The Blue Crab
BY W. P. HAY, M.S., PROFESSOR OF BIOLOGY, HOWARD UNIVERSITY.
(Appendix to the Report of the Commissioner of Fisheries.)
THE following report embodies the results of two summers' work (1902 and 1903) in the crab-producing region bordering Chesapeake Bay. The information was gathered incidentally in connection with a thorough study of the diamond-back terrapin, and on that account is by no means as complete as could be desired. Many of the theories advanced by the fishermen and packers regarding the blue crab have not yet been subjected to close examination, although every opportunity has been taken for this purpose. In some cases the reports secured were so contradictory that it is not deemed safe to express an opinion concerning them. Quite a number of facts, however, have been brought to light, and they are here presented in the hope that they may prove valuable to those engaged in the fishery or to those whose duty it is to secure the enactment of laws to regulate and prolong it.
Distribution and Habitat.
The natural range of the blue crab is from Massachusetts Bay to some as yet undetermined point on the east coast of South America. On the coast of the United States it is common from Cape Cod to the southern extremity of Texas, and throughout the greater portion of this long coast line it is very abundant. Its favorite habitat is in the water of some bay or at the mouth of a river, and it seems to prefer shallow water to that of much depth. Consequently, such bodies of water as Delaware Bay, Chesapeake Bay, and the protected channels along the coast of Virginia and other South Atlantic and Gulf States, fairly swarm with these creatures. Chesapeake Bay is especially favorable and has long been famous, not only for the great number of crabs which it produces, but also for their large size and exceptionally fine flavor.
Although the blue crab is essentially an inhabitant of salt water, it is frequently found in water that is only slightly brackish or even apparently quite fresh. Specimens have been recorded from the Hudson River as far north as Newburg, and on creditable authority I have learned of the presence of an occasional individual in the Potomac River and the Eastern Branch opposite the city of Washington. At Crisfield, Md., and at other points along both the eastern and western shores of Chesapeake Bay, I have frequently observed the blue crab in ponds and ditches, often at a distance of a mile or two from the bay and in water that was nearly fresh.
Power of Movement.
Either in the water or on the land the blue crab is an animal of great activity and has considerable power of endurance. Progression through the water is effected by means of a sculling motion of the broad, oar-like posterior legs, and under ordinary conditions is slow, the effort of the animal being apparently only to keep itself afloat while it is borne along by the current. Under these conditions the movement is either backward or sidewise. The shell is held with the posterior portion uppermost, the legs are brought together above the back and strike forty strokes per minute. When alarmed, however, the animal strikes out with great vigor and rapidity, moving its paddles too swiftly for the eye to follow; it moves though the water almost as rapidly as a fish and quickly sinks below the surface. When on the bottom and undisturbed, the crab may be seen to walk slowly about on the tips of the second, third and fourth pairs of legs, the large pincers being held either extended or folded close under the shell and the paddles either raised and resting against the back of the shell or assisting the movement by slow sculling strokes. In such cases the movement is in any direction -- forward, backward or sidewise -- although the usual direction is sidewise. If the animal becomes alarmed it moves away by a combination of the walking and swimming motions and often disappears like a flash. In fact, so rapid is the movement that it is almost impossible to see how it is accomplished. It is too steady and uniform to be a series of leaps, and the animal seems too far above the bottom to be running upon it; yet all the legs are in motion except the large first pair. Of the latter, the one on the side toward which the animal is moving is held straight out sidewise, while the other is folded under the shell.
Method of Concealment.
The coloration of the crab is such as to harmonize very perfectly with the surroundings, and the animal attempts very little concealment if there are other objects on the bottom. Often, however, a clear, sandy bottom or some oozy pond will be found to be almost alive with crabs which have buried themselves until only their eyes and the antennae are exposed. In thus hiding, the crab goes nearly vertically backward into the bottom and then, by a few movements, turns slightly, so that the shell rests at an angle of about 45 degrees. The material above settles down and effaces all traces of the entrance. It usually happens that the bottom affected by the crab is firm enough to render this operation somewhat slow, and it rarely attempts to escape pursuit in such a way. It seems probably that concealment is usually adopted as an ambush from which a sudden attack can be made on some passing fish.
In certain places, notably shallow ponds and stream which become nearly dry at low tide, the crab may be observed to dig rather large conical holes, apparently as reservoirs and to take up its position in the deepest part. The work of making such an excavation often requires two or three hours, usually commencing soon after the tide has begun to ebb strongly and continuing until the edge of the excavation is nearly exposed above the water. The animal works from some suitable point, carrying away load after load of material clasped between the large claw and the low surface of the front of the shell. It loosens up the surface with the tips of its second, third and fourth pairs of legs, grasps all it can carry and then moves off a few inches in the direction of the side which bears the load and deposits it so that it will not roll back. Thus the hole is gradually
The blue crab has very seldom been seen to come out on land voluntarily, although it is able to sustain life for several hours when removed from the water. In low swampy situations I have occasionally seen an individual moving about in the dense grass or hanging to the grass just above the water, and in Miss Rathbun's paper, "The Genus Callinectes," there is a description by Mr. Willard Nye, Jr., of the migration of a large number of crabs from a small pond to the ocean over a beach 400 feet wide. They had been imprisoned in the shallow water and were forced by cold weather to make the excursion to deeper places.
