genitalia of each species and subspecies attain 

 the adult form (the animals reach the sub- 

 adult stage) is constant, regardless of the lo- 

 cality in which the shrimps occur. 



In P. a. aztecus the petasma and the thely- 

 cum often develop at a slower pace, relative to 

 body length, than in the sympatric P. d. duo- 

 rarum. For each length listed, I have compared 

 the more advance representatives of P. a. 

 aztecus with the least advanced members (i.e.. 



least developed genitalia) of P. d. duorarum, 

 and the differences presented herein seem to 

 be valid. 



Although there are still no infallible cri- 

 teria on which recognition of every single ju- 

 venile under 10 mm. c.l. may be based (for 

 instance, it is difficult to distinguish females of 

 P. brasiliensis from females of the sjTnpatric 

 P. d. duorarum) , the characters cited here al- 

 low the identification of many small and prac- 

 tically all larger juveniles. 



IDENTIFICATION OF SEX IN VERY SMALL JUVENILES 



In individuals as small as 4 mm. c.l., about 

 18 mm. t.l., sex in the three taxa can be de- 

 termined by the position and size of the endopod 

 of the first pair of pleopods. The endopod in 

 males is located more proximally on the basis 

 and is a little longer than in females (figs. 2 



and 3). Also, in males of that length, sternite 

 XIV bears a posteromesial ridge (figs. 4a and 

 5a), whereas in females sternite XIV is rather 

 evenly produced to a central elevation (figs. 

 4b and 5b). 



L 



0-5 mm. 



Figure 2.—Penaeus aztecus aztecus. Anterior (dorsal) 

 view of endopod of first right pleopod. a. $ 

 5.5 mm. C.I., White Oak River, N.C. b. 9 

 5.5 mm. c.l., Missbsippi Sound, Miss. 



Figure 3.—Penaeus duorarum duorarum. Anterior (dor- 

 sal) view of endopod of first right pleopod. 

 a. ^ 4 mm. c.l., Mississippi Sound, Miss, b, 

 9 4 mm. C.I., Mississippi Sound, Miss. 



