Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.3.6. Reproductive System
The ovaries of a female blue crab are connected to each other just behind the foregut and extend forward and back¬ward through the body. Thus, in blue crabs, as in American lobsters, the ovaries appear roughly H-shaped. In early stages of its development, each ovary of a blue crab is thin and white, with a short lateral arm. It still appears this way im¬mediately after the female has shed her shell and, as a soft crab, has mated. But ovarian growth starts soon thereafter and, several months later, results in a very large ovary, which is orange because the eggs are full of orange yolk. Each ovary now may extend far out to the side of the body and into the first abdominal segment.
From the ovaries, paired oviducts run forward and down¬ward for a short distance, then widen to form an oval-shaped structure known as a seminal receptacle.. Here are stored the sperms that the female blue crab receives from the male blue crab during mating. Each seminal receptacle slants backward and downward and then narrows into a short tubular vagina, which runs ventrally to an opening on the sixth thoracic segment. Although the oviducts and the dorsal portion of the seminal receptacles are soft and unlined, the ventral portion of the seminal receptacles and the vagina are hard, being lined with chitin. At the time of ecdysis, this lining is shed, along with other portions of the exoskeleton.

In an immature blue crab the seminal receptacles are small and white. Yet, in a mature crab immediately after copulation, the seminal receptacles are enormously distended, at times equal in size to the heart; and they are pink in color, due to the presence of a gelatinous "sperm plug" that keeps the sperms secured within the receptacles. Later, after the sperm plug has been absorbed, the receptacles are again white.
At the time of ovulation, when ripe eggs are released from the ovaries and move down the oviducts, sperms fertilize the eggs either within the oviducts or within the seminal recepta¬cles. When fertilized eggs emerge from the vagina, they be come attached to the pleopods of the female and remain there until ready to hatch into the first larval stage. Yet many sperms remain within the seminal receptacles and many eggs within the ovaries, so usually a second "laying" occurs, after which the ovaries appear collapsed and grey or brown in color as they begin to degenerate. Yet even now, enough sperms remain within the seminal receptacles to fertilize several more batches of eggs, were the eggs able to ripen.
In an immature blue crab the seminal receptacles are small and white. Yet, in a mature crab immediately after copulation, the seminal receptacles are enormously distended, at times equal in size to the heart; and they are pink in color, due to the presence of a gelatinous "sperm plug" that keeps the sperms secured within the receptacles. Later, after the sperm plug has been absorbed, the receptacles are again white.
At the time of ovulation, when ripe eggs are released from the ovaries and move down the oviducts, sperms fertilize the eggs either within the oviducts or within the seminal recepta¬cles. When fertilized eggs emerge from the vagina, they be come attached to the pleopods of the female and remain there until ready to hatch into the first larval stage. Yet many sperms remain within the seminal receptacles and many eggs within the ovaries, so usually a second "laying" occurs, after which the ovaries appear collapsed and grey or brown in color as they begin to degenerate. Yet even now, enough sperms remain within the seminal receptacles to fertilize several more batches of eggs, were the eggs able to ripen.

In the male blue crab the testes consist of a pair of slender, convoluted, opaquely white arms lying on the dorsal surface of the midgut glands. Medially, the terminal portion of each arm passes around the posterior end of the foregut and joins with the other arm to form a short cross-bar. Just anterior to the cross-bar a tiny tube, the vas efferens, connects each arm of the testes with a much-coiled vas deferens. The vas efferens is difficult to find since it is concealed within the testis and the coils of the vas deferens.
The vas deferens consists of several portions. The first known as the anterior vas deferens, is white and tightly coiled and lies close to the middorsal line between the foregut and the heart. Here the sperms are gathered in egg-shaped bun¬dles, called spermatophores, and stored. In the second .portion, the median vas deferens, the coils form a large mass and appear pebbled pink, due to their content of material that subsequently is deposited in the seminal receptacles of the female during copulation and forms a sperm plug.

The third portion, the posterior vas deferens, is long, con¬voluted, greenishly translucent, and almost empty except during passage of the spermatophores. The final portion, the penis, is a short, translucent tube at the base of the last pair of thoracic legs. The penis lies permanently within a groove in the first pair of abdominal appendages, the copulatory pleopods. These, in turn, are inserted into the seminal recep¬tacles of the female during copulation. Also fitted into the groove on the copulatory appendages of the male is his sec¬ond pair of pleopods, which during copulation act as pistons to push the spermatophres along the groove, where they break up and release the sperms.

The T-shaped abdomen and elongated, grooved copulatory pleopods of the male blue crab are his most distinguish¬ing external sexual characteristics. In contrast, the abdomen Of the mature female blue crab is broad and rounded, and her pleopods are relatively short and fringed with hair-like setae, to which eggs are attached during development. Coloration also serves to separate the sexes. Normal coloration of both male and female blue crabs consists of a dark blue-green or gray-green carapace and bright blue and blue-green legs, with scarlet markings. Except for the appendages and the female abdomen, the underparts are white. In the male, the greater portion of the chelipeds, or claws, is blue-or gray-green, with dull purple "fingers," whereas in the female there is more blue on the chelipeds and the "fingers" are bright red.
It may interest the reader to learn that just as there are blue American lobsters, so also there are blue blue crabs. Some years ago a specimen of blue dab' was described as having a carapace of robin's egg blue and appendages of pale blue with traces of pale red. The under surface of the body was white. Also, just as parti-colored American lobsters exist, so do parti-colored blue crabs. One such specimen was described as being gray on the left side and brownish on the right. A tendency toward albinism occurs in blue crabs, as it does in American lobsters.