A total of 655 freshwater species
(just over a quarter of all described carideans ) are presently known.
Freshwater species of carideans belong to eight families/subfamilies, numerically dominated by the Atyidae, with 359 species/subspecies
(Table 1). Although this family is considered in many textbooks as restricted to freshwater habitats, some anchialine genera are known (e.g. Antecaridina
), whilst juveniles of Atya
have been found under fully marine conditions in Atlantic waters.
Although the most speciose genus Caridina
occurs in six biogeographic regions, many genera and species are either only known from their type locality or have a narrow geographical distribution (e.g. Lancaris
is restricted to Sri Lanka, see Cai & Bahir, 2005). Some species are morphologically adapted to live in fastflowing water, such as the Caribbean Atya scabra
(Leach), which lives beneath rocks under waterfalls and in rapids, whilst other species, such as many Caridina
species are adapted to lakeshore weed beds, usually displaying a more gracile habitus. Cave dwelling taxa are well represented with many exclusively stygobiont genera. Of particular ecological interest are the only two freshwater commensal species (a widespread mode of life in marine shrimp species): Limnocaridina iridinae
Roth-Woltereck from the mantle cavity of a unionid clam from Lake Tanganyika (Roth-Woltereck, 1958) and a Caridina
species from Lake Towuti in Sulawesi living with freshwater sponges (Cai, pers. obs.).
The second most speciose family is the Palaemonidae
(Table 1), with many more marine and brackish water species known than there are freshwater taxa, all of the latter being restricted to the subfamily Palaemoninae. The numerically dominant genus is Macrobrachium, restricted to fresh and brackish water
[…]. Other species-rich genera are Palaemonetes
, a taxonomically poorly defined world-wide genus, and Palaemon
. Some species of Palaemonetes
are exclusively freshwater, such as the North American Palaemonetes kadakiensis
Rathbun, but several estuarine species can tolerate fully freshwater conditions. Several species of Palaemon
have also been recorded from marine, brackish and freshwater environments, e.g. Palaemon concinnus
Dana (see Marquet, 1991).
It is difficult to estimate the true species richness of freshwater shrimps, as every year new taxa continue to be described, mainly in the two most numerically dominant genera, Caridina
. As a result, species discovery curves are not flattening out (Fig. 2), and it can be expected that many more species await discovery
. New genera also continue to be erected, for instance for morphologically disparate species previously placed in Caridina
(e.g. the genera Lancaris
). Genetic studies have only recently started in freshwater shrimps, with for instance the work of Baker et al. (2004) highlighting the presence of several cryptic lineages in Australian Paratya
, some of which may well represent true species.