Nature Elective

Crawdad
Family Astacidae

First posted August 1, 2004 Last updated October 21, 2004


A crawdad at Caz in a defensive posture
Photo by David Nelson, specimen from Austin Creek, July 23, 2004

The crawdad, also know as the crawfish, mud brother, and by many other names, is a constant hit with the campers at Caz: they are easy to catch and fun to play with. A common classroom biology subject, it is also the only animal at camp that is commonly eaten. And not just in the French Quarter in New Orleans: it has a world-wide distribution and is eaten in many countries. Crawfish feed on dead animals as well as live pollywogs, small fish, and most anything that they can catch. They are decapods, which means that they have 10 legs.

The underside reveals the (from front to rear) the mouthparts, 10 legs, swimmerets under the tail
Photo by David Nelson, specimen from Austin Creek, July 23, 2004

Crawfish Structure

The crayfish is typical of most shrimplike crustaceans and is characterised by a joined head and thorax, or midsection, and a segmented body, which in the species at camp is a reddish brown, although the larger ones have white markings around the base of the main claw, or chelipeds.

The crayfish are usually seen are about 3 inches to 6 inches long, but if you look among the smaller rocks you can find miniatures under 1 inch, or monsters greater than 6 inches in the larger pools. There are about 150 species in North America, and there are more than 500 throughout the world. Among the smallest is the 1inch long Cambarellus diminutus of the south-eastern United States, and among the largest is Astacopsis gouldi of Tasmania. It can grow to reach 16 inches and its weight is about 8 pounds.

The crawfish is an excellent example of body segmentation, a theme seen throughout the animal kingdom. The head has three pairs of sensory antennae (two pair are very small and usually overlooked; see the photographs below) and a pair of eyes on movable stalks. The "legs", or more properly, appendages, or pereiopods, all attach to the thorax. As noted above, there are 10 legs. Note that the animal is bilaterally symmetric, which means its right side and its left side are the same. This is another theme seen throughout almost all of the animal kingdom. The 10 legs, therefore, are organized as five pairs of legs. The first pair are the large claws, called chelipeds. Watch a crawdad at the bottom of a pool at Caz and you can see that the crawdad extends in front of its body while moving. These strong pinchers are specialised for cutting, capturing food, attack, and defence. A pinch can hurt! The next two pair of legs each have miniature claws and are used for feeding as well as walking. Watch a crawdad in a pool and you will see it pick up food with the large claws and then pass it to these smaller claws, then to its mouthparts. When you pick one up, be careful these don't nip you! They are not powerful enough to hurt, just tickle. The rear two pair of legs end in points, not claws, and are used only for walking. The large muscular abdomen is attached to the tail, which is used in short bursts to escape enemies, which include raccoons (and campers!). The crayfish also has five pairs of swimmerets which are under the abdomen. All of these "legs" can be regenerated if broken off.

Crayfish have a shell, or carapace, which is its outside skeleton, or exoskeleton. This jointed shell provides protection and allows movement, but limits growth. As a result, the crayfish regularly gets too big for its skeleton, sheds it, and grows a new larger one. (See below for a great science activity that you can do with the shed shell.) This is called molting, and occurs six to ten times during the first year of rapid growth, but less often during the second year. For a few days following each molt, crayfish have soft exoskeletons and are more vulnerable to predators.

You have a skeleton on the inside, called an endoskeleton. These bones help you to move and protect your brain, heart, lungs, and other soft parts.		The crawfish has its skeleton on the outside, called a shell or an exoskeleton. This shell does the same things as your skeleton: helps it move and protects is brain, heart, and other soft parts.

This is a diagram of a crawdad, from the top, or dorsal, side. A is the cephalothorax and B is the tail. C marks the second antennae; not shown are the two pair of first antennae. D indicates the eyes, E is the rostrum, a projection out over the eyes. F refers to the main claws, called the chelipeds, specialized for fighting, defending, and holding onto food. G shows the peripods, or walking legs. They also are used in feeding and probing crevices for food. The front two pairs have claws, the back two pairs are pointed and clawless. H is the carapace, or shell over the cephalothorax, that protects the internal organs. J is the uropod, or center part of the tail, and K marks the telsons, or lateral parts of the tail. Swimmerets, not shown, are on the underside of the tail, I.

Here you can see the two pair of first antennae that are not shown in the diagram above. They originate from just above and between the secondary antennae. Photo by David Nelson, specimen from Austin Creek, July 23, 2004

It is fun to try to catch a crawfish, place them in a shallow container and watch them crawl around. Try to draw one and you will find youself "seeing" more than you did before!

 

Crayfish Behaviour

Crayfish often conceal themselves under rocks or logs. They are most active at night, when they feed largely on snails, algae, moss and other vegetation, insect larvae, worms, fish, tadpoles, almost any kind of dead animal matter, old shed carapaces (crawdad shells), and even other crawfish. Studies show that adults (one year old) become most active at dusk and continue heavy feeding activity until daybreak. Young crayfish are more likely to be the ones out during bright sunny days, while the older crayfish are more active on cloudy days and during the night. Although they mainly feed in the water, they do venture out a little onto land, but they are vulnerable to predation and breathe by gills, so terrestrial activity is limited. Terrestrial activity allows them to escape from isolated pools when the creek dries up. General movement is always a slow walk. If you sit quietly on a rock, you can see them walk about the bottom, feeding and fighting, holding their large claws out in front. Small crawfish will always back away from a larger one. If startled, crayfish use rapid flips of their tail to swim backwards and escape danger.

Most crayfish live short lives, usually less than two years. Therefore, rapid, high-volume reproduction is important for the continuation of the species. Many crayfish become sexually mature and mate in the October or November after they're born, but fertilisation and egg laying usually occur the following spring. The fertilised eggs are attached to the female' swimmerets on the underside of her jointed abdomen. There the 10 to 800 eggs change from dark to translucent as they develop. The egg-carrying female is said to be ‘in berry,’ because the egg mass looks something like a berry. Females are often seen "in berry" during May or June. The eggs hatch in 2 to 20 weeks, depending on water temperature. The newly-hatched crayfish stay attached to their mother until shortly after their second molt. If you want to find baby crawfish, look in the small stones and gravel along the edges of the pools of Austin Creek.

Crayfish as Food

Most people know that crayfish are good to eat. In some places in the US, such as Louisiana, crayfish are a specialty and are served up boiled in spices, baked, fried, or served with noodles, rice, etc. Crawfish etouffee is a famous dish in New Orleans! It is delicious! Here is a recipe from Nurse Di who grew up in Louisiana:

2 tablespoons flour
2 tablespoons vegetable oil
3 cans chicken broth
1 medium bell pepper, chopped
2 medium onions, choped
3 celery ribs, chopped
1 can cream of mushroom soup
1 (15 ounces) can tomato sauce
1 tablespoon parsley flakes
1 teaspoon Accent seasoning
3 cloves garlic, minced
1 teaspoon salt
1/2 teaspoon pepper
1/4 teaspoon basil
1/4 teaspoon poultry seasoning
1 teaspoon Worcestershire sauce
1 teaspoon lemon juice
2 lbs crawfish tails
1/2 cup chopped green onions
hot cooked rice

References

The Brigham Young University website is full of scientific information.

Acknowledgements

The crawdad diagram is from the Louisian Marine Education Resources website.
The description of the structure, behaviour, internal anatomy, and science is taken or adapted from the Crayfish Corner.