CAMBARUS
1. CLASSIFICATION
PHYLUM ARTHROPODA
CLASS CRUSTACEA
SUBCLASS MALACOSTRACA
ORDER DECAPODA
FAMILY ASTACIDAE
Genus cambarus (in western united states, the pacific form, pacifastacus, may be used).
2. General observations. the crayfish, a common inhabitant of freshwater streams and ponds,
often emerges at night and makes characteristic burrows and mounds in wet fields or gardens. along with its marine relatives, lobsters and crabs, the crayfish feeds chiefly on decaying organic matter, (it is also locally known as crawfish) observe the living crayfish in an aquarium.
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· How does the animal right itself when turned over?
· Pick your crayfish up by the dorsal shield (carapace). Hold it firmly!
· Why does the animal buck and flip its tail down so strongly?
· What is the function of this movement when the animal is in water?
· How do you suppose crayfish dig in soft
Observation respiratory currents of a crayfish resting in a shallow pan of water and note how they are produced. Add a few drops of India ink near the animal and watch the particle flow into the respiratory stream.
SUGGESTION
In many areas of our southern and prairie state, it is possible to watch foraging and digging crayfishes (“crawdads”) at night, seek them out with a flash light in marshy or moist open fields and wach their pattern of movement, digging, and rapid escape. Dig a few from their tunnels and observe their action in a laboratory or home aquarium. Baked or boiled crayfishes represent another biological aspect of these organisms well worth investigating,
3. External anatomy (FIGURES 12.12 and 12.13 ) if you Specimen is preserved in formalin, avoid fumes by rinsing it overnight in tap water before dissection.
a. Segmentation. Observe the tagmata, or grouping of segments, into three general regions (1) head (somites i-v , with antennules, antennae, mandibles, and two pairs of maxillae) (2) thorax (pereion) (somites vi-xiii, with three pairs of maxillipeds, one pair of chelipeds, and four pairs of walking legs or pereiopods) and (3) abdomen (pleon) (somites XIV-XI, with five pairs of swimmerets or pleopods and a uropod and telson (Tail)
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· Are all appendages jointed?
· How do the pincers operate? (Better use a pickled specimen)
· What are the advantages of a hollow exoskeleton?
· How does the surface area for muscle at tachement compare between a hollow exoskeleton and a solid endoskeleton?
· What are the disadvantages of an exoskeleton?
Observe the pag and socket joints between abdominal segments. The exoskeleton of each body segment is divided into a dorsal tergum, a lateral pleuron, ad a ventral sternum. Find these parts on the abdominal and thoracic someites of your specimen
b. Body parts. Before removing the appendages and studying their relationship, identify the major body parts visible dorsally, (FGURE 12. 11) head, antennule, antenna, cephalothorax, carapace, cervical grovve, rostrum, eyes, cheliped, pereiopods, uropod, and telson, then observe the crayfish from the lateral view and located the fill chambers under the pleura of the caracpace (BRANCHIOSTEGITES). Study the animal ventrally and observe its 19 paired appendage. First, find the anteriormost sensory antennules, biramous (two branched ) structures with many joints and a balancing organ or statocyst n the flatteened dorsal surface of the basal joint. Next are theantennae, a pair of long, slender, many jointed appendages with excretory openings at the basal segment of each. Then come the mandibles or chewing jaws, followed by two pairs of maxillae for food handling. Three pairs of mxillipeds follow,. These serve for manipulation and sensory perception f food. Then, completing the largest claws, the first pair of walking legs, followed by the remaining four pairs of walking legs (pereiopods, a term taken from pereion, or thorax, the thoracic legs), five pairs of swimmerets, and , finally, the uropods, the terminal pair of appendages, which with the central telson provide a broad propulsive paddle, also used for e
gg protection
Next review the location of the following body openings: moth , anus (one the ventral surface of the telsion, the central portion of the tail) and excretory pore on each antenna, find the external sex openings. the male genital ducts or onoducts open at the base of the last leg. A trough for the transfer of spermatophores (SPREERM CAPSULES) into the female seminar receptacle is formed from the fused tubular first and second swimmerets (figure 12.150. the oviducts open at the base of the second walking legs , another pore at the base between the fourth walking legs serves to receive the spermatophores, the first swimmeret in feels is Small lor absent, the second is a typical swimmeret rather than a specialized organ as in the male. The female telson and filamentous swimmerets hold the egg cluster to form an external brood pouch for the eggs and young. Females carrying such a mass of eggs are said to be “in berry”
c. Appendages (Figures 12.14, 12.16). after this initial examination of your specimen, you should be ready for a detailed review of the appendages
Learn not only functional and structural differences between appendages but also their relationship to a common prototype which is through to be a simple, biramous (two-branched) appendage such as the swimmeret, species thought to be primitives shows a repetition of parts with little specialization as seen in the brine shrimp. In more highly evolved animals, the specialization as seen in the brine shrimp. In more highly evolved animals, the specialization of appendages , especially on the head, is apparent. There are striking differences between a brine shrmp with its many similar biramous appendages (homologous matamerism) and a crab, shrimp, or crayfish with highly modified appendages (heretonomous metamerism), apparently derviced from more simple appendages,. structrueal changes in serially homoloous parts can often be traced by comparing them with a unspecialised segment or appendage.
Examine a sample dissection of crayfish appendages if one is available. Read the following instructions carefully before starting your dissection and study figure 12.14
Each limb consists of three basic portions: the stem or attachment, the protopod, and two branches (endopod and exopod ) are attached. Each appendage fundamentally follows this structural pattern, though with considerable modification in the head region. In some cases , homology can be determined only by embryological study, owing to the loss of some portions during growth and to the development of more specialized appendages, in certain cases, as in antennules, the precise homology of the segments is still uncertain
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Carefully cut away the lateral extension of the carapace covering the fills, the branches tegite, from one side to expose the internal sheet of gills (figure 12,17)
Begin your dissection with uropod and work forward, the simple structure of the abdominal appendages more closely resembles what is presumed to have been the primitive crustacean biramous appendage, and makes a more suitable starting point, each appendage should be examined critically with a hand lens or under a dissecting microscope. Observe the structure, check it against its stated function, and note the increased degree of structural specialization as you proceed toward the head. You may find it more instructive to pin the appendages alongside the animal under water in a waxbottom dissecting pan, in order to view them from the same orientation, afterwards the appendages can be dried and glued onto a cardboard sheet and labeled for later reference (figure 12.16). be sure to remove a bit of attachment membrane along with each appendage to ensure having the intact structure. Refer to figure 12.14 for detailed structure and terminology
Beginning with the uropod, remove one appendage at a time the side on which you have exposed the fills, identify the three basic parts (if all are present ) and pin out the appendage, carefully orienting the protopod to the left, the endopod anteriorly, and the exopod posteriorly. Compare the normal plane of movement of each part and of each limb with the others to get an idea of how the appendage functions. From the uropod proceed anteriorly through the simple pleopods, noting again the sexual differences and water circulating role of these organs. The walking legs, removed next, are of interest for their endopod specialization (the exopod disappears entirely during development ) and for their respiratory role, special gills , podobranchlea, are attached to certain legs (which ones?) the coxopod has a specialized extension or
epiopod to which the gill filaments attach
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