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LIFE CYCLE
The following section is largely condensed from the remarkably comprehensive "Biology of the Lobster" (J.R. Factor, editor 1995), which offers a thorough review of previous lobster research. Every effort has been made to update the conclusions and to add new perspectives whenever subsequent research findings warranted changes. The life cycle is depicted in Fig. 6 and 7 and a brief review follows below.
MATING
Mating is a complex ritual that is intimately tied to social interactions with the dominant male and to the desirability of his shelter (see Atema and Voigt 1995, Lawton and Lavalli 1995). In naturalistic settings, only the dominant male mates with the available premolt females (Cowan and Atema 1990). To initiate courtship, males must establish a shelter large enough for two lobsters. Next, they must advertise their presence, dominance and, when possible, proven mating success to females. Since a good shelter is essential for mating success, they must demonstrate their dominance over both males and females. To dominate the most desirable large females, the male must be larger or at least of comparable size (Cowan and Atema 1990).
The "female lobsters choose the dominant male and initiate cohabitation in his shelter" (Atema and Voigt 1995). Chemical cues just prior to molting and mating appear to facilitate entering the male shelter by lowering male aggression. Field and laboratory work describe females approaching sheltered males many times before entering. Not all females can enter, however, and the sheltered males react to entering females in many ways from acceptance to vigorous rejection (Cowan and Atema 1990; as cited in Bushman and Atema 1997).
"In a naturalistic environment with two males and five females, the dominant male accepts only premolt females and cohabits with them each for a few days to weeks with a mean duration of 12 days in naturalistic aquaria Ö The female molts sometime during cohabitation and mating follows after 30 minutes. He climbs on her back Ö and begins to turn her over Ö The male then inserts his gonopods Ö into the female seminal receptacle and deposits his spermatophore with a few thrusting movements. Actual copulation lasts only a few seconds, after which the female rights herself with a tail flip out from under the male" (Atema and Voigt 1995). The male then eats most of the female's molt shell. "Females begin to show resistance to courtship after 12 hours Ö The 1-2 week cohabitation period is sufficient for the vulnerable postmolt female to harden her shell and still allow the male sequential access to other females" (Cowan and Atema 1990).
Although intermolt mating has been observed, it occurs mostly in prespawning, non-inseminated females or in females that have depleted their spermatophore (Waddy and Aiken 1990). The function of intermolt mating may be to allow females to replenish sperm they either failed to receive in the postmolt period or lost as a result of previous fertilization and spawning. Once inseminated, female receptivity ceases and males are usually no longer attracted (Atema and Voigt 1995).
Spawning is independent of insemination. Spawning is the passage of the egg from the ovary to the exterior of the female. Females position themselves on their back during spawning and the abdomen is flexed forward to form a brood chamber that catches the eggs as they are externally fertilized (Talbot and Helluy 1995, Templeman 1937).
Females which molt and mate in the summer usually spawn in the fall. However, if they molt and mate in the fall, then they may not mate until the following summer (Waddy and Aiken 1995). Large females have been shown to molt, then undergo two successive spawns before molting again (Talbot and Helluy 1995).
Spawning usually occurs earlier in warmer waters and depending on environmental conditions egg production can range from a few hundred to more than 100,000 eggs. Fecundity (# of eggs) is also related to the size of the female and older females generally can maintain larger broods. Spermatophores may be stored for as long as two years and normally only a fraction of the total stored sperm is used in a single spawning. More than one male may mate with a single female and sperm from both males may be used to fertilize eggs in the subsequent spawning. Eggs are attached to the mother and to other eggs by means of stalks formed from the egg coat, which are extremely durable. Nevertheless, mature "females normally lose 30-50% of a clutch during the long brooding interval of 9-16 months" (Talbot and Helluy 1995).
"Hatching and larval release occur following a 9- to 12-month period of embryonic development" (Ennis 1995). In order to decrease this egg development time, females have been known to migrate into warmer waters. In Canada, ovigerous females released in shallow water (Campbell 1990) migrated into "deeper water (>200 m [or about 650 ft]) exposing the developing eggs to the maximum temperature available during the winter months. Females return to shallow water the following summer to hatch their eggs when the surface temperature is high" (Ennis 1995).
