Sustaining a thriving lobster fishery through science and community.
| Table of Contents
What's in a name?
Molting & Growth
Nervous & Sensory Systems
Muscular System & The Lobster's Tail
Larvae & PostLarvae
The Lobster's Future
Adult lobster life is similar to that of adolescents except for the array of physiological, ecological, and behavioral events that are related to reproduction. Virtually everything we know about the sex life of the American lobster is based on laboratory observations where detailed accounts of courtship and mating activities can be made using artificial shelters pressed up against glass windows. Although the difficulties of studying lobster courtship and mating behavior in nature are not insurmountable, they are formidable. The biggest obstacle to surmount before we can witness lobster mating in the field is the lobster's private habits. Mated pairs of lobsters generally live under rocks and in deep, dark crevices where they remain hidden. Furthermore, lobsters are nocturnally active, so when they do come out of their shelters it is difficult to monitor their activities without disturbing their normal behavior.
To encourage normal behavior in the laboratory, special tanks have been designed and built to facilitate direct observations. These tanks are fully equipped with a gravel bottom, ambient running sea water, and abundant shelter space. Live food is provided by stocking the tanks with fish and invertebrates commonly found in association with lobster shelters in nature. Lobsters, captured by local fishermen, are measured, sexed, given individual identification markings, examined for any distinguishing characteristics, placed in the tanks, and monitored over a twenty-four hour cycle for several months. Observations made in the lobster aquaria revealed fascinating and complex behavioral interactions between lobsters.
When two adult male and five adult females (each weighing approximately 1.25 pounds) are housed together, each female, in turn, forms a brief (one to two week) pair bond with the dominant male. When it gets close to the time for her to shed her shell, the female lobster initiates the pair bond by making repeated approaches to the shelter of the dominant male. The female "checks out" the male by poking the tips of her claws in his doorway and flicking her antennules repeatedly. In response, the interested male backs up slightly, raises his abdomen, and begins to beat his pleopods in metachronal waves. (Figure from: Atema, J. (1986) Review of sexual selection and chemical communication in the lobster, Homarus americanus. Canadian Journal of Fisheries and Aquatic Sciences 43(11): 2283-2290. Drawing by Ellen Chase Botkin. Used with permission.)
After a few days of shelter checking, the premolt female enters the male's shelter. When she enters, the female holds her claws out with the tips of the claws pointing down into the ground. The male approaches and taps her claws alternately with his own and the cohabitation begins. The pair continue to box from time to time, usually with the male hitting the female's claws, but sometimes he jabs her in the side or even seems to nudge her. "Boxing" may provide a way for the male to "know" how soon the female will shed her shell. The male may be able to sense the female's claws becoming hollow as her muscles shrink to squeeze out of her old shell. As any excited lobster would, the male fans his pleopods more and more frequently as the day of the female molt approaches. Pleopod fanning draws water through one shelter entrance and out the other and may serve the dual function of aerating and broadcasting the scent of the shelter.
One morning, after the pair has been cohabiting for a few days, the female lobster molts. Immediately before molting, the female pushes, boxes, and jabs at the male lobster, seemingly trying to push past him to escape from the shelter (A). Or maybe she's loosening up the old shell to break away from the new. Soon, her carapace lifts up away from her body and she rolls over on her side to start pulling out of her shell. It takes a pound and quarter lobster about one-half hour to escape from its old shell. Ecdysis appears to be an exhausting experience and when the female is finished she has no skeletal support and flops around trying to stand up. Lobster harvesters refer to animals in this molt condition as "rags" because of the consistency of their shells. After another half-hour, the lobster is able to support herself on her legs, largely through the support of a temporary hydrostatic skeleton. At this point, she approaches the male, who has been standing inches away, looking on, and pleopod fanning furiously. She then slowly turns so that her abdomen faces him. The male then mounts and helps the female to roll over using his middle pair of legs (B). Once the female is on her back, both lobsters outstretch their abdomens and pleopod fan against one another (C). Then the male intromits his first pair of modified pleopods and transfers a packet of sperm (spermatophore) to the female's seminal receptacle (D). After mating, the female tail flips out from under the male and rests near the back of the shelter (E). The male resumes his intense, turbulent pleopod fanning and begins to consume her molt shell after yet another half hour (F). (Figure from: Atema, J., S. Jacobson, E. Karnofsky, S. Oleszko-Szuts, and L. Stein. (1979) Pair formation in the lobster, Homarus americanus: Behavior development, pheromones and mating. Marine Behaviour & Physiology 6: 277-296. Used with permission.)
Cohabitation ends when the female leaves the dominant male's shelter for the last time. There is no evidence indicating that she is kicked out. Apparently, she leaves when she is ready. She then takes up residency alone in a new shelter where she stays while her shell hardens completely, which takes two months in the warm summer waters, but much longer in colder regions and during winter months. When her shell has calcified sufficiently, the female extrudes eggs, the number of which is related to her size, and broods them on her pleopods for a nine to twelve month gestation period.
Meanwhile, the male shelter continues to be an active place. When the first female moves out, a second one moves in, shares the male shelter, molts, mates, recovers, and moves out. Then the next female moves in, and so on until all of the receptive females in the aquaria have molted and mated. This type of mating system is unusual and can be thought of as serial monogamy from the female perspective because the female forms only one exclusive pair bond during the breeding season, but has a sequence of mates over a lifetime. However, from the male perspective, the lobster mating system is better categorized as serial polygyny, because although the male forms exclusive pair bonds, he does so with a succession of females over the course of the breeding season.
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