96 U.S. NATIONAL MUSEUM BULLETIN 29 2 



fingers; distal portion of merus in some specimens dark blue with 

 maroon articulating membrane at distal end. Second pereiopod with 

 proximal two podomeres and lower proximal portion of ischium laven- 

 der, otherwise mostly purplish black except distal portions of merus 

 and carpus, and proximal portions of carpus and propodus with 

 bands of bright, almost iridescent, blue; lower surface of palm with 

 distal white spot, and upper and lower surface of dactyl with proximal 

 white spot; large tubercles on opposable margins of fingers white; 

 upper surfaces of carpus and palmar portion of propodus \vith broad 

 cream-tan band. Third, fourth, and fifth pereiopods lavender basaUy 

 with upper margins of ischium and merus purple; carpus purple, 

 propodus and dactyl bluish purple, latter with corneous tan tip. 

 Distal surfaces of merus and carpus with yellow spots. Pleopods 

 pinkish tan. 



The most conspicuous variation in coloration is in the chela which 

 may be ohve bro\\Ti with black tubercles. In observing the animal in 

 the water, the most conspicuous areas are the white or cream distal 

 segments of the third maxilhped and the white spot at the opposable 

 base of the dactyl when the fingers are slightly gaping. 



Material examined. — The Dominican collections contain 26 

 males (carapace lengths 10.9-92.0 mm) and 31 females (cl 12.2-64.2 

 mm), including 3 with eggs (cl 13.7-44.2 mm). Inasmuch as none of 

 the numerous juvenile specimens of Macrobrachium in the collections 

 could be positively assigned to M. carcinus, the smallest size at which 

 sex can be recognized could not be determined. 



Ecological notes. — Macrobrachium carcinus was observed on 

 Dominica in the comparatively quiet mouth of the Layou River, in 

 rapidly flowing drainage ditches, beneath large stones in swift portions 

 of streams, and under stones at the margins of pools along even the 

 smallest creeks. It was seen or collected from sea level to 1,900 feet. In 

 streams where there is suitable cover, this shrimp probably attains 

 even higher altitudes. 



Like most, if not all, of the shrimps on Dominica, the members 

 of this species are more active at night than during the day. In the 

 pools, after dark, they leave the cover of the overhanging rock under 

 which they are secluded during the day and move freely about the 

 bottoms, and it is reasonable to assume that it is during the night 

 that they migrate across riffle areas in moving from one pool to another. 

 In repeated observations in riffle areas during the day, tliis slu-imp 

 was never observed in open water and has never been seen in shaUow 

 rifiles between pools. 



Our observations suggest that there exists a correlation between 

 the size of the pool and the number of M. carcinus populating it. 

 In several small pools (under 15 square feet in surface area and 



