Tens of millions of years ago, probably when dinosaurs still stalked the earth, armies of salt water crustaceans inhabited the shallow seas, salt marshes, and estuaries so abundant in that watery age. During the Eocene when the Atlantic Ocean periodically receded from the North American continent, these marine crustaceans began to evolve the ability to survive in brackish water habitats that became more and more common, and eventually they evolved into freshwater organisms. The ecological niche they colonized had few, if any, competitors, and many species evolved. Southeastern North America has the most species on the continent–evidence that this is the region where crayfish first colonized fresh water. 68 species of 8 genera are known to occur in Georgia today.
Illustration of crayfish anatomy from Crayfishes of Georgia by Holcombe Hobbs.
Biologists like to study crayfish because they represent good case studies of evolution. Barriers often form in river drainages, isolating different populations of crayfish, and accordingly this affords frequent opportunity for speciation to occur. Scientists use cladistics (the comparison of anatomical characteristics) to determine the evolutionary relationships between species. Of course, DNA testing is an even more accurate way to determine these relationships.
Holcombe Hobbs was the foremost crayfish expert in Georgia, and he wrote a very thorough book in 1981–The Crayfishes of Georgia which is available online http://si-pddr.si.edu/dspace/handle/10088/5545 for free. He knew of 65 species in state. In the 30 years since his book was published, only 3 additional species have been discovered in Georgia surveys, showing just how thorough he was.
I find most interesting the burrowing habits of crayfish which create little mud chimneys in wet fields and stream sides. Crayfish also burrow at the bottom of streams, but these don’t result in visible chimneys.
Illustration of crayfish burrows from The Crayfishes of Georgia. Note the chimney-like hills.
It’s likely the ancestors of crayfish lived in intertidal zones near the sea shore. At low tide they survived dessication by digging burrows in the sand and mud, similar to many modern day organisms. Later, this habit helped them survive drought when they colonized upland freshwater habitats. Crayfish are yet another example of a Pleistocene survivor because they were able to survive the many dry climate phases of the Ice Ages. Today, upland crayfish dig burrows down to the water table. Other species dig burrows directly in stream beds. The ones that dig on land usually tunnel around tree roots. According to Dr. Hobbs, these can be difficult to excavate when collecting specimens. Some species defend their burrows aggressively; others retreat deeper into side chambers. During droughts and cold weather, crayfish plug the tops of their chimneys to stay warm and moist. If the water table falls below the level of their burrow, they become dormant.
Much crayfish habitat has been destroyed by the creation of reservoirs which are like deserts for this animal. They don’t like deep water. However, man has created a lot of crayfish habitat. Crayfish do like to live in roadside ditches.
Here’s a list of major rivers and drainage systems in Georgia and the number of known crayfish species each holds.
St. Mary’s–7 species
When my family first moved to Athens, Georgia in 1976 (I was 13), I delighted in rambling around the woods that surrounded the partially developed residential neighborhood, not far from Cedar Shoals High School. It was mostly second growth pine and oak interspersed with old fields and plum thickets. There was even a pond site with a small waterfall that led to a chain of beaver ponds. (Developers have since destroyed this last described landscape and built a shopping center over it.) One day, a newfound friend and I took a dip net to one of the creeks we frequently followed. Along the creeks, we always saw the handlike prints of raccoons, and occasionally the empty shell of a crayfish that had provided a meal for the former. Simply by swinging the net across the bottom of a stony section of the creek, we collected two dozen crayfish in a short period of time. We put the crustaceans in a bucket and brought them h0me, setting the container in a backyard lawn. The next morning, the crayfish had vanished–a mystery for us then because we didn’t know crayfish are nocturnal and can travel over land at night.
Crayfish taste delicious–much like shrimp but without the iodine flavor. Eat them fresh, not previously frozen. Fresh, the tail meat is delectable, and the juice from the heads (actually the brain) is rich. Previously frozen, they smell bad upon preparation. The tail meat is still palatable, though nothing special, but the juice from the heads tastes like a mixture of mud and rotten fish.
The Crayfishes of Georgia
Smithsonian Publications 1981