During the molting periods the crab will always hid itself, if possible, under some submerged timber, rock, or bunch of grass. Here it will remain quietly until after its shell has been shed and the new shell has hardened.
The color of the crab is more or less variable, and it is believed by the fishermen that the animal is able to change its hue slightly to approximate the color of its surroundings. Light grayish-green individuals are said to be taken on sandy bottoms, while the dark olive-green are said to be found among the grass. This theory, however, is not very well borne out by crabs held in captivity in the live boxes, for there they retain their original colors, and even after they have cast their shells exhibit quite as much variety as before.
Food.
The blue crab's food is of a varied character, but the animal is pre-eminently a scavenger and a cannibal. In the shallow waters of ponds and small tidal streams it preys to a certain extent upon small fish, which it stalks with some cunning and seizes by a quick movement of its large claws. In such situations, too, I have sometimes observed it nibbling at the tender shoots of eel grass or other aquatic vegetation, or picking at the decayed wood of some sunken log. Its favorite food, however, is the flesh of some dead and putrid animal, to obtain which it will travel a considerable distance from its hiding place. A piece of stale meat or a rotten fish will attract the crabs for several yards around, and they will swarm over the morsel until it is entirely devoured. The offal from stables and water closets which project over the water furnishes the crabs with many a meal, and in such spots numbers of the animals may be observed lying in wait for food.
Wherever crabs are abundant they constitute a source of great annoyance to fishermen, for they are adepts at stealing bait from the hooks, and will return time and again after having been drawn to the surface of the water and apparently frightened away.
An injured crab, if thrown into the water, will be speedily set upon by its associates and torn to pieces. Even one that is uninjured, if small or in the soft-shelled condition, is likely to be captured and eaten by stronger individuals.
In eating a bit of food the crab first grasps it in the large claws and pushes it back under the front of the shell, where it is seized between the tips of the second pair of legs and pushed forward and upward to a point where it can pass between the third maxillipeds to the jaws. These strong organs masticate the food while the other mouth-parts prevent the escape of the smaller particles. It is then swallowed and the complicated set of teeth in the stomach reduce it to a thin fluid mass before it is allowed to pass into the intestine.
Digestion in the crab seems to be a rapid process, for the food disappears so quickly from the stomach that this organ is usually found to be perfectly empty within a few minutes after having received a full meal. It is a common idea among the fishermen that food is not retained in the crab's stomach at all, but this I have disproved by numerous dissections.
Reproduction.
The sexes of the crab are separate, and reproduction is effected by means of eggs, which are laid by the female after copulation.
Crabs may be found pairing at almost any time during warm weather but there seem to be five or six periods between early June and the beginning of cold weather, when the act is at its height. During these times mated crabs, "doublers," as they are called by the fishermen, are found in considerable numbers, either lying on the bottom in shallow water or swimming at the surface. It appears that the male crab is able to distinguish the female which is about to shed her shell, and having found such a one, seizes her and carries her about with him, sometimes for a day or two, until the shedding of her shell is imminent. He then places her in some sheltered place and stands guard over her ready to repel the advances of any other male. At this time the female invariably is of the virgin form, and copulation has not taken place. When she sheds her shell, however, she has passed into the ovigerous form, the broad semi-circular abdomen of the virgin form. She is now ready for copulation, and is immediately approached again by her mate. When the female is ready to produce her eggs for this act it seems that she seeks the ocean or the mouth of some large bay. In Chesapeake Bay mating crabs are abundant at least as far north as Annapolis, but a crab with eggs is very seldom found there. On the other hand, at Cape Charles City, Va., at Hampton, Va., and neighboring points, egg-bearing females are far more abundant that either males or virgin females during the latter part of summer, but apparently do not often come into shallow water.
The eggs of a crab are very minute, about one-one hundredths of an inch in diameter, and they are very numerous, it having been estimated that a single female may produce as many as 3,000,000. As soon as the eggs are laid they adhere to the fringes of hair on the swimmerets and form a mass which is nearly a third as large as the female's body. They are carried about thus until they hatch, when the young, after clinging to the mother for a short time, loosen their hold and begin a free existence.
Molting.
