unlikely this species could ever have reached 

 Cuban waters, much less farther south, since it 

 does not seem to live off the southernmost end 

 of Florida. 



Large concentrations of white shrimj) are foimd 

 in less than 20 fm. ; it is rare at greater depths 

 where brown shrimp (P. a. aztecus) and, in some 

 areas, pink shrimp [P. d. duorarum) occur in quan- 

 tities sufficient to sustain profitable fishing. Never- 

 theless, white shrimp do invade water deeper than 

 20 fm. Springer and Bullis ( 1952) reported them at 

 43 fm. off Alabama and Louisiana, and later Bullis 

 and Thompson (1965) caught P. setiferus in 

 ■±5 fm. off Louisiana. This latter depth is consid- 

 ered very close to the lower batliymetric limit of 

 tlie species. 



Extensive biometric studies that took into ac- 

 count 45 different characters have failed to show 

 any significant morphological differences between 

 the white shrimp from the Atlantic and those from 

 the Gulf of Mexico. 



Some time during the past, the range of P. 

 setiferus became discontinuous — probably toward 

 the close of the Pleistocene with the consolidation 



of the Florida Peninsula. The Pleistocene range of 

 the white shrimp probably extended from tlie 

 Carolina Coast southward across the Suwannee 

 straits into the Gulf, and with the elevation of 

 the peninsula and the closing of the straits, the 

 population was divided. The northern segment has 

 succeeded in moving south to middle Florida, as 

 far as the Saint Lucie estuary. For some reason, 

 perhaps ecological, however, the Gulf population 

 has not become established beyond the eastern por- 

 tion of Apalachee Bay along the west coast of the 

 peninsula. 



Although recent genetic interchange between the 

 Atlantic population and the Gulf population seems 

 unlikely, the two, as stated above, cannot be dis- 

 tinguished. The three characters — relative lengths 

 of the rostrum, petasma, and third pereopod — 

 which Burkenroad (1934) indicated might prove 

 to be different in the two stocks, vary with increas- 

 ing size, and the first also with attainment of sex- 

 ual maturity in the female. The proportional 



(r.l.) 

 length of the rostrum - — p^ is variable and de- 

 creases as the adult shrimp grow longer; but it 



80- - 



G 60 



z 



LU 



Z) 



o 



z 



LU 



u 



40 



20 



% 



\ 



\ 



2 ''2 



ROSTRAL TEETH FORMULA 



% 



% 



10/ 

 /2 



Figure 13. — Percentage frequency of the different combinations of rostral teeth in Penaeus (L.) setiferus (L.) from 



the Atlantic Ocean and the Gulf of Mexico. 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



470 



