Physiological Processes (Continued)

Circulatory System

The heart is a single-chambered sac consisting of striated muscles with several openings called ostia. It is suspended in and surrounded by a blood sinus called the pericardium which lies directly above the pyloric stomach on the dorsal (upper) surface of the animal, just under the carapace. Invertebrates have a dorsally positioned circulatory system and a ventral nerve cord, whereas vertebrates have a ventrally located circulatory system and a dorsal nervous system. The blood, or hemolymph, passes from the pericardium, through the ostia, and into the heart. At the beginning of a contraction, the ostia close (via ostial flaps), the intracardial pressure increases, which opens the cardioarterial valves so that the heart can empty. Blood is pushed into the major arteries, most of which are directed forward to supply sensory organs and vital systems. Arteries also run toward the abdomen, both ventrally and dorsally, to supply blood to the pereiopods, ventral nerves, gut, and abdominal muscles. From these arteries the blood enters sinuses, or blood cavities. These sinuses bathe the various organs. No veins are present to return the blood to the heart. Instead, blood returns to the heart via interconnecting spaces known as venous sinuses which open back into the pericardium. Because of this architecture, the lobster's circulatory system is known as an "open" system. After contraction, the heart muscle relaxes, intracardiac pressure drops, the cardioarterial valves close, the heart is distended by action of the ligaments, the ostia open, and the hemolymph enters from the pericardium.

The heartbeat is controlled by the cardiac ganglia and is known as a neurogenic system (of neural origin), as opposed to that of humans - - a myogenic system (of muscular origin). The cardiac ganglia provide a pacemaker mechanism, whereby the neurons fire in a coordinated burst of impulses at the beginning of each contraction cycle and then remain inactive for the balance of the cycle. The burst pattern is stable over thousands of cycles. In contrast, our myogenic heart has its rhythmic heart beat due to muscular contractions. The heart of an adult lobster beats at a rate of 50-136 beats per minute.

Reproductive System

The ovaries of the female lobster are located in the cephalothorax, extend through the abdomen, and are similar to the form of an "H". The cross-bar is found at the forward margin of the heart, just behind the pyloric stomach (A). Paired oviducts emerge just below the heart and run downward toward the base of the third pair of thoracic legs. There they meet the seminal receptacle, a bluish structure extending from the third walking legs to the fourth. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

The testes (B) of the male lobster are variable in shape and may be either longitudinally paired or H-shaped. Similar to the female oviducts, in the male, paired ducts, called the vas deferentia, emerge just below the heart and run downward toward the of the fifth walking legs. Along this course, they become thicker and glandular, and then they become narrow and thin-walled to form the duct. The thick, glandular portion of the vas deferentia secrete a gelatinous matrix over the sperm to form dense packets of sperm called spermatophores.

During mating, the male transfers a spermatophore to the female. Males of the spiny and slipper lobsters plaster the spermatophore to the outside of the hard-shelled female's abdomen, where it darkens and becomes known as the "tar". Clawed lobsters copulate shortly after the female sheds her old exoskeleton. The male turns the soft female over, cradles her body to hold her up off the bottom while he mounts her and inserts his modified first pair of pleopods (known as the gonopods) into the seminal receptacle of the female. The appendix masculina - - a hardened structure located on the second pair of pleopods in males - - helps to push the gelatinous spermatophore through the groove created by the gonopods and into the seminal receptacle. The male then attaches an additional gelatinous material to the outside of the female's receptacle. Once this transfer is accomplished, mating is complete and the female tail flips out from under the male as he releases her. The gelatinous material hardens within the first day, effectively plugging the female so the spermatophore won't be lost. The sperm plug falls off several days later, leaving behind the hardened spermatophore. Females can also mate in the hard-shelled condition. This may be important for very large females who molt infrequently.

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