36 U.S. NATIONAL MUSEUM BULLETIN 292 



Of the species that are characteristic of fresh water but which 

 invade the sea or have marine larvae, some are restricted to the 

 comparatively sluggish waters near the mouths of streams (pi, 2b). 

 Jonga serrei and Macrohrachium acanthurus occur in estuarine or 

 subestuarine habitats. They are definitely tolerant of fresh water but 

 are found to be progressively less abundant as the stream gradient 

 increases, and on Dominica neither was found above 150 feet elevation. 

 On that island, both are absent in those streams that reach the coast 

 with their beds above the high tide level (pi. 2a). Macrohrachium 

 acanthurus is most abundant on Dominica in sluggish streams that 

 flow over a muddy bottom. 



Juveniles of most of the remaining species, in their migrations 

 upstream, may be found in a wide variety of habitats, but the adults 

 seem, for the most part, to be somewhat selective in sharing the 

 stream beds with other species. At high elevations on Dominica, 

 Atya innocous shares the streams with Guinotia dentata, and, while 

 it is obviously the dominant element and in some streams the sole 

 element of the decapod fauna at altitudes above 2,500 feet, it also 

 occurs throughout most of the streams on the island, becoming less 

 abundant as the size of the rivers increases. Typically, it is an in- 

 habitant of cascading waters where it is numerous in small pools and 

 among rocks. 



Below about 2,500 feet, A. innocous is joined by Macrohrachium 

 heterochirus, M^hich seems to be confined to riffle areas, a habitat in 

 which it occurs almost to sea level. Macrohrachium carcinus ascends 

 the streams to about 2,000 feet, and it lives in pools or under 

 large stones in the larger rivers. In subestuarine habitats, it seeks 

 shelter among the littoral debris. Below about 1,300 feet, Micratya 

 poeyi, Potimirim glabra, and M. crenulatum join the former tln-ee, 

 sharing the stream bed with them almost to sea level. Macrohrachium 

 crenulatum inhabits pools in the smaller streams and seeks the com- 

 paratively quiet, littoral portions of shallow riffles in the larger 

 streams. Both Micratya and Potimirim are riffle dwellers but occa- 

 sionaUy may be found among rocks at the edges of pools. Xiphocaris 

 elongata occurs from elevations of approximately 1,200 feet to sea 

 level. Adidts usually inhabit pools although they do not hesitate to 

 move through the swiftest currents; in contrast, the young seek the 

 margins of streams where the current is not so swift, and they occur 

 in numbers in pools at lower elevations. On Dominica, no adults 

 were observed below 150 to 200 foot elevations. Below 500 feet, Atya 

 scahra and Macrohrachium jaustinum join the stream fauna. The former 

 frequents cascading reaches of smaU streams while the latter is largely 

 confined to quieter waters of pools and protected littoral areas of 

 larger streams. Macrohrachium jaustinum invades the subestuarine 

 habitats in which M. acanthurus and Jonga abound. 



