The genus Alpheus Fabricius 1798
Alpheus is by far the most diverse and morphologically heterogenous genus, and probably the largest genus within the order Decapoda, with over 280 described species but estimated 1000 species wolrdwide.
Alpheus can be distinguished from all other alpheid genera by the combination of the following features:
Features 1-3 may be used to distinguish Alpheus from Synalpheus, features 1 and 2 from Pomagnathus and Metalpheus, and features 1, 4 and 5 from Racilius. However, there is some evidence that Pomagnathus, Metalpheus and Racilius are embedded within Alpheus (Anker et al., 2006).
Alpheus is morphologically a highly heterogenous assemblage, with most variation lying in the chelipeds (both major and minor) and frontal margin of the carapace (rostrum, orbital hoods and annexes). The 19th-20th century French carcinologist Henri Coutière subdivided Alpheus into 7 informal species groups, based mainly on the features of these two body regions. These groups have guided most subsequent workers, such as the alpheid authorities Albert H. and Dora M. Banner (Banner and Banner, 1982). Recently, Williams et al. (2001) proposed a first phylogenetic hypothesis for Alpheus based on molecular data, revealing the long-suspected polyphyletic status of some of these groups. The present state of knowledge of the 7 Alpheus groups is summarized below.
It is not surprising that with so many species, Alpheus is also ecologically by far the most diverse shrimp genus, with species living in crevices of coral rubble, under intertidal rocks, in self-bored tunnels in basaltic rocks, associated with sea anemones, among seagrass roots, in sponge canals, coralline algae, in excavated burrows in mangrove mud, in burrows shared with partner gobies, in tunnels of echiuran worms, in burrows in silty sediments of the deep sea, or in freshwater, in burrows made under river banks. Most species appear to be free-living in crevices of coral rubble, under rocks or in self-dug burrows, and many of them usually live in male-female pairs. Alpheus range in size from 10 mm TL (total length) in some sponge-dwelling members of the A. crinitus group to over 80 mm TL in some giants from the Philippine mangroves.
The taxonomy of Alpheus is extremely challenging, mainly due to presence of numerous cryptic taxa (see Knowlton and Keller, 1985; Bruce, 1987; Bruce, 1999; Anker, 2000; Anker, 2001; Nomura and Anker, 2005) and the ignorance of the importance of colour patterns in most previous studies. Major regional reviews exist, for example Crosnier and Forest (1966) for the eastern Atlantic; Chace (1972) and Christoffersen (1979) for the western Atlantic; Kim and Abele (1988) for the eastern Pacific; Tiwari (1963) for Vietnam; Liu and Lan (1980) for China; Miya (1974) for Japan; Banner and Banner (1982) for Australia; Banner and Banner (1981) for the Red Sea; Banner and Banner (1983) for the western Indian Ocean; Chace (1988) for the Philippines; Banner and Banner (1985) for Indonesia. However, most of them are far from being exhaustive; some lack illustrations, contain misidentifications and should therefore be used with extreme precaution. The two notable exceptions are the very good, well illustrated contributions by Crosnier and Forest (1966) and Kim and Abele (1988) that are extremely useful starting references for the eastern Atlantic and eastern Pacific, respectively.   Another unsolved nomenclatural issue is the currently uncertain status of the type species of Alpheus (De Grave, Anker and Felder, in prep.).