Crab Migrations
Many species, including crabs, undergo long-distance migrations in order to release offspring in areas favorable for proper early development. The risks associated with migrating can be costly, including acquiring and allocating energy for both migration and oogenesis (egg development) and increased predatory risk. Two of the most spectacular migrations of crabs are blue crab migration at Chesapeake Bay and red crab migration Chrismas Island. However, blue and red crabs choose different strategies (e.g. timing, route, mechanisms) in order to successfully complete migration and maximize survival of offspring.
At Chesapeake Bay, blue crab mating typically occurs from May to October in lower salinity areas of the Chesapeake Bay. After mating, males remain in these lower salinity areas. However, because blue crab larvae require high salinities for proper development, females will begin moving south to the spawning grounds near the mouth of the Chesapeake Bay and near-shore coastal areas. Periods of peak spawning (larval release) along the Mid-Atlantic Bight have typically been documented from late July through August. After spawning, many females use flood tides to return to the lower estuary, where they may produce subsequent broods, but do not move back into lower salinity zones. Some mature females may also move into near-shore and offshore coastal waters after spawning.
The Chesapeake Bay is a large estuary and for many crabs the distance between mating areas the spawning grounds can be considerable (> 100 km). Peak spawning typically occurs in the late summer; therefore, crabs beginning to migrate in the early fall may not be able to spawn that year. According to a study, in which, blue crab were tagged, released and recaptured, it appears that most female blue crabs in the upper Chesapeake Bay appear to begin the spawning migration in the early fall. Furthermore, prior research has indicated that after mating female blue crabs fed extensively in the early summer months, first allocating energy to muscle growth, then to egg development and hepatopancreas reserves. This would enable crabs to build muscle mass required for long-distance migration and produce eggs en route to the spawning grounds.
Another spectacular crab migration is the red crab migration at Christmas Island, South Indonesia. At the beginning of the wet season (usually October / November), most adult red crabs suddenly begin a spectacular migration from the forest to the coast to breed. Breeding is usually synchronized island-wide. The rains provide moist overcast conditions for crabs to make their long and difficult journey to the sea.
During peak migration times, sections of roads where crabs cross in high numbers may be closed to vehicles for short periods of time. While the rains provide the moist preconditions for the march to begin, the timing of the migration breeding sequence is also linked to the phases of the moon. Eggs are released by the female red crabs into the sea precisely at the turn of the high tide during the last lunar quarter. The sea level at the base of the cliffs and on the beaches, where the females release their eggs, at this time varies the least for a longer period, and it is therefore safer for the females approaching the water's edge to release their eggs.
Males lead the first wave of the downward migration and are joined by females as they progress. Larger males arrive at the sea first (after about five to seven days) but are soon outnumbered by females. The crabs replenish moisture by dipping in the sea. The males then retreat to the lower terraces to dig burrows. The density of burrows is high (one to two per square meter) and males fight each other for burrow possession. The females move to the terraces and mating occurs. After mating, males dip again and begin returning inland. The females produce eggs within three days of mating and remain in the moist burrows on the terraces for 12-13 days while they develop.
A single female can brood up to 100,000 eggs. The eggs released by the females hatch immediately on contact with the sea water and clouds of young larvae swirl near the shore before being washed out to sea by waves and tides. Millions of the larvae are eaten by fish, plankton feeders such as manta rays and the enormous whale sharks which visit Christmas Island waters to feed.
After about a month in the ocean, and after growing through several larval stages, the surviving larvae have developed into prawn-like animals called megalopae. The megalopae gather in pools close to the shore for one to two days before changing into young crabs and leaving the water. Although only five millimeters across, the baby crabs begin their march inland, taking about nine days to reach the plateau. In many years, very few or no baby crabs emerge from the sea, but the occasional very successful year (perhaps only one or two every ten years) is enough to maintain the red crab population to a high level.
Every fall, millions of red crabs will undertake an arduous, miles-long migration to the Indian Ocean where they reproduce. Scientists had long puzzled over what changes in the crabs' bodies took place to enable this stark change in behavior. Webster and Steve Morris from the University of Bristol in England looked at the so-called crustacean hyperglycemic hormone (CHH), which, among other things, controls the conversion of stored energy in the muscles (called glycogen) into usable fuel (called glucose). It's the equivalent of a marathoner consuming a sugary gel to keep their muscles going.
Crab migrations are spectacular events of nature that many scientists have spent a lot of time to study. To this day, we have discovered some of the mysterious secrets behind these great migrations. However, there are plenty of questions that needed to be answered; and, we certainly are expecting to know more about them.