Ovigerous females sometimes aggregate in certain shallows of Grand Manan at the time of hatching (Campbell, 1990). However, "the presence of stage I larvae in surface waters indicates that hatching occurs over a broad expanse in offshore waters" (Ennis 1995, see Katz 1994). "In these deep water lobsters there is no evidence of a migration of ovigerous females to coastal waters comparable to that near Grand Manan" (Ennis 1995).
Hatching generally takes place during a 4 month period from late May through much of September. The hatching season tends to begin earlier and continue somewhat longer in the southern part of the lobster's range. During hatching, 1-2000 larvae may be released at any one time and the time required to hatch and release a full clutch of eggs can vary from 15-31 days (Ennis 1995).
"The 6-8 weeks of planktonic existence is arguably the most complex and least understood phase of the complex life cycle of Homarus americanus. This planktonic phase includes three larval stages plus a postlarval stage, during which the critical transition from pelagic to benthic lifestyle occurs" (Ennis 1995).
"The first-stage larva emerges from the prelarval molt either in conjunction with rupture of the egg membranes or up to 24 hours later, and remains attached to the cuticle" [female] (Ennis 1995). Release of free-swimming Stage I larvae is accomplished in less than one minute and occurs generally after darkness. "Immediately upon release, larvae swim upward and swarm within a few centimeters (about 1-2 in) of the surface" (Ennis 1995). Their bluish transparency matches their pelagic environment and may help to shield them from predators.
Stage I larvae are about 8 mm (0.3 in) long (Ennis 1995) and possess limited swimming ability. Depending on temperature, Stage I duration can vary from 2 days (220C or 720F) to 14 days (100C or 500F). "Stage II larvae are only slightly larger, approximately 9 mm long [or 0.35 in], and closely resemble those of the first stage" (Ennis 1995). This stage ranges from 4 to 15 days. Stage III larvae are similar but they are approx. 11 mm (0.4 in) long and they have completed a tail fan. Stage III lasts from 5 to 25 days (Ennis 1995). Curiously, larvae captured in the wild almost invariably have a shorter duration for each molt stage compared with laboratory reared larvae (Fig. 8).
All three stages possess very little horizontal swimming ability but can maintain depth control. "In coastal waters, the vertical distribution and movements of lobster larvae and postlarvae are confined to the upper 2-3 m [less than 10 feet] of the water column, whereas in offshore waters they appear to be unrestricted by depth within the upper 30 m [less than 100 ft]" (Ennis 1995).
"The wet weight of larvae increases 70-80% at each molt, but remains relatively stable between molts" (Ennis 1995). Wild caught Stage III larvae are generally larger by up to 0.5 mm (Ennis 1995). In the laboratory, "temperature has little effect on stage-specific survival of lobsters in Stages I and II (> 60 %), but survival rates in Stage III and the postlarval stage are reduced to <26% at 100C [500F], compared with those reared at higher temperatures Ö In the wild, the highest survival rate is associated with hatching earlier in the season, when surface temperature is increasing rapidly. This results in the most rapid growth overall and the shortest duration of planktonic life" (Ennis 1995).
POST-LARVAE (4-5 mm or 0.16-0.20 in CL)
Stage IV is a postlarval stage when the animal is first recognizable as a lobster. The time it takes from hatching to attain postlarval stage varies with temperature from 11 days at 220C (720F) to as many as 54 days at 100C (500F) (Ennis 1995). At these temperatures the duration of the postlarval stage can range from 11 days to 49 days (Ennis 1995). "The postlarval lobster resembles a miniature adult, although the proportions differ, and it is now ready to make the transition from a pelagic to a benthic lifestyle" (Ennis 1995). It is brown pigmented which helps to camouflage it in the benthic substrate (see Atema and Voigt 1995).
Postlarvae possess well-balanced, stable equilibrium and they "swim with much greater agility, precision, and speed than do the earlier stages Ö Diving and bottom-testing behavior begin 2-6 days after molting to the postlarval stage" (Ennis 1995).
The postlarvae show a strong preference for substrate with preformed crevices and macroalgal cover (Ennis 1995). Settlement can be delayed quite considerably when unsuitable substrate is provided and molting to the first juvenile stage may also be delayed (Botero and Atema 1982). As postlarvae age or when their water has been preconditioned by fish predators they tend to settle more quickly and choose shelters less selectively (Boudreau et al. 1993). In the presence of a thermocline or thermal gradient, settlement will also be delayed and even settled postlarvae will respond to a reduction in temperature by leaving the bottom. "Remaining above the thermocline could reduce considerably the energy costs associated with the settlement process. It should be possible to determine the most likely zones of lobster settlement by mapping the thermal structure of the water column" (Boudreau et al. 1992). In general, "postlarvae are very well adapted behaviorally for locating bottom that is suitable for settling in terms of enhancing the survival of the early benthic stages" (Ennis 1995, see Cobb and Gulbranson 1983).
