DECAPOD CRUSTACEANS OP THE WEST INDIES 99 



on being startled by nets lowered to capture them, they swam off 

 rapidly and gracefully, abdomen first, their appendages trailing. It 

 was nothing short of thrilling for the observers to watch these large, 

 ghostlike, striped animals gliding across the sandy bottom of the 

 river mouth. 



19. Macrobrachium crenulatum Holthuis 



Figures 22, 25c, i 



Palaemon olfersii. — Sharp, 1893 [part], p. 123. 

 Bithynis olfersii. — Rathbun, 1901 [part], p. 124. 

 Macrobrachium olfersii. — Schmitt, 1933a [part], p. 315. 



Macrobrachium crenulatum Holthuis, 1950a, p. 95 [type-locality: Rfo Peje Bobo, 

 Panama]; 1952, p. 107, pi. 27a-d; pi. 28. 



Diagnosis. — Carapace with antennal and hepatic spines, without 

 branchiostegal spine. Rostrum reaching about as far as end of an- 

 tennular peduncle, dorsal margin faintly convex, tip not upturned; 

 armed with 11 to 14 rather regularly spaced dorsal and 3 or 4 ventral 

 teeth; posterior 4 to 6 teeth of dorsal series placed on carapace behind 

 level of orbital margin. Eyes large, cornea well pigmented. Second 

 pereiopods of adult male very dissimilar and unequal; major cheliped 

 with fingers slightly longer or shorter than palm, curved dactyl 

 forming wide gape, one large tooth on proximal part of opposable 

 margin of each finger, each finger densely covered with nonaligned 

 spinules on both surfaces and bearing numerous tufts of long, stiff 

 hairs along cutting edges; palm distinctly compressed, about twice as 

 long as wide, armed with longitudinal rows of strong spines, those on 

 mesial margin forming spiny crest diminishing rather abruptly at base 

 of fixed finger but not near midlength of palm, spines of upper and mesial 

 surfaces partially concealed by hairs; carpus shorter than either palm 

 or merus but much more than half as long as palm. Third pereiopod 

 with propodus two and one-half to three times as long as dactyl. 

 Color pattern usually characterized by light transverse patch on 

 posterior part of third abdominal tergum; fingers of second pereiopod 

 and distal podomeres of third to fifth pereiopods not conspicuously 

 banded; second pereiopods dark colored. A medium-sized species, 

 maximum postorbital carapace length about 30 mm. 



Color in life. — At least a part of the tremendous variation in 

 color in this species is due to adaptations to different backgrounds. In 

 comparison with this variation in actual color, the color pattern is not 

 nearly so variable. Since most of the animals observed in the field 

 were adapted to a dark background, this phase is described in detail. 



Carapace of males mostly reddish purple to bright blue above but 

 with reddish-black longitudinal stripe on rostrum and irregular, 

 similarly colored blotch ventrolateral to posterior rostral spine. 



