The massive Coconut Crab
The coconut crab, Birgus latro, it is the biggest hermit crab living on land, weighting up to 4 kg in adult stage. The common name come form the reputation of climb palm trees, take coconuts and remove the fibers, with they incredibly strong chelae, to use them to build a “nest” or as a bedding, to avoid hot temperatures in order to reduce water lost.
Birgus latro appear in tropical regions with high density of palms trees. It can be found in indonesia and most of the islands in the Indian Ocean. The densest population of Coconut Crabs in the Indian Ocean area is in Christmas
Island, Seychelles, and in the northern atolls of Chagos archipelagos; coconut crabs can also be found in the Eastern Pacific, in Cook Islands and Palmerston Island and in small number in Gambier islands.
Adaptations to land living
Coconut crabs are adapted to the life on land in adult stage, coming back to the sea only to fecundation and release larvae.
One of the main adaptations is the breathing system, coconut crabs have branchiostegal lungs located in the cephalothorax. These “lungs” are gill tissue, reordered in a sack shape in order to increase the surface area, and therefore being able to exchange oxygen with the air instead of water. However, this organs will not function without a certain level of water irrigation, in order to maintain humidity. Coconut crabs also have a small set of rudimentary gills, which allow them to breathe under water for a small amount of time, they will drown if stay in water for more than an hour.
The sense of smell is also well developed in this kind of crabs. They antennae has been modified and it is most similar to the one present on insects. Coconut crabs will move their short antennae vigorously in order to increase the smell reception. Their olfactory system it is accurate to life on land, coconut crabs can sense the odor their food resource from a long distance.
Coconut crabs are hermit crabs, what characterize hermit crabs is their soft abdomen, therefore they use a Gastropod shell as protection. The adult stage of B.latro have a hard exoesqueleton with a tough abdomen, no shell is needed.
Life cycle and Reproduction
Coconut crabs reach sexual maturity when they are five years old, and with a size of 25 mom TL approximately. They copulate on land, during the night time to avoid predators. Afterwards females are left with a packet of sperm, they had to the water,
release the eggs and fecundation takes place.
Females carry the fecundate egg mass in their abdomen until the spawning, 35 to 45 days after fecundation, in summer time. The spawning takes place in the water, during the night in rocky shores. Larval release is almost instant once the eggs reach the water, and then females go back to land.
Larvae life last for 15 to 28 days. It is composed by 4 larvae stages, sometimes a 5th larvae stage is develop, it is believed that is in order to expand the larvae life. After the first 4-5 Zoea stages the larvae metamorphose in a Glaucothoe, a benthic an amphibious stage. In this stage the organism have to find a shell as a protection against predators and desiccation. The Glaucothoe stage last for 28-50 days, after this period the organism metamorphose again, but this time in a juvenile.
The juvenile stage it is a hermit crab, adapted to live in the shore, spending time in the sand and water for a few weeks, and
then they will keep using a shell as protection but life in land close to the shore. Then they start adult life in the land, where they can live for more than three decades, feeding out of fruit, rotten meet and even have been documented coconut crabs eating alive birds and rats.
Coconut crab are listed as a “Data deficient specie” by the IUCN Red List. Conservation strategies are running in some regions of the Indian Ocean, these gather capture minimum size, ban of capture of eggs and individual in some islands, among others.