Juveniles

The juvenile stage of the lobster's life starts when it metamorphoses into the basic form of the adult and settles to the bottom and lasts until the lobster reaches sexual maturity. Laboratory experiments have demonstrated that postlarval and juvenile lobsters can burrow in many types of substrate, but until recently, it was unknown which substrates offered the best protection against predators. Using a combination of both laboratory and field techniques, researchers found that cobble substrates, made up of rocks of varying sizes (common in many rocky seashore areas), protected the newly-settled and juvenile lobsters against most predators. Peat from salt marshes, eelgrass, and mud substrates all offered some degree of protection, but not as great as that of cobble. Underwater research techniques, particularly that of suction sampling which employs a large tube that acts as a vacuum cleaner, have confirmed laboratory observations and have shown that newly settled lobsters are most abundant in cobble habitats.

The larger, more mobile juveniles, starting from when they reach a size of about four inches, are not really different from adults. They eat the same kind of prey, live in the same areas, and, in general, have all of the same behavior as sexually mature lobsters. Newly settled juveniles, however, are different from adults in all of the above attributes. There are also morphological differences between these younger and older juveniles: while the older juveniles have a heavy crusher claw and quicker cutter claw, the younger juvenile's claws are both small and of the same dimensions. Even the muscle fiber bundles within each claw are identical - - but these fiber bundles change slowly over the first year of benthic life, gradually becoming mostly slow, tonic fibers in the crusher claw and fast, phasic fibers in the cutter claw.

Because of their vulnerability to predators, the recently settled juvenile lobsters are in a potential bind. In order to grow and thus become less vulnerable, they need to find food which would suggest that they need to forage. But long-term laboratory observations in "ant farm" aquaria suggested that these small juveniles never left their shelters. Similar SCUBA observations also suggested that these small juveniles might be shelter bound for nearly their entire first year of life. Lobsters measuring 20 mm in carapace length ("CL") or greater (three or more inches long) were the only small lobsters ever observed out of their shelters.

Fortuitously, the "ant farm" laboratory observations showed that the small lobsters spent a significant proportion of their time pleopod fanning which created a strong current through their shelter. The water within this current often carried planktonic organisms in it, and the lobsters would move their mouthparts, legs, and claws as if they were attempting to catch the plankton. Subsequent videoanalysis of these currents and the movements of the mouthparts and legs confirmed that these lobsters were indeed capturing the plankton with a combination of raptorial techniques carried over from the larval stage days and suspension feeding techniques used by other crustaceans. Feeding experiments demonstrated that postlarval and early juvenile lobsters could survive and grow rapidly on planktonic diets up to a size of 12 mm CL (about two inches long). If the diet were supplemented by organisms found within their shelters, such as worms and amphipods, it is possible that they could remain in their shelter for their entire first year and grow to a size of about 20 mm CL. Field experiments using differently sized tethered (or leashed) lobsters have demonstrated that predation pressures are greatly reduced for lobsters above 15 mm CL. Interestingly enough, this is also the same size at which the development of the two claws is complete. Thus, instead of going out for dinner, early juveniles eat in-and that is how they stay alive to become adults.

As mentioned, at about 20 mm CL juvenile lobsters begin to emerge from the shelters and begin short, near shelter foraging bouts. As they increase in size, the area foraged becomes larger and larger. The lobsters also move offshore with increasing size. Their larger, functionally separated claws allow these larger juvenile to crush mollusks, urchins, and all of the same foods as the sexually mature adults. These juveniles also alter their behavior, so that instead of responding to threats by escape maneuvers, they aggressively display with their claws. While fights-to-the-death between lobsters are rare, they do exhibit a progressive sequence of display behaviors, which, if an opponent refuses to withdraw, can escalate into a full-blown attack where antennae, claws, legs, or even eyestalks can be ripped off. These fights have extremely interesting consequences for the combatants. Laboratory-staged fights between evenly sized juvenile lobsters have demonstrated that the winner of a fight is truly a winner and the loser is a true loser. If a previously-fought lobster is then placed into another staged fight, its behavior is dependent upon its status from the previous fight. In other words, previous losers lose or refuse to fight with a new opponent. In the case where losers are pitted against other losers, no displays or fights occur. Winners again win, unless they are pitted against another winner - - in that case, the fights escalate and the lobsters cause extensive damage to each other. Usually the dominant or subordinate status only lasts until the animal molts. Then, if a subordinate molts first, survives, and becomes larger than the dominant, it will often become the new dominant animal.

It is currently believed that these seemingly long-term changes in status and behavior are determined by the level of two hormones known as serotonin and octopamine. Winners have higher blood levels of serotonin, while losers have higher levels of octopamine. If a lobster who has previously lost a fight is injected with serotonin, it becomes extremely belligerent and will attack lobsters much larger than itself. Even after being severely damaged (having claws and legs ripped from their joints), these serotonin-injected lobsters will still continue to fight. Studies such as these imply that aggressive behavior may have a hormonal component, and the lobster is currently being used as a model to better understand human aggression.

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