Crayfish

Description:

The external morphology of freshwater crabs varies very little, and the form of the gonopods (first abdominal appendage, modified for insemination) is common in classification. The development of freshwater crabs is characteristically direct, and the eggs hatch as juveniles, and larval stages that occur inside the egg. Total offspring comprises only a few hundred eggs (as compared to hundreds of thousands laying marine crabs) each of which is quite large, with a diameter of about 1 mm (0.04 inch.) . The population of freshwater crabs has required altering the water balance, the salt can absorb urine and have several adaptations to reduce water loss. In addition to their gills, freshwater crabs have a "pseudo-lung" in its gill chamber allowing them to breathe in the air. These changes have been pre-adapted to terrestrial life, although they need to return to the water periodically ammonia excretion.

Habitat:

Freshwater crabs are found in all tropical and subtropical regions of the world. They live in a variety of water bodies, from fast flowing rivers, swamps, and on tree trunks or cave. They are primarily nocturnal, going out to eat at night. Most of them are omnivorous, although a small number are specialized predators such as Platyhelphusa armata of Lake Tanganyika, which feeds almost exclusively on snails. Some species are important food sources for various vertebrates. Some freshwater crabs are secondary hosts of trematodes of the genus Paragonimus, which cause paragonimiasis in humans. Most endemic species are reduced, that is, there are only a small geographic area. This is due in part to their low dispersal ability and low fertility, as well as habitat fragmentation caused by the human population. In West Africa, the species that live in savannas have a wider spread than the species of the rainforest, in East Africa, the mountain species have limited distribution, while the lowland species are more widespread .