Anatomy of finfish and shellfish

Unit 12 - Arthropoda

12.2.7. Reproductive System
In the American lobster the ovaries of the female appear in the form of a letter H, with the cross bar at the forward margin of the heart and with longitudinal lobes extending forward and backward through much of the animal. The stage of ovarian development is apparent from the color, bright yellow or flesh-colored early in development, then salmon, light green, and finally a rich dark green by maturity. After cooking, the mature, egg-filled ovaries are bright red and are known as the coral.
From the ovaries, paired ribbon-like oviducts emerge at a level just below the heart, then quickly narrow as they run outward to the body wall and downward to the base of the third pair of thoracic legs, where they terminate on the inner surface of the basal segment. Externally and medial these openings is a triangular, bluish structure extending from the base of the third to just beyond the base of the fourth pair of thoracic legs. This is the seminal receptacle, a small pocket in the exoskeleton that receives sperms from the male during mating.

In males of the American lobster the testes, which are pale tan-grey in color, may be H-shaped, like the ovaries of the female, or longitudinally paired, without a cross bar. From the testes, paired ducts, the vasa deferentia, emerge beneath the heart, at approximately the same place that the oviducts emerge from the ovaries. Like the oviducts, the vasa deferen¬tia run outward to the body wall before turning downward. At this point they become S-shaped, with their posterior margin thickened and glandular, capable of secreting a gelatinous material that coats the sperms as they pass through the duct. The vasa deferentia then become briefly bulbous and muscular and, following this, narrow and thin-walled, forming an ejaculatory duct that opens at a papilla on the inner surface of the base of the fifth (and last) pair of thoracic legs.

The most obvious external difference between male and female American lobsters lies in the shape of the first pair of abdominal appendages. In the male these are the copulatory pleopods, relatively long, hard, grooved, and tapering. In the female these pleopods are small and soft. Yet there are other sexual differences, for mature males are heavier and have lager claws and a longer, more swollen carapace than have mature females.
Also in spiny lobsters, the sexes can he separated by differ¬ences in the abdominal appendages. In males the pleopods have one leaf-like terminal segment. In females the pleopods have two terminal branches, those of the first pleopods being leaf-like, while those of more posterior pleopods have one ref1ike branch and one rod-like branch used for attachment of eggs. In addition, the fifth pair of thoracic legs in male spiny lobsters terminates in a single, simple segment like that of more anterior legs, whereas in females the fifth pair of thoracic legs terminates in a small claw used in cleaning the attached eggs. Mature males tend to be larger than mature females.
Before leaving the subject of structure in American lobster, we may give some thought to coloration, for it can be surpris¬ingly variable and frequently serves a protective function, enabling a lobster to blend with its background.
Normal, or "wild-type," coloration of American lobsters is mottled olive-green or dark blue-green above, with small black or-green-black spots and often red tubercles and spines. On some lobsters the sides of the body and tail, as well as large portions of the claws, may be dusky orange, often dot¬ted with green-black. Other lobsters are almost entirely dusky orange, with green-black spots. Such variations in color exist among lobsters of widely differing sizes, from the one-pound individuals commonly purchased in fish markets to the lobsters of 10 to 15 pounds or more that are caught on the southeastern part of George's Bank and in areas to the south.
American lobsters may be of other colors as well. Some, known as calico, or leopard, lobsters, are light yellow with purple-blue marbling or spots. Other lobsters are rich indigo blue, with bright, clear blue on the sides of the body and on the extremities. Sometimes lobsters are pale red, hardly dis¬tinguishable from the cooked animal when seen from above. Yet, whatever their color topside, live American lobsters tend to be very lightly pigmented, or even cream-colored, underneath.
Occasionally, fishermen catch American lobsters that are cream-colored above as well as below, but with dark eyes and often with red pigment on the underside of the claws. Or such a cream-colored lobster may have faint traces of blue in its shell, as did one that was exhibited in Boston at the New England Aquarium. Such lobsters are frequently called al¬binos, although true albinos lack all pigment in eyes and shell. True albino American lobsters apparently have never been taken.
No single factor is responsible for the differences in color of American lobsters. The basic color pattern is inherited, just as are color and texture of hair in man and other mammals. But in an American lobster the actual color that develops may depend partly upon the type and strength of illumination to which the animal is exposed and even more upon its diet.
Thus, Professor F. H. Herrick, who in 1895 published .a classic monograph on the American lobster, observed that bluish coloration in this animal can result from prolonged exposure to sunlight. Recently, John T. Hughes and George C. Matthiessen of the Massachusetts Division of Marine Fisheries reported that lobsters held for a period of years at the Division's lobster hatchery and rearing facility in Oak Bluffs, Martha's Vineyard, and fed primarily quahaugs, clams, scallop viscera, and alewives, turned a deep sky-blue color, which eventually faded into a pale blue—grey. When, however, these bluish lobsters were then fed exclusively on green crabs, they reverted somewhat to the wild-type colora¬tion after the next molt and became identical in coloration with the wild-type following the second molt.

