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LOBSTER PHYSICAL REQUIREMENTS
Based on a review of the scientific literature, most of the research involving lobsters in the last century has focused on mortality tests. From these tests have come a detailed understanding of the physical and environmental conditions a lobster can endure before it dies (Fig. 3A and B). Hundreds, if not thousands, of lobsters have been killed over the years in every conceivable combination of fatal experimental conditions. From these tests, it is now clear that many of these variables work in unison and are determined to a large extent on pre-conditioning or acclimation (see Cooper and Uzmann 1980 and Jury et al. 1994).
Without a doubt, temperature is the factor which exerts the most control on all other responses. "Temperature is the major factor controlling size at maturity, egg maturation, incidence, timing and synchronization of spawning, success of [egg] attachment and incubation and time of hatching" (Crossin et al. 1998).
The survivable temperature range in the laboratory for the American lobster is quite broad, ranging from -10 to 30.50C (30.20 to 86.9 0F.). They can also survive abrupt temperature increases of 160C (roughly 290F) or decreases of 200C or about 360F (Lawton and Lavalli 1995).
As a practical matter, lobsters normally inhabit waters where temperatures are as low as 50C (410F) or as high as 200C (680F). When the water temperature drops to approximately 50C (410F) adult molting is blocked (Waddy et al. 1995). In addition, temperatures must decline to 80-100C (460-50 0F) during winter to maintain a balance between the synchronization of the molt and ovarian cycles of the female lobster (Aiken and Waddy. 1986).
Lobster larvae have a much lower tolerance for temperatures than adults. The postlarval stage is attained at temperatures below 100C (500F), but molting to Stage V rarely occurs and survival is quite low (Ennis 1995). Temperature at hatching is even more restricted. Over a 5-year period ending in 1979, bottom water temperatures at hatching were as low as 80-90 C (460-480F) in samples collected in Northern New England. Temperatures ranged from 110-13.60C (51.80-56.50 F) in Buzzards Bay and Block Island Sound where larvae were more abundant (Fogarty 1983).
The lower lethal limit of adult lobsters exposed to various dilutionís of seawater is between 8-14 ppt (parts per thousand) depending on temperature, oxygen and acclimation conditions (McLeese 1956). Salinityís below 10 ppt cause significant physiological changes that are extremely stressful even if the animals manage to survive short-term exposure. Heavy mortality has been reported after extreme spring runoffs in estuarine systems (Thomas and White 1969). Males seem to be able to tolerate lower estuarine salinity better than females. It also appears that for larvae, "high temperature reduces tolerance to high salinity, but increases tolerance to low salinity over the 15- to 35-ppt range" (Ennis 1995).
Larvae and molting individuals are more sensitive to reduced salinity (Charmantier et al. 1988). Larval lobsters are sensitive to salinityís below 20 ppt, but these seem to be harmful only after exposure exceeding >20 days (Templeman 1936). "Tolerance to low salinityís decreases during larval development" (Ennis 1995). At a salinity of 11.6 ppt, very few Stage I larvae survive longer than 24 hours and none molt. No stage III or IV larvae survived salinityís below 12.5 ppt. No larval molting occurred beyond a salinity of approx. 40 ppt (Ennis 1995).
Lobsters can survive in waters with low levels of dissolved oxygen (hypoxia), with the possible exception of locations experiencing severe organic enrichment. The lower lethal oxygen level for juveniles and adults ranges from 0.2 mg O2/liter at 50C (410F) to 1.2 mg O2/liter at 250C (770F) in 30 ppt salinity (Cooper and Uzmann 1980). Oxygen requirements are highly sensitive to low salinity and the American lobster consumes more than twice as much oxygen at 10 ppt than it does at 20 ppt (Jury et al. 1994a).
Larval lobsters appear twice as sensitive as juveniles and adults to reduced levels of dissolved oxygen. For larvae, "dissolved oxygen concentrations <1.0 mg O2/liter Ö and pH levels <5.0 and >9.0 are lethal" (Ennis 1995).
Webster's New World dictionary defines shelter as "something that protects, as from the elements, danger, etc." For the lobster, shelter takes two forms: natural, "ready-made" shelters and those that must be constructed or burrowed.
Clawed lobsters "create remarkably similar burrows with one, two or more openings Ö The burrows begin as U-shaped tunnels (Fig. 4A) and may be expanded later into additional openings. The 'entrance' tunnel is usually a crater like, or wide-mouthed, depression with a mound of sediment at one end. The 'rear exit' and additional openings simply open onto the flat surface of the sediment" (Lawton and Lavalli 1995).