In practically all the lower animals whose bodies are
As the crab approaches the shedding period it begins to show its condition by various external "sign," which are well known to the fishermen and are of great importance to them. The first indication is a narrow white line which appears just within the thin margin of the last two joints of the posterior pair of legs. This line is so narrow and so obscured as to be barely visible, but it is immediately detected by the expert, and the individual bearing it is classed as a "fat crab," or more vulgarly as a "snot." Within three or four days the white line gives way to an equally narrow and obscure red line, and a set of fine white wrinkles makes its appearance on the blue skin between the wrist (carpus) and the upper arm (meros). Such a crab is known as a "peeler," and may be confidently expected to cast its shell within a few hours. As the time progresses the marks become more and more evident, and a reddish color (especially in virgin females) begins to appear at the margins of the segments of the abdomen. Then, on the under surface of the carapace, extending from the neighborhood of the mouth around the sides and backward to the posterior margin, there appears a narrow fracture, so that the whole upper surface of the shell can be raised up from the back like a lid, to expose the soft body beneath. Such a crab is termed a "shedder" or a "buster." At this time the animal usually lies motionless, but if disturbed is still capable of movement, and may crawl or swim slowly away. It is incapable of showing any great muscular force however, and can inflict only an insignificant pinch with its claws.
The actual casting of the shell is now a matter of only a few minutes; a quarter of an hour will usually suffice, though the operation may be prolonged to three or four times that period if the crab is disturbed or if it is suffering from some recent injury. In the latter case it is often unable to complete the process and dies. By convulsive, throbbing movements the hinder pair of legs begin to be withdrawn from their encasement and are finally freed. Meanwhile, the other legs have been started out and the body has begun to protrude more and more from the shell. At last everything is out except the front of the body and the large claws, but the latter, on account of the great discrepancy between their size and that of the narrow articulations through which they must be withdrawn, require some further effort before they can be freed. The thing would hardly be possible at all were it not for the fact that on the upper surface of the large segment of the arm (meros) a broad triangular surface of the shell becomes loosened and rises up like a flap to make way for the crowded tissues within. Some of the hard shell of the other lower (proximal) segments also seems to become softened and elastic, so that by a steady pull the great pincers are finally drawn through. Thus the crab has backed out of its shell and meanwhile it has grown, for if it is caught and measured it will be found to be considerably larger than it was before.
The skin is soft and the animal looks and feels flabby and helpless. The back is wrinkled, and the "horns," or large lateral spines, are curled curiously forward. Within a few minutes, however, the body fills out, the horns straighten, and the growth at this interval is complete. The crab is now known as a "soft-shell," and from the aesthetic standpoint is at the height of its glory, for all the brilliant coloration of the various parts is undimmed by any of the shell deposits, the soft integument seeming to bear the bright pigments at the very surface.
Under natural conditions the crab usually selects some place of concealment in which to pass the period of shedding, and probably does not leave it until the new shell has hardened, but it is by no means helpless, even immediately after ecdysis has occurred. On the tips of legs which seem too soft to support any weight whatever it can walk away, or, if forced to make the effort, can swim. The new shell hardens quickly. Within twelve hours it becomes parchment-like and the crab is called a "buckler," "buckram," or a "bucklum"; in two or three days it is as hard as ever and once more starts out in search of food.*
Autotomy.
Autotomy, or the automatic throwing off of the appendages, is very characteristically shown in the blue crab and is of frequent occurrence. Very often if a large individual, in the hard-shelled condition, is captured and held by one leg it will snap the limb off and make its escape. Likewise, if one of the legs is injured toward the tip the entire member will be dropped off. The breakage always occurs at the same point -- across one of the segments near the base of the leg -- and is a provision of nature to prevent the animal from bleeding to death. It is practiced ordinarily only by the hard-shelled crabs; an injury to a soft-shelled individual usually causes death. Under other conditions, however, -- notably, a sudden lowering of temperature -- the act has been observed, and in one of the early attempts to procure soft crabs for market, by confining the hard crabs in an inclosure until they had shed their shells, severe cold weather reduced the entire catch to a log of legless bodies ("buffaloes," they are called by the fishermen).
Autotomy seems to be limited to the legs, for, so far as I have been able to determine, none of the other appendages are ever thrown off, although if they are forcibly removed they will be regenerated.
Regeneration of the parts cast off usually follows autotomy, but, according to the researches of several biologists, will not take place indefinitely. Three or four times seems to be the limit. The process of regeneration is quite rapid. At the first molt after a limb has been cast off, provided that the injury does not occur immediately before a molt, the new limb appears as a small bud in which all the missing segments may be found, coiled in an elongate spiral. At the next molt the segments straighten out and the new limb, except for its smaller size, looks like the one which was cast off. Another most, possibly two, will be sufficient to restore the limb to its full size.
*It is believed by the fishermen that the molting of the crabs is influenced largely by the moon and the tides, but the evidence to support this theory is very contradictory.