Larvae and Postlarvae

Planktonic larval life begins as the female lobster raises her abdomen and vigorously fans the pleopods to which the prelarvae are attached. This fanning breaks the cement attachment to the prelarvae and propels them upwards toward the water surface. Usually, the female lobsters releases only a small batch of prelarvae over a period of several nights to several weeks, possibly to prevent the larvae from being consumed all at once by various fish predators. When the attachment is broken, the prelarvae are released and they shed the cuticle binding their legs, in a true molt, to enter into the first larval stage. Lobster larvae are more developmentally advanced than are the larvae of other crustaceans. The minute lobster larvae are freed from their mother's pleopods, they are ready to begin feeding, while most other crustacean larvae hatch in non-feeding stages, or naupliar stages (from the Greek, nauplios meaning sailor). These naupliar stages subsist on remaining yolk reserves from the egg before molting into a protozoeal (from the Greek, proto-, meaning first and zoon, meaning animal) stage. During the protozoeal stages, these types of crustaceans develop their digestive tract and feeding apparatus. They then molt into a mysid stage, named after the non-decapod swimming shrimp, Mysis. Lobsters undergo the naupliar and protozoeal stages while in the egg and hatch out directly into the mysid stage.

The first stage larva is about eight millimeters (one-sixth of an inch) long, completely transparent, with large dark eyestalks. Because of its transparency, all of its internal organs can be seen through its shell, or carapace. These larvae are typically concentrated at or near the water surface during the night and, depending upon the intensity of sunlight, may be near the surface or submerged to greater depths during daylight hours. This movement up and down through the water is estimated to be on the order of fifteen to thirty meters (45 to 90 feet) each day and is due to the response of the larvae towards light, which changes during and between each stage, so that they are either attracted to or repelled by light. Because other planktonic organisms, both animal and plant, perform similar vertical migrations, the lobster is always in the presence of a rich food supply. We know little of how the larvae feed, but it is commonly thought that they are raptorial (from Latin, raptus meaning "to seize"), chasing after their intended prey and seizing them with their legs or mouthparts. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

Movement is achieved by the larvae beating oar-like outer branches, or exopodites, of their thoracic appendages (both legs and maxillipeds). Beating these exopodites results in an upwardly directed movement, while cessation of the beating results in sinking. Backward movements can be achieved by a rapid flexion of the abdomen, but these flexions are not as effective as in later, benthic (bottom-dwelling) stages due to the relative size of the abdomen. The swimming of the larvae seems haphazard, with frequent somersaults and spiral swimming paths. Some of this undirected movement may be due to currents and waves, but it may also be related to the absence of a functional statocyst (gravity organ) until after metamorphosis.

Second stage larvae differ somewhat from first stage, in that they are about twice as large (one-third of an inch), have large pigment cells (called chromatophores), bear pleopod buds upon their abdominal segments, and have functional claws on their first three legs. The mandibles are also more developed and they bear more sensory hairs on all appendages. But they are still transparent and have large, dark eyestalks. These larvae behave similarly to the first stage larvae. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.

Third stage larvae are larger still, being about one centimeter (half an inch) in length. They have even more and larger pigment cells, the pleopods are nearly fully formed, but still not functional, uropods appear on either side of the telson, and their first pair of chelae are much larger. The eyestalks are longer and more slender than in the previous stages, the mandibles are thicker and bear more prominent teeth, and they have even more sensory hairs. But they, too, are still transparent. However, their behavior differs in that they are less light sensitive and so are found in the upper waters. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

Depending on the water temperature, the larvae may take from eleven to over fifty days to molt through the three larval stages and into the metamorphic postlarval stage. During this time, which is usually between the months of June through August, the larvae are in waters with prevailing winds directed onshore. By the time they are preparing for metamorphosis, they are inshore. The metamorphic molt results in a postlarva that looks like a miniature lobster and is pigmented with a brownish-red color. It has lost the oar-like exopodites on all of its legs, retaining them only on the maxillipeds, and swims by beating the pleopods of the abdomen. This swimming is well-directed and strong enough that the postlarvae can swim against some currents. But, typically the postlarvae orient to the currents, which again will bring them closer inshore to find a suitable settlement habitat. The postlarvae are strongly light seeking at the beginning of this stage; however, as molting into the fifth stage draws near, light becomes repellent, and they begin to seek shelter in the benthic, or bottom, environment. It is currently thought that the postlarvae make repeated dives down to the bottom to examine the habitat there; if they judge it to be unsuitable or only marginally suitable (as in the case of sand where they cannot build a shelter, or mud where shelters are precarious), they return to the surface to be carried elsewhere and to try again. The postlarvae are capable of delaying the molt into the fifth stage for a period of time and will do so in the laboratory if they are not provided with a suitable settlement habitat. (Figure from: Herrick, F.H. (1911) Natural History of the American Lobster. Bulletin of the U.S. Bureau of Fisheries 1909: 149-408 and (1895) The American Lobster: A Study of Its Habits and Development. Bulletin of the U.S. Fisheries Commission 15: 1-252.)

This transition from planktonic to benthic realm is the most dangerous period in their life. Only a small percent of lobsters actually survive the transition from the top to the bottom. Waiting for them on the bottom are crabs and bottom-dwelling fish ready to snatch them up for a quick meal. The claws of the postlarvae are small and useless for warding off predators. Their only defense is to flee by tail flipping, but this escape response often is not fast enough to get away from many fish. The only chance the early juveniles have of surviving is if they can get into a burrow quickly.

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