92 U.S. NATIONAL MUSEUM BULLETIN 29 2 



brown dorsally, fading ventrally to lavender, with posteriorly directed 

 wedge of bluish green from orbital area to level of fourth obliquely 

 vertical stripe; stripe on rostrum often olive brown instead of purple. 

 Tergum of first abdominal somite reddish brown with small trian- 

 gular area of bluish green posteriorly; tergum of second abdominal 

 somite with broad posterior triangle of reddish brown fading apically 

 (anteriorly) to reddish tan near midlength of tergum, remainder of 

 tergum pale bluish green; tergum of third through sLxth abdominal 

 somites similar to second, but reddish-tan apical portion of triangle 

 extending to anterior margin of each somite. Pleura of abdominal 

 somites mostly lavender with reddish-brown ventral borders and 

 wedges of pale bluish green. Telson and uropods reddish brown 

 with inner ramus of latter slightly lighter, and extremities of both 

 rami bright red. 



Eyestalk reddish tan above. Inner flagellum of antennule purple 

 and outer pale tan; peduncle of antenna lavender tan, flagellum pale 

 tan; antennal scale with lavender lamellar area separated from 

 thicker, cream, lateral portion by purple band. Third maxilliped 

 pinkish tan with purple dactyl; first pereiopod pinkish tan -with light 

 chela; second pereiopod brownish purple with olive fingers tipped 

 with orange; remaining pereiopods pinkish tan with upper distal 

 portion of each podomere, except dactyl, reddish purple; dactyl 

 tipped with cream; uropods cream and pale lavender. 



Material examined. — The Dominican collections contain 48 males 

 (carapace lengths 4.0-36.5 mm), 35 females (cl 4.0-20.6 mm), in- 

 cluding 1 ^\^th eggs (cl 16.2 mm), and 6 juveniles (cl 1.9-3.9 mm). A 

 carapace length of 4.0 mm was rather arbitrarily selected as the small- 

 est size at which sex can be determined. In one male with a carapace 

 length of 4.2 mm the appendix masculina on the second plcopod is 

 subequal to the appendix interna, but it is slightly longer in specimens 

 having carapace lengths of 4.0 to 5.2 mm. 



Ecological notes. — This shrimp characteristically frequents the 

 comparatively quiet waters near the mouths of those streams that 

 enter the Caribbean over a bed that is below sea level or does not have 

 riflles at its junction with the sea. Only at one station (81, on the 

 Indian River, at 150 ft. elevation) was it found more than 10 feet 

 above sea level. 



During the day, the shrimps are found among the debris and roots 

 of shoreline plants that are exposed in the water. After dark, they move 

 to the tops of debris accumulations or into open water. During the 

 daylight hours, no M. acanthurus can be seen in the Layou River mouth 

 (station 15), but at night thousands of young and many adults may be 

 observed swimming and crawling about the surface of the debris or on 

 the river bottom. 



