Freshwater crab Freshwater crab There are around 1,300 species of freshwater crabs, distributed throughout the tropics and subtropics, divided among eight families. They show direct development and maternal care of a small number of offspring, in contrast to marine crabs which release thousands of planktonic larvae. This limits the dispersal abilities of freshwater crabs, so they tend to be endemic to small areas. As a result, a large proportion are threatened with extinction. There are more than 1,300 described species of freshwater crabs, out of a total of 6,700 species of crabs across all environments. The total number of species of freshwater
Aquatic ecosystem lagoons and mangrove swamps. In the deep water, hydrothermal vents may occur where chemosynthetic sulfur bacteria form the base of the food web. Classes of organisms found in marine ecosystems include brown algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks. Fishes caught in marine ecosystems are the biggest source of commercial foods obtained from wild populations. Environmental problems concerning marine ecosystems include unsustainable exploitation of marine resources (for example overfishing of certain species), marine pollution, climate change, and building on coastal areas. Freshwater ecosystems cover 0.78% of the Earth's surface and inhabit 0.009% of its total water. They generate nearly 3%