyday lives can live for a short while in air because they can secrete a surfactant that coats the gills, keeping them damp and functional -- you can sometimes see this surfactant as a foamy substance covering the crab's mouthparts.

The question of osmoregulation is separate from that of desiccation. All crustaceans (marine, freshwater, or terrestrial) excrete ammonia over their gills and via excretory organs called nephridia ("nephridium" = plural). In most of the malacostracan crustaceans, including crabs, the nephridium is referred to as an antennal gland or a green gland. This gland filters the fluid of the body cavity and opens to the outside via a pore at the base of the second antenna, hence the name "antennal gland". The antennal gland regulates the excretion of metabolic wastes, and by maintaining ion concentrations in the fluid of the body cavity functions in osmoregulation.

The cuticle of a crab also has an osmoregulatory function. It provides a barrier between the animal's body and the external environment, preventing water loss; it is especially important for freshwater and terrestrial crabs. Land crabs still do lose water over their carapace, and replace this lost water by drinking.

Water is also lost when the animal excretes ammonia over its gills. So far, no crabs are known to excrete a concentrated urine, which would conserve water. However, the coconut crabs (land-dwelling relatives of hermit crabs) have been shown to excrete uric acid, which is a more water-conserving form of nitrogenous waste than ammonia.

If you need additional information on crab physiology, try to get your hands on a textbook on invertebrate zoology. There are several good ones; here are a couple that I can recommend:

Ruppert and Barnes, 1994. Invertebrate Zoology, 6th edition. Saunders College Publishing.
Brusca and Brusca, 2003. The Invertebrates, 2nd edition. Sinauer Associates, Inc.

Allison J. Gong
MadScientist