Burrowing "behavior is similar for the American and European lobsters" (Fig. 4B). A wall of excavated bottom material is generally in front of the entrance, being most conspicuous in front of recently excavated burrows. Movements into and out of the shelter tend to level off the mound. Where bottom configuration is not limiting, the walls of the burrow are generally semicircular in shape. The lobster typically rests against the inner wall with its antennae directed toward the entrance. The height of the burrow entrance is usually less than the width and the inner portion is enlarged into a chamber" (Cooper and Uzmann 1980).
"Ventilation is necessary to prevent low oxygen and high carbon dioxide stress during burrow occupancy." The construction of lobster burrows and their openings probably promotes passive ventilation. Fluid will move through the burrow from the end where the flow is slower (smaller, blunter, or lower opening) to the end where the flow is faster (larger, sharper or higher opening) Ö For lobster burrows in rock substrates, currents will cause water to flow between the interstitial spaces [between the rocks or gravel] thereby providing the necessary ventilation" (Lawton and Lavalli 1995).
The relationship between shelter size, substrate and lobster size (Fig. 4C) has been investigated by Wahle and Steneck (1991, 1992). Their findings "suggest that body size-substratum scaling considerations are important in habitat selection." Studies by Lavalli and Barshaw in 1986 (see also Barshaw and Lavalli 1988) indicate that small lobsters may be attacked before they even have time to construct a burrow and they remain vulnerable to excavating predators like crabs even inside the burrow. Wahle and Steneck (1991) conclude that "the American lobster appears to be restricted to shelter-providing habitats in its early benthic life, but this restriction apparently relaxes as it grows." Furthermore, Steneck (personal comm.) has shown that adult lobsters remain vulnerable to fish when tethered in deep water (i.e. Cashes Ledge). So shelter availability continues to be an important consideration.
A reduction of the food supply to larvae results in reduced survival and increased development time (Ennis 1995). However, postlarval lobsters are more resistant to starvation and survive at least 12 days of food deprivation (Ennis 1995). In fact, "the better nutritional condition and the higher growth rate of postlarvae are probably due to a higher quality of prey in the field and indicate that starvation or poor nutrition is unlikely to be a major source of natural mortality" (Juinio and Cobb 1994).
The natural diet of larval and postlarval lobsters includes the wide variety of phytoplankton and zooplankton available to them. They are generally considered to be opportunistic feeders, although they show a strong preference for live prey with a bias towards copepods and diatoms. (Fig. 5). Unlike the earlier larval stages, the Stage IV postlarva "shows increased dependence on protein and accumulates lipid stores. This probably confers considerable advantage in adapting to a benthic habitat by enabling newly settled lobsters to rely temporarily on stored reserves as they make the transition from planktonic to benthic existence" (Ennis 1995).
Plankton provides an adequate diet for the growth and survival of shelter-restricted juveniles and supplements the diet of emergent phase juveniles (Barshaw 1989, Lavalli 1991). Despite the habitat differences, diet is fairly consistent for emergent and vagile phase juveniles and is dominated by mussels, lobsters, Atlantic rock crabs and gastropods (Weiss 1970).
Stomach contents of juvenile, adolescent and adult American lobsters suggest that while they consume the same type of prey, the relative proportion of the prey items taken is dependent on the size of the lobster. Smaller lobsters consume more hydroids, gastropods, crustaceans, polychaetes and brittle stars than do larger lobsters. Plants may be actively selected, forming a functional nutritional component of the diet (Weiss 1970, Hudon 1987).
"The principle components of the diet of adult lobsters are various crustaceans and mollusks, with polychaetes and echinoderms increasing in relative importance in certain areas or times of the year Ö there is a rapid increase to peak feeding activity between June and July; feeding activity then remains high in September even as temperatures begin to fall; and females maintain a higher level of feeding activity than males, at least until mid-February" (Lawton and Lavalli 1995). Feeding slows as the result of molting in males and greater demands on females for reproduction and molting.
"With the widespread introduction of escape vents on lobster traps, it is now likely that most lobsters feed from traps before they are finally captured by the commercial fishery. In areas of intense fishing pressure, fishing bait may provide a significant [energy] subsidy, supplementing the natural food resources available on lobster grounds" (Lawton and Lavalli 1995)