Blue Crabs

It’s thought that life evolved in the sea, and the saltiness of most animals’ blood reflects this ancient history. How one marine species preserves the salt balance in its blood, even in nearly fresh water .

A reader wrote in with this question, “Blue crabs thrive in salt water but I’ve heard that crabs placed in fresh water will soon die. My question is . . . why?”

The chemical composition of the blood of most animals bears a resemblance to seawater. The salt ions in blood are critical to sustaining life. The salt content of their blood accounts for the fact that most marine animals can’t survive in fresh water. In fresh water, a marine creature risks losing its internal salts to the outside environment. Very few marine animals have evolved ways to regulate their internal salt concentration in both salt and fresh water . . .

But blue crabs can do this, to some extent. They’ve evolved a kind of “salt pump” in their gills that allows the crabs to keep the salt level in their blood constant — even in water that’s almost fresh. That’s why, although young blue crabs develop in seawater, adults of the species are able to live in bays and estuaries where the water is less salty.

But even the remarkable gill pumps of a blue crab can’t draw salt from fresh water. So, to answer the question — crabs will die in fresh water because they can’t keep enough salts in their blood.

The following books and articles were used in preparing this script:

  • Steven Funderburk, et al. (eds). Habitat Requirements for Chesapeake Bay Living Organisms. Second Edition. Prepared by: Habitat Objectives Workgroup Living Resources Subcommittee and Chesapeake Research Consortium, Inc. Solomons, Maryland. October 1992. pp. 6-1 to 6-8.
  • J.C. Rankin, and J. Davenport. Animal Osmoregulation. A Halsted Press Book, John Wiley and Sons. New York and Toronto. 1981. pp. iii-iv, 1-2.
  • Roger Eckert et al. Animal Physiology, Mechanisms and Adaptations. W.H. Freeman, New York. 1988. pp. 387-88.

Blue crabs are remarkable in their ability to travel from marine to nearly fresh water environments. To understand why, you first need to look back through time to the dawn of life on Earth.

It’s thought that life evolved in the sea, and most animals’ blood reflects this. Blood is very much like salt water as it contains similar levels and types of salt ions. And those levels and types of salts (e.g. sodium, calcium, and potassium) are crucial for most biological processes in an animal’s body.

Because blood has a fairly high salt concentration — just as seawater does — some animals must invest a lot of energy keeping their internal salt concentrations constant. For instance, fresh water fish have evolved mechanisms to keep their internal salts from diffusing out to the fresh water environment around them.

Animals that live in marine environments, on the other hand, have evolved mechanisms to keep the higher levels of salts outside their body from coming in to their body. These mechanisms are generally related to the animal’s excretory system. The system is either set up to get rid of a lot of extra water (fresh water environment) or to conserve water (salt-water environment).

It is very, very rare for marine or estuarine animals to have evolved mechanisms that allow them to move between full strength sea water and fresh, or nearly fresh, water. “If you find an example of an animal who moves between fresh and salt water, it is almost always related to a crucial life history trait such as reproduction (salmon are one example of a fish that can move between fresh and salt water — and it’s because they spawn in fresh water and live the rest of their lives in a marine environment).” Blue crabs are one of a very few examples of marine or estuarine animals that can tolerate a range of full-strength seawater to low salt environments.

Female blue crabs spawn in the marine environment where salt concentrations are full strength. There, crab larvae grow and develop. As they grow larger, they travel on ocean and estuarine currents to the habitats where they’ll spend their adult lives. Many blue crabs are taken by currents up into estuaries. Estuaries are bodies of water (like bays) where salt water from the ocean and fresh water from rivers mix together. So, adult blue crabs typically face a wide range of salinities–from full strength sea water to nearly fresh water–over the course of their life time, and in some cases within a period of days!

One mechanism that blue crabs have evolved to help them keep their internal salt concentrations constant in the face of these fluctuating external salt levels, is “salt pumps” in their gills. These gill pumps can draw salt from water around the blue crab, even at very low concentrations. That means the crab can keep its internal levels of salts stable even when the outside water has a very low salt concentration. The gill pumps work both ways — they can also pump salt from inside the crab out to the surrounding water if the crab finds itself in salt water that is higher in salt concentrations than that found in its own body.

This ability to move between low salt and marine environments is highly unusual for marine and estuarine animals. In fact, it is likely that species diversity is very low in estuaries because so few animals have evolved the ability to tolerate the wide range of salinities found there.

This diversity is good news for blue crabs though. Estuaries are extremely productive–they have a high level of nutrients, and plants and animals that do live there are plentiful. So blue crabs are able to exploit this highly productive habitat because of their ability to move between nearly fresh and full-strength seawater environments.

Callinectes sapidus (cal-in-EK-tees SAP-i-dus) is the scientific name for blue crab. Callinectes means beautiful swimmer, and sapidus means tasty.

What you have in your mind?