Color in all decapod crustaceans results primarily from the presence of pigments known as carotenoids (after carrots, from which they were first isolated) in the tissues and shell. The major carotenoid of decapod crustaceans is astaxanthin, which is bright red in color. When combined, or conjugated, with protein, the red color of free astaxanthin is replaced by a color characteristic of the particular conjugated protein that is present. For example, in the shell of American lobsters, the most abundant pigment usually is a conjugated protein of astaxanthin that is blue. Eggs of American lobsters contain a green conjugated protein. Green crabs about to molt have a green conjugated protein in the old shell and a brown one in the epidermis and pigmented layers of the new shell.
The reason that diet plays such an important role in de¬velopment of color in American lobsters and other decapod crustaceans is that carotenoids present in the conjugated pro¬teins of these animals have to be either ingested or produced in the animal's body from ingested carotenoids. These pig-ments cannot be synthesized from noncarotenoid material, except by plants.
Shrimps, lobsters and crabs turn red when they are cooked because heat breaks down the linkage between astaxanthin and protein, and the astaxanthin is freed. Shrimps, lobsters and crabs that are red before being cooked do not have free astaxanthin, but rather an astaxanthin—protein complex that is red in color.
With regard to coloration, there is an important difference between shrimps, on the one hand, and lobsters, crayfishes and crabs, on the other. This concerns the way in which the colors are manifest. Shrimps have a light, fragile, quite trans¬parent shell, through which the underlying integument is visible. In the integument are numerous pigment-containing cells known as chromatophores. Under the influence of cer¬tain hormones that originate within the central nervous sys¬tem and are released into the hemolymph, the pigments within the chromatophores either concentrate in the center of the cell or migrate to the periphery, as the case may be.
Chromatophores have many branches, and thus a cell in which the pigments are dispersed looks very different from one in which the pigments are assembled into a tiny mass at the center. Furthermore, the area covered by chromatophoral pigments when they are dispersed is much greater than when they are concentrated, so the degree of pigment dispersion largely determines the overall coloration of a shrimp. This may change, rapidly and frequently, in response to changes in illumination and color of background, a fact that explains why common names for shrimps often include some that are descriptive of very different colors.
In lobsters, crayfishes and most crabs, the shell is thick, strong, and largely opaque, due to pigments that are deposited within the shell. Hence, in these decapod crustaceans, the color of the animal is fairly constant, depending primarily upon the color of pigments within the shell rather than upon the degree of dispersion of pigments within the chromatophores. Only in. certain restricted area is the shell of a lobster, crayfish, or crab more or less transparent, and here the color of the underlying pigments can be seen. In a few crabs, notably the fiddler crab Uca pugilator and the ghost, or sand, crab Ocypode, the shell is fairly light and semi¬transparent, and overall coloration results largely from pig¬ments within the chromatophores.
Sometimes the left half of an American lobster (or of its close relative, the European lobster, Homnrus gammarus) may be of one color and the right half quite a different color. Professor Herrick and several later investigators described a number of such particolored lobsters: light yellow/bright red; dark green/pale red; blue/white; green-black/light orange; dark green/sky blue; dark blue/light red; dark green/red; white-red/purple.-blue.
In one case a bilateral difference in color of American lobster was correlated with a bilateral difference in sex. In 1959, Dr. Fenner A. Chace, Jr., and Dr. George M. Moore described an American lobster that on its left side was orange, with mot¬tling and spots of dark green-brown and on its right side was similarly mottled and spotted but mostly in shades of blue over a light, blue ground color. Externally, the lobster ap¬peared female on the right side and male on the left side. When the lobster was dissected, it was found to have well¬ developed female reproductive organs on the right side and male reproductive organs on the left. In three earlier reports by other scientists, American lobsters having both male and female reproductive organs were described, but in no case was the bilateral difference in sex associated with a bilateral difference in coloration.