Malacostraca
This class contains over 23000 species. The term Malacostraca comes from the Greek roots malakós,
meaning "soft" and óstrakon, meaning "shell". The name is misleading, since the exoskeleton is only
soft immediately after moulting.
The Order Decapoda (deca = 10) includes crabs, crayfish, and lobsters. In crabs and lobsters the
cuticle is stiffened by calcification, i.e. the deposition of calcium carbonate.
Birgus latro (Cococnut Crab, Palm Thief, Robber Crab)
Since Birgus latro can be up to 1 m in length and up to 4 kg in weight, is the largest terrestrial-living
arthropod in the world. Its claws are strong enough to open the husk and shell of coconuts so that it can
feed off the flesh. It is also known as the robber crab, term that refers to its reputation for removing
nuts from the tops of coconut palms, but it is also known as the palm thief, because some coconut
crabs are rumored to steal shiny items such as pots and silverware from houses and tents. It is widely
distributed throughout the western Pacific and eastern Indian Oceans, occurring almost exclusively on
oceanic islands or small offshore islets adjacent to large continental islands. For istance is commonly
found at the Salomon Islands, republic of Vanuatu, Marshall Islands and others; however the species in
the pic come from Sipadan island. Being arthropods, coconut crabs can grow only by moulting: adults
moult annually, and juveniles stages more often. These animals are generally nocturnal and remain
hidden during the day, emerging only during the night to forage. Except as larvae that are aquatic,
Birgus latro cannot swim, because adults are adapted to a terrestrial life. They use a special organ called
a branchiostegal lung for air breathing and gas exchange. This organ can be interpreted as a
developmental stage between gills and lungs, and is one of the most significant adaptations of the
coconut crab to its habitat. The branchiostegal lung contains a tissue similar to that found in gills, but
suited to the absorption of oxygen from air, rather than water. Therefore these arthropods have a well
developed sense of smell and can detect food odors (such as bananas and coconuts) over large
distances.
As in the pic 1 (sx), Coconut crabs live in underground burrows, rock and tree crevices, depending on
the local terrain and environment. During the day, Birgus latro stays hidden to protect itself from
predators and reduce water loss from heat. The coconut crabs' burrows contain very fine yet strong
fibers of the coconut husk which the animal uses as bedding. The burrow entrances are plugged with
sand, which often create a small mound on the surface where are the crabs.
As in the pic 5 (sx), Mark-recapture tecnique. Coconut crabs are captured, marked using a carapace-
etching tool and released for studying their habitat and behaviour.
As in the pic 6 (dx), Birgus latro climbs up trees to eat coconuts or fruit, to avoid the heat or to escape
predators. These arthropods are well known for their ability to cut down and open coconuts from the
trees with their strong claws, eating the content on the ground. However they are omnivorous,
commonly eating the fallen fruit of the screw-pine (Pandanus) and the Coconut Palm, but they like
other trees's fruit as well.
As in the pic 7 (sx), A female crab carrying a sponge-like egg mass beneath the abdomen. The
extrusion of eggs occurs on land in crevices or burrows near the shore. Shortly after a simultaneous
fertilization, the female lays her eggs and glues them to the ventral surface of her abdomen by means of
three feathery pleopods, carrying the fertilized eggs underneath her body for a few months. This egg
"sponge" contains about 100.000 eggs and there is a positive correlation between the number of eggs
produced and the size of the female. The egg sponge is afforded no physical protection from the
environment by the abdomen and is therefore susceptible to inundation by fresh or salt water and
exposure to dessicating conditions.
At the time of hatching, the female individual releases the eggs into the ocean at high tide.
Birgus latro is closely related to the terrestrial hermit crabs of the genus Coenobita (click here to see it).
Notes on molting phenomena
Although a crustacean grows, its exoskeleton does not, so the animal must molt its old exoskeleton in
order to accommodate its expanding body. The process of molting is hormonally controlled. In
preparation for molting, the tissue layer under the exoskeleton detaches and secretes a new exoskeleton.
At this point, the animal has two skeletons – the outer one and the inner one. When the new skeleton is
completely formed, the old skeleton splits along specific weak points and the animal pulls out, leaving
its old skeleton intact except for the split. Mobility is limited immediately after a molt because the
exoskeleton is not rigid enough to keep the limbs stiff. Although crustaceans molt throughout their
entire life, they molt less frequently with age.
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