instead have slightly larger and fewer teeth. The 

 shorter distomedian projection and the distal fold, 

 however, are the most striking characters that set 

 the males of P. a. aztecus apart from those of P. 

 Ira-siliensis. In the former the distal fold is rela- 

 tively small and unarmed, or, more frequently, 

 armed with small spines on the outer surface, 

 whereas in P. bra.slJiemk the fold protrudes in- 

 ward to form a large auriclelike projection that 

 bears strong spines on the inner surface. 



P. a. aztecus also differs from P. d. diwrarum 

 and P. hrasiliensis in northern waters by the 

 broad dorsolateral sulcus and the often sinuous 

 rostrum. Large brown shrimp also differ from 

 the other two in ha\ing a proportionately longer 

 abdomen. Furthermore, in P. a. astecus the third 

 pereopod is thinner and longer — reaching at least 

 to distal end of the antennular peduncle and in 

 large individuals exceeding it by as much as one- 

 tenth length of carpus — than in P. d. duorarum 

 and P. hrasUiemis. The relatively longer length 

 of the third pereopod in brown shrimp is illus- 

 trated in table 3, which includes the lengths of 

 various podomeres (dactyl to ischium) for the 

 three carapace-length intervals, and in figure 29, 

 which shows the length of the carpus at various 

 carapace lengths in P. a. aztecus, P. d. duoranim. 

 and P. hrasilien.sis. 



As stated earlier, ii is difficult to distinguish 

 small juvenile P. a. aztecus from P. d. diwrarum 

 and P. hrasiliensis. Many juvenile brown shrimp, 

 however, have a broad dorsolateral sulcus and, 

 thus, may be readily identified. Also, small brown 

 shrimp are usually more slender and their ex- 

 ternal genitalia are proportionately less well de- 

 veloped than those of the other two .shrimps. 

 Juvenile males of P. a. aztecus may also be rec- 

 ognized by having a low, rounded midrib on 

 sternite XIV surrounded by a very shallow, 

 horseshoe-shaped groove (fig. 50). Furthermore, 

 in males of P. a. aztecus 11 mm. c.l., .51 mm. t.l. and 

 larger, the ventral costa of tlie petasma lacks dis- 

 tomarginal spines, wliieh are present in P. d. 

 duorarum. Juvenile females of P. a. aztecus may 

 be recognized by characters of the tlielycum. 

 Among females with a carapace length of fO to 

 11 nun. c.l., 47 to 51 mm. t.l., those of P. a. 

 aztecus may be distinguished by the presence of 

 a knob on the midposterior margin of sternite 

 XIII. In females 12 mm. c.l., 55 mm. t.l., a small 

 triangular projection extends caudad from the 

 midposterior margin of sternite XIII, and when 



L 



1mm. 



J 



Figure 50. — Penacus (M.) aztecus aztecus 

 Ives. Sternites XIII and XIV, S 10 mm. 

 c.l., Laguna de la JIancha, Veracruz, 

 Mexico. 



attaining 14 mm. c.l., 64 mm. t.l., the anterior arms 

 of the median carina are clearly distinct on the 

 posterior process. At 16 mm. c.l., 73 mm. t.l., the 

 posterior process has assumed its typical diamond - 

 like shape. At 17 mm. c.l., 77 nun. t.l., the two ribs 

 projecting from the median carina are strongly 

 developed. The thelycimi also attains the adult 

 fonn at a greater size in females of P. a. aztecus 

 than in P. d. duorarum and P. hrasiliensis. 



The length of the rostnun also aids in distin- 

 guishing juveniles of P. a. aztecus from those of 

 P. d. duorarum. Brown shrimp, some as small as 

 8 mm. cL, 36 mm, t.l., have rostiiun proportion- 

 ately longer than those in P. d. duorarum; the ros- 

 trum of the brown shrimp extends anteriorly as 

 far as distal one-third of the thickened i>ortion of 

 the lateral antennular flagellum. 



P. a. aztecus is separable from the closely allied 

 P. paulensis by the broader dorsolateral sulcus, 

 with its rounded rather than sharp dorsal lip. Fe- 

 males of brown shrimp have wider anterior and 

 posterior j^rocesses, and in males the distal iiortion 

 of the ventral costa is strongly arched and the at- 

 taclied margin is armed with numerous contiguous 

 teeth. 



Reproduction 



SUBADULT STAGE-SEXUAL MATURITY 



The smallest male I found with joined petasmal 

 endopods was 19 mm. c.l., 85 mm. t.l., but indi- 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



539 



