Physiological Processes Molting and Growth Since the shell of the lobster is hard and inelastic, it must be shed periodically in order for the animal to grow. The act of escaping from the old shell is known as ecdysis (from the Greek, ekdysis, meaning "getting out") or, more commonly, as shedding. The term molting is used to refer to the entire cyclical process of preparing for, undergoing, and recovering from ecdysis. For lobsters, molting is a continual process because lobsters show indeterminate growth; that is, they grow throughout their lives and therefore spend much of their time either preparing for or recovering from ecdysis. (Females take a break from the molting process when they are carrying eggs or when they are preparing to extrude another set of eggs.) To prepare for ecdysis, the lobster lays down a new exoskeleton underneath its old one. This new exoskeleton is complete in every detail, with every spine, bump, hair, and pigment present. If any limbs have been lost, they begin to regenerate. Minerals are removed from the old exoskeleton and stored in large gastroliths on either side of the cardiac stomach wall. Blood is removed from the claws and other limbs which causes them to shrivel and the lime in the joints of these appendages dissolves. By the time the lobster is several days away from shedding, certain areas of the exoskeleton, where reabsorption of minerals has occurred, are very soft and appear bluish in color. Just prior to molting, the lobster drinks in water and absorbs it through its now very permeable gut. This water intake causes the new exoskeleton to swell, pushing apart the old carapace. During the molting process, the lobster lies on its side and its shell opens along the cephalothorax/abdomen interface. Softened joints allow the lobster to withdraw its shriveled limbs from the old skeleton - - all appendages, including the legs, gills, mouthparts, antennae, antennules, eyestalks, and pleopods are withdrawn. In addition, the digestive system is shed and the ossicles holding the gastroliths in place are lost, resulting in the gastroliths falling into the cardiac sac and being dissolved by the digestive juices. The freed minerals then pass back into the hemolymph (the lobster's combined blood and lymphatic fluid) and are redeposited in the new soft shell. (Figure redrawn by Sapir Ad from a photograph by Dave Aiken published in Factor, J.R., Editor (1995) The Biology of the Lobster Homarus americanus. Academic Press, NY. 528 pps.) Escape from the old shell takes anywhere from several minutes to more than a half hour. Keeping other factors constant, the time required for shedding depends on the size of the animal. (It takes longer to pull bigger claws out of those narrow joints!) If shedding is prolonged unnaturally, it usually results in the animal's death because a lobster in the midst of ecdysis is entirely helpless being virtually unable to move, and consuming a great deal of energy at a time when its respiratory surface, which also sheds its skeleton, is unable to function. Once freed from its old shell, the lobster immediately resumes water uptake and also pumps water through its branchial chambers at a maximum rate. The absorbed water is transferred to the blood and the body tissues, particularly the midgut gland and muscles. The lobster remains secluded for several days while its shell hardens and then re-emerges to engage in its normal activities. However, an adult lobster's shell is not fully hardened for several months. Between sheds, the lobster adds tissue in place of the water it absorbed and, thus, effectively grows into its new shell. BACK | CONTINUE