Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.3.1.External Anatomy
We have seen that although crabs appear to be tailless, they have a very small tail, which they keep tucked underneath their body. Due to its small size, this tail and its appendages cannot be used for locomotion. The thoracic legs of a crab are used for walking. In certain crabs, including the blue crab, the last pair of thoracic legs is flattened and paddle-shaped and is used for swimming.

While the tail of shrimp or lobster is among the meatiest and most succulent portions of the animal, the tail of a crab contains little meat. The dorsal abdominal muscles are small and very weak, being used solely to extend the tail backward. Virtually the only time at which these muscles are used is during mating, when the abdomen of both male and female must be drawn backward to permit the transfer of sperms.

The ventral abdominal muscles of crabs are somewhat heavier and stronger, particularly in the mature female. While carrying eggs, she used these muscles to curl her broad, rounded abdomen over the mass of eggs. When not carrying eggs, she uses these same muscles to hold her abdomen tightly in a depression on the ventral surface of her body. Male crabs have a “locking device” consisting of small tubercles on the fifth thoracic segment that secure the triangular or T- Shaped abdomen in a depression on the ventral side of the thorax Covering both head and thorax of a crab dorsally is a hard carapace. Thus, the boundary between the two body regions is obscure and, as in shrimps and lobsters, one generally speaks of a cephalothorax rather than of the two separate regions. The cervical groove, indicating the boundary be¬tween head and thorax, lies just behind the center of the carapace, where it runs generally forward and to each side.

Ventrally, the boundary between head and thorax is well marked, as is the division of the thorax in to segments, al¬though only the last five may readily be visible. Also the attachment of the thoracic legs to the exoskeleton is clearly apparent, one pair on each of these last five thoracic seg¬ments. The first pair is modified as chelipeds, or claws, while the remaining four pairs are adapted for 'balking or, in some cases, for walking and the last pair for swimming. In two families of primitive crabs (Dromiidae, Dorippidae), the last pair or last two pairs of thoracic legs ar held dorsally, often supporting a piece of sponge or bivalve shell or some other type of sheltering material.
The cephalothorax of a crab is characteristically short and broad and, in some species, greatly extended to the sides. In the blue crab, Callinectes sapidus, the paired, widely expanded branchial regions of the carapace terminate in a long, sharp lateral spine. Here the exoskeleton turns sharply inward and downward, to end just above Ube legs. As a result of anterior—posterior compression and lateral expansion, the branchial chambers are short and wide. Within these cham¬bers the gills are, of necessity, arranged in a broad oval, rather than linearly as in shrimps and lobsters. Indeed, at their base the most anterior pairs of gills in a blue crab "face" forward.
Contrary to widespread popular belief, crabs can walk for¬ward or diagonally, and some species do so quite often. But usually crabs move sideways, particularly when hurrying. The attachment of one pair of chelipeds and four additional pairs of thoracic legs within the short space available at the side of a crab favors sidewise movement over forward move¬ment. When a crab runs sidewise, the legs on the leading side pull the body by flexing, while the legs on the trailing side push the body by extending.
One genus, the semiterrestrial ghost crab, Ocypode, can run at great speed. In tests by Dr. Dennis R. Hafeman and Dr. J. I. Hubbard, the species Ocypodc ceratoplithalrna ran at an average speed of 1.825 meters per second, or over four miles per hour, on the firm sand of a tidal beach. When on the hard deck of a ship, the crabs ran even faster, the average speed being 2.33 meters per second, or 5.2 miles per hour. These are the highest recorded speeds for any crustacean. During the tests the crabs did not use their last (fifth) pair of thoracic legs or their chelipeds, except for balancing. The second, third, and fourth pairs of thoracic legs did the moving, with the second and fourth legs on the leading side usually being extended first, to be followed by the third leg. On the trailing side, the same sequences occurred, but with a phase lag of about a third of a cycle.
Some observers have reported that when Ocypode is run¬ning, it does so with one side leading for a while. The crab stops abruptly, rotates its body, and then runs with its other side leading. The process of rotation is repeated. In this way, the flexor and extensor muscles of the legs on each side are alternately used and rested.
In the anterior portion of the cephalothorax of a crab are the mouth parts, grouped around the opening to the esophagus. These mouth parts are generally similar to those of shrimps and lobsters. The outermost pair is the third maxillipeds, used for holding food. Under and in front of these are two more pairs of maxillipeds and two pairs of maxillae, also used for holding food, and a pair of mandibles, or jaws, which push the food into the esophagus.