This work is based on the examination of 

 some 5,000 specimens collected throughout the 

 ranges of the taxa considered. The great ma- 

 jority of them are in the collections of the 

 U.S. National Museum, and many are sorted 

 according to size (carapace length). Other 

 specimens examined are in the collections of 

 the American Museum of Natural History; 



Chesapeake Biological Laboratory; Instituto 

 Nacional de Investigaciones Biolc^gico Pesque- 

 ras, Secretaria de Industria y Comercio de Mexi- 

 co; Gulf Coast Research Laboratory; Museum 

 of Comparative Zoology, Harvard University; 

 Peabody Museum of Natural History, Yale 

 University, and Virginia Institute of Marine 

 Science. 



Burkenroad (1939) pointed out several di- 

 agnostic features which aid in the recognition 

 of the larger juvenile grooved shrimps from 

 the western Atlantic, and some of these were 

 based on secondary sexual characters. Wil- 

 liams (1953) used the length of the adrostral 

 sulci, characters of the rostrum, and color to 

 identify juvenile P. (L.) sctiferus, P. (Meli- 

 certus) d. duorarum, and P. (Melicertus) a. 

 aztecus. The studies by Eldred (1958) on P. 

 d. duorarum and Perez Farfante (1969) of 

 western Atlantic Penaeus constitute the only 

 information available on the progressive 

 changes in the development of the thelycum 

 and petasma of Penaeus in eastern America. 

 The shape and length of the rostrum, the length 

 and width of the adrostral sulci (in shrimp 

 from the Caribbean and the Atlantic coast of 

 South America), and the width of the dorso- 

 lateral sulci of the sixth abdominal somite are 

 usually helpful. In males, sternites XIII and 

 XIV also exhibit peculiar features in some of 

 the grooved shrimps. The most reliable char- 

 acters for the identification of juveniles, how- 

 ever, seem to be the structure of the petasma 

 and thelycum. 



Facilitating this investigation was a pre- 

 vious knowledge of the local occurrence of the 

 species and the time of the year when juveniles 

 of the three shrimps are found in estuarine 

 waters. For instance, P. d. duorai'wm is the 

 only one of the three that occurs in Tampa Bay; 

 only P. a. azteciis lives north of Chesapeake 

 Bay, and only P. d. duorarum and P. brasi- 

 liensis frequent the waters of the Bermudas. 

 Furthermore, P. a. aztecus juveniles first ap- 

 pear in estuaries of North Carolina and Texas 

 much earlier in spring than those of the other 

 Penaeus. Finally, comparison of progressively 

 younger specimens with those of sizes readily 



identifiable made possible the recognition of 

 the earlier developmental stages. 



I present here a group of diagnostic char- 

 acters that will permit identification of indi- 

 viduals with a c.l. (carapace length — the lin- 

 ear distance between the postorbital margin 

 and the midposterior margin of the carapace) 

 of as little as 8 mm., and t.l. (total length — 

 measured from the tip of the rostrum to the 

 tip of the telson) of about 35 mm. The typical 

 features exhibited by each species and sub- 

 species during a large part of the juvenile 

 period are listed in the tables below, which 

 are arranged in size sequences. For the rec- 

 ognition of small individuals (less than 11 mm. 

 c.l.), all of the characters listed should be con- 

 sidered because occasionally one alone may not 

 prove to be diagnostic. 



Figure 1 depicts the first pleopod, the post- 

 eroventral portion of the thorax, the thelycum, 

 and the petasma — introducing the terminology 

 utilized in the tables. Figures 2 and 3 illustrate 

 the characters used in the identification of sex, 

 and the remaining figures show the peculiar 

 characters of the rostrum, dorsolateral sulcus, 

 and the external genitalia, as well as the pro- 

 gressive changes in the latter, which allow 

 the recognition of each taxon. The features 

 cited were found to be reliable throughout the 

 geographic range of each of the shrimps con- 

 sidered. Whereas the carapace length at which 

 these characters first appear may vary slightly, 

 they are present in all individuals at the car- 

 apace length cited. The rate of development 

 of the petasmata and thelyca varies, conse- 

 quently these structures do not attain the adult 

 form when the shrimps reach a given length 

 (e.g., 15 mm. c.l.) but rather within a definite 

 range of lengths (e.g., 15 to 20 mm. c.l.). It 

 seems that the range within which the external 



