BIOLOGY AND NATURAL HISTORY 195 



(smaller specimens) in color. The tail is edged in green. Its antennae and 

 "horn" (rostrum) are comparatively longer than those of the other species. 

 Two species belonging to Division II are taken by North Carolina fisher- 

 men: P. aztecus and P. duorarum. P. aztecus is the most common of the 

 grooved shrimps. In color it is brownish to orange, although the color is 

 often so light as to make the shrimp seem white. Its tail is usually edged 

 in purple to red-purple. Although F. duorarum may be indistinguishable 

 from P. aztecus in color, it often is somewhat blue to blue-gray in hue. Its 

 tail is usually edged in blue. A character relied on locally by the fishermen 

 to differentiate between these species, "channel shrimp" and "brown shrimp" 

 is a more or less distinct red or blue spot on each side of the third abdominal 

 segment. This may be a valid field diagnostic of P. duorarum, for such a spot 

 is certainly lacking in P. aztecus. 



As has been implied, P. aztecus and P. duorarum seem to be relatively 

 more abundant in North Carolina in recent years than they have been in the 

 South Atlantic and Gulf regions as a whole. The only available estimate 

 (Weymouth, Lindner, and Anderson, 1933) places the percentage of all 

 shrimp other than P. setiferus at about 5 per cent of the total catch with 

 P. setiferus accounting for the remaining 95 per cent. An estimate for North 

 Carolina for the year 1948 based upon tax collection figures places the per- 

 centage of P. setiferus at less than 50 per cent, with the bulk of the catch 

 made up of P. aztecus. Although it is possible that the grooved shrimp have 

 become more abundant generally in recent years, population percentage 

 figures from the various states are not available.^ 



DISTRIBUTION 



The geographic distribution of the Atlantic species of Penaeus is listed 

 by Burkenroad (1939). Adults of P. setiferus are found from Fire Island, 

 New York, to Louisiana, Texas, and Vera Cruz, Mexico, and in Cuba and 

 Jamaica. The records of the three species of grooved shrimps show con- 

 siderable difference in distribution of the various forms. P. duorarum alone 

 occurs in Africa; P. aztecus alone occurs north of Cape Hatteras and south 

 of Rio de Janeiro. P. brasiliensis is absent from the northeastern Gulf of 

 Mexico. 



Burkenroad (1939) finds a preference for low salinity (or a greater de- 

 pendence of post larvae upon low salinity nursery grounds) in P. aztecus 

 and P. setiferus; a high salinity preference (independence of brackish nur- 



I. Since this section on the shrimp was written, a new and very productive fishery for one of 

 the grooved shrimps, P. duorarum (North Carolina "channel shrimp") was discovered in Feb- 

 ruary, 1950, in the Tortugas region west of the Florida Keys in the Gulf of Mexico. (See Clarence 

 P. Idyll, New Fishery for Commercial Shrimp in Southern Florida. Comm. Fish. Rev. Vol. 12, 

 No. 3, Mar. 1950, p. 10-16). 



