Crawfish Tunnel Mound

Thursday, June 21, 2007      In the Wild    by Bob Thomas   .....

Crawfish are detritivores. That is, they break down organic material such as leaves and other plan parts (but they also eat living plants and animals such as worms, insects, etc.). All day long, they sit around munching on leaves. The benefit is that if we didn’t have things like crawfish doing this, we would soon have leaves and other organic material stacked up well over our heads.
While they are munching on leaves, crawfish are also consuming zillions of microbes, thus ingesting microbial protein. This being a crawfish’s main source of protein, next time you consume crawfish tails, just say to your friend, “This is the best processed microbial protein I’ve ever tasted,” because that is basically what you are eating - though it has been transformed into a crawfish.
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What are crawfish chimneys? Crawfish chimneys are those “smokestack”-looking things that appear in ditches, fields, and our yards each spring. Everywhere you see one (sometimes a crawfish will make two), there is a crawfish living in a burrow underneath. Their tunnels may extend down into the earth 3 ft or more, sometimes being a single burrow going straight down, but more often being a main tunnel with a couple of side tunnels, each with a room at the end. They are normally full of water.
Sometimes one sees that the color and texture of the chimney mud is different at different levels of the chimney. This is a sign that there are different types of soil layers below the surface. As the crawfish burrows down, it brings up soil from different layers and deposits the pellets of mud at the top of the chimney.
How do they make them? No, they don’t have little backhoes or dredges. They actually use their legs and mouth parts to dig up mud and make it into a little ball called a pellet. Each pellet is taken to the surface (the crawfish moves through the burrow looking like a fullback carrying a football) and placed on top of the existing chimney. The next pellet is set beside the first. This continues, much like a brick layer putting bricks on a surface, then making another layer, etc. until a building is totally bricked. The crawfish makes the chimney out of many, many pellets of mud. Take a close look the next time you see a chimney and you will clearly see this neat system.
Why do they build chimneys? This is not completely known, but there are a couple of theories.
The crawfish has to dig its burrow. As it does so, it has to do something with the mud. If it took the mud outside the burrow and crossed the ground to dispose of it, it would be vulnerable to predators. To avoid that, they learned to build the chimney, thus never leaving the entrance to the burrow. If you ever approach a chimney and the crawfish is visible, you will see how quickly it can fall/run back down the tunnel.
Many animals are known to construct their burrows in such a fashion that they enhance the flow of air through the tunnels. It is known that the water in crawfish tunnels may get low in oxygen. It may be that the chimneys help air flow into the burrows so that oxygen can be absorbed in the water.
What happens to burrowed crawfish during droughts? Crawfish routinely plug the openings of their burrows with mud during dry periods. This is especially true of horizontal burrows near the surface, since the crawfish can’t move deeper to remain wet. As the water table drops in more vertical burrows, the animal moves further down (this is the same response they have to cold weather - they move down to warmer water levels).
A major difficulty is that, over time, oxygen may be depleted in the burrow water. When this happens, the crawfish may position itself just above the water, thus keeping the gills wet and absorbing oxygen from the air in the burrow.
Where do crawfish lay their eggs? They lay their eggs in their burrow, and hold them attached to little appendages under the tail called swimmerets, or pleopods. As long as oxygen levels in the burrow water are high, the adult keeps the eggs under water. As the oxygen drops, the adult keeps the eggs moist, but gets them out of the water, thus allowing them to absorb oxygen from the air. The eggs usually hatch in the burrows, and begin to grow. Since there is a restricted amount of food available in the burrow, the youngsters consume infertile eggs and the carcasses of deceased siblings. Indeed, they will kill one another in order to survive. Once the rains begin and pools reform on the surface, all the crawfish move out of the burrow and into the surface environment. The babies usually stay with mom for a short period, but eventually get on with life and disperse throughout the realm.
How many crawfish per burrow system? Usually one, but there may be a male and female present. Once eggs are laid, the male typically stays near the entrance and the female remains deeper in the burrow.

Published in In The Wild, Louisiana Levant Magazine, June 21, 2007.

http://loyno.edu/lucec/natural-history-writings/crawfish-and-their-chimneys