Crayfish - also called crawfish, mudbugs, or crawdads - are an important and often overlooked part of the floodplain forest ecosystem at Congaree National Park. Some species live in streams and lakes, where they burrow into banks or hide under woody debris. Other species live under the forest floor and make mud "chimneys" around burrow openings. Crayfish burrows provide important habitats for other small animals. Crayfish also have diverse diets. They can serve as scavengers that help recycle dead plant material or predators that feed on other invertebrates and small fish. Crayfish in turn are important food items for bass, snakes, raccoons, barred owls, and other animals - including people!
As crustaceans, crayfish are invertebrates related to shrimp, crabs, lobsters, and pill bugs. More specifically, crayfish are Decapods, which is Greek for "ten feet." Of the 500 crayfish species found worldwide, about 350 species are found in North America, and more than 37 species are found in South Carolina. Surprisingly little is known about crayfish ecology worldwide. Many crayfish are secretive and difficult to identify. While some species can be locally invasive, scientists generally recognize that native crayfish –and all the habitats and organisms that depend on them –are relatively vulnerable to human impacts.
In 2005, Congaree National Park partnered with scientists from the South Carolina Department of Natural Resources and Clemson University to study crayfish diversity and distribution at the park. Scientists sampled crayfish in six distinct habitats:
- Streams, like Cedar Creek, are channels with flowing water year round.
- Guts are channels that have flowing water during floods but dry out in between floods.
- Ox-bow Lakes, like Weston Lake, have permanent, standing water.
- Gum Ponds have well-defined banks and lots of tupelo (gum) trees. They are usually (but not always) flooded.
- Sloughs are low, poorly-defined areas that dry out more frequently than gum ponds.
- Floodplains are defined for this study as the flat, relatively elevated areas between water features.
Researchers used several methods to capture live crayfish before releasing them back into their habitat.
- Burrow Net - Scientists experimented with a small net that was inserted into a crayfish burrow. This prevents the need to dig up the entire burrow.
- Dip Netting - Scientists simply used nets to catch crayfish found while wading.
- Electrofishing - Scientists sent a mild electric shock into the water to temporarily stun (but not kill) crayfish. Stunned crayfish were scooped up in a dip net.
- Minnow Traps - Scientists used small wire minnow traps that were baited with dog food, hung in the water, and checked daily.
- Seine Netting - Teams of scientists dragged a wide net, with weights on the bottom, along the channel to catch anything swimming, including crayfish.
Scientists compared the results from each method to see which one worked the best. Electrofishing was generally the most effective method overall. Other sampling methods tended to only catch crayfish of a certain species, size, or sex at a given site. Minnow traps, for example, tend to catch large males and exclude both small males and females. Large males are easy to identify, however, so scientists actually find this method useful in some cases.
Scientists found seven crayfish species at Congaree National Park. This gives the park more crayfish biodiversity than other, larger, Federally-protected areas in the region. The seven species included the following:
- Devil Crayfish (Cambarus diogenes) - This species was usually found burrowing in floodplains.
- Sickle Crayfish (Cambarus reduncus) - Two individuals were found burrowing in floodplains.
- Digger Crayfish (Fallicambarus fodiens) - This species was found in streams, guts, ox-bow lakes, gum ponds, and sloughs.
- White River Crayfish (Procambarus acutus) - This species was found in all habitats.
- Cedar Creek Crayfish (Procambarus chacei) - This species, which was found only in stream habitats, is a species of conservation concern in SC.
- Eastern Red Swamp Crawfish (Procambarus troglodytes) - This species was the most abundant species at the park, and was found in streams, guts, ox-bow lakes, gum ponds, and sloughs.
- Unknown (Cambarus sp.) - A single female was found that could not be confidently identified. It is either Cambarus spicatus (the Broad River spiny crayfish, which is a species of conservation concern in SC) or Cambarus acuminatus (Acuminate crayfish).
Life History of the Eastern Red Swamp Crawfish
In 2006 scientists from the South Carolina Department of Natural Resources and Clemson University studied the life history of the Eastern Red Swamp Crawfish (Procambarus troglodytes;ERSC) at Congaree National Park. Much of ERSC life history was previously unknown to science. The scientists intensively sampled ERSC habitats four times during the year. A total of 1,019 ERSC were collected and measured. Three annual populations, or "cohorts," were clearly grouped by size. Changes in the number and size of crawfish during the year allowed researchers to estimate life span, reproductive cycles, relative productivity (growth rate) and habitat use.
ERSC reach maturity (total length about 6-8 cm and total weight about 7-14 grams) within 1 to 1.5 years, but generally live less than 2.5 years. Activity varies seasonally. Significant numbers of ERSC were found in winter and spring. Almost none were found in summer and fall, when they were presumably retreating in burrows. Reproductively active males were only found in May, indicating that ERSC only reproduce once a year. The timing of this reproductive cycle is likely linked to water level and temperature. The winter is wet enough, but too cold. The summer is warm enough, but too dry. The spring is perfect because it is warm and wet.
ERSC productivity was significantly higher in habitats with shallow, changing water levels (sloughs, guts and gum ponds) than in habitats with permanent water (streams and ox-bow lakes). This is likely because habitats with shallow, changing water levels lack fish predators that feed on young. These habitats may also have better sediment for burrows, which is where ERSC eggs and young initially develop. Water level changes, which are seasonally driven, may also trigger instinctive burrowing and reproductive responses. Although ERSC reproduce more in these habitats, as they grow they move throughout the floodplain, presumably during floods. This movement is important for reducing local competition and mixing the population.
ERSC life history is similar to that of the Red Swamp Crawfish (Procambarus clarkii), the species typically raised for human food and fish bait. The Red Swamp Crawfish is not native to the Carolinas, however, and may cause problems if it escapes into the wild. The ERSC may thus be a good, local commercial alternative.
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