THE CRAYFISH

External Anatomy of the Crayfish  (Cambarus sp.) 

            Crayfish are found in streams, rivers, lakes and ponds where the water contains adequate amounts of calcium salts.  They are primarily nocturnal, hiding in crevices and under rocks during the day and emerging to feed at night.  They will eat almost anything organic - plant or animal, living or dead. 

            Obtain a preserved crayfish (either a female or a male) and place it in your dissecting pan.  The body is divided into an anterior cephalothorax (fused head and thorax) and a posterior abdomen (Figure 11). The chitinous exoskeleton protects the crayfish from predators.  Because the exoskeleton surround the body, it must be molted periodically to allow for growth. Until the new skeleton hardens, the animal is helpless. Observe a demonstration of crayfish exoskeleton.

The carapace is a saddle-like covering over the cephalothorax.  A transverse groove separates the fused head from the thoracic region.  The rostrum is an anterior, pointed extension of the head. A pair of stalked eyes is located on the lateral side of the rostrum. One pair of antennae ale located on the head as well.

 The abdomen consists of several segments and is terminated by the telson (Figure 11).  Examine the appendages and note that the appendages are modified to serve a variety of functions: feeding, walking and swimming. 

Figure 11: External features of the crayfish (lateral view).

The male and female crayfish have an equal number of appendages. However, in male crayfish the first two pairs of swimmerets have been modified.  They are elongated and can be brought together to form a trough-like channel, used for the transfer of sperm from the male to the seminal receptacles of the female.  In female crayfish, all of the five pairs of swimmerets are of the same size and are used for swimming. Use Figure 12 to assist you in determining if your specimen is a male or a female.

Figure 12:  Male (A) and female (B) crayfish  (ventral view).

Cambarus sp.