The juvenile life history stage has been subdivided into three phases as follows (Lawton and Lavalli 1995):
Shelter-restricted juvenile phase - (4 to 14 mm or 0.16 to 0.55 in CL) lobsters that have recently settled to the bottom. They retain the capacity for suspension feeding and rapid, highly effective tail-flipping. Emergent juvenile phase - (15 to 25 mm or about 0.6 to 1.0 in CL) exhibit a bottom foraging mode, yet only limited movement outside of the shelter. Further growth and differentiation of the claws. Vagile juvenile - (25 to ~40 mm or about 1.0 to 1.5 in CL) a progressive change from movements restricted to the immediate vicinity of shelter to a wider ranging surface active foraging mode and the potential for habitat shifts.
ADOLESCENTS (~40 to 50 mm or about 1.5 to 2.0 in CL)
The adolescent phase is marked by physiological but not functional maturity. Social interactions begin to dominate seasonal movements and lobster population distribution. Onset of clear, size-related sexual differentiation. Males may become physiologically mature at a smaller size than females.
ADULT (larger than 50 mm or about 2.0 in CL, but much larger in some areas)
The adult phase is characterized by the onset of functional maturity. Except during courtship or severe environmental stress most "adult lobsters live alone in close fitting-shelters where they spend most of their time" (Atema and Voigt 1995).
"In shallow water, lobsters generally emerge from their shelters about 1 hour after sunset and show greatest activity in the following two hours, after which they gradually return to their own shelter or a nearby alternate shelter. Some animals are resident in one shelter for up to 9 months including over wintering, others move among different shelters in a general area, and yet others are transient Ö Lobsters seem to 'know' their physical environment. It has been suggested that they use their activity period primarily to forage for information, not food: to update their knowledge of the physical (and social) environment" (Atema and Voigt 1995).
"In the 2 months preceding the vulnerable molt period, lobsters gradually increase the number of shelters regularly occupied to 1.9 during the last 2-week period preceding molting Ö Increased shelter use corresponds with increased premolt activity Ö Lobsters engage in extensive housekeeping, cleaning debris and silt out, pushing sand and rock and modifying the entrances. They may block entrances for up to 2 weeks with rocks or other objects Ö Despite these preparatory efforts Ö other lobsters often break though the barricades and evict the newly molted animals Ö Lobsters may buy time by molting in early morning, when other lobsters do not move about, thus avoiding eviction until the next night" (Atema and Voigt 1995).
Most inshore lobsters appear to be relatively local in their distribution moving less than 25 km (about 15 miles) seasonally (Lawton and Lavalli 1995). "In the Gulf of Maine, lobsters engage in small scale movements from shallow water into deeper water, apparently in response to strong winds and turbulence rather than the seasonal thermal regime" (Lawton and Lavalli 1995). Adult inshore females seem to move to deeper water earlier in the fall than adult males. "In Connecticut, long-term tag-release studies suggest that only a small percentage of the inshore lobsters migrate to deeper waters or offshore canyons" (Lawton and Lavalli 1995). Of the 21,136 tagged lobsters recaptured only 24 were found off the continental shelf. "Thus, migratory behavior of inshore lobsters in southern New England may be less common than was previously thought." (Lawton and Lavalli 1995)
"Probably 20% and possibly 30-40% of offshore lobsters annually migrate into shallow water in directed shoal ward movements the spring and summer off southern New England Ö Lobsters from offshore portions of the Scotian shelf and eastern Gulf of Maine undertake seasonal movements to shoal areas on Georges and Browns banks (Fig. 9). These movements from deep to shallow water may act to maintain lobsters within a temperature range of 80-140C [460-570F], allowing for more rapid growth than would be possible for lobsters either remaining inshore, where temperatures drop below 00C [320F] in winter, or remaining in deep offshore habitats, where temperatures rarely rise above 120C [540F] (Cooper and Uzmann 1971, Uzmann et al. 1977).