■''^o 



1 mm. 



Figure 47. — Penacus (J/.) a:tcciis aztccus 

 Ives. Median protuberance with horns on 

 posterior margin of sternite XIII, $ 55.5 

 mm. C.I., off Matagorda Bay, Tex. 



5 mm 



Figure 48. — Penaeus (M.) aztecus aztecus Ives. Thely- 

 cum, 9 39 mm. c.l., northwestern Gulf of Mexico. 



5 fm. The only reference to the presence of P. 

 a. aztecus farther north on Florida's west coast is 

 that by Eldred et al. (1965), who reported two 

 postlarvae 145 km. off the mouth of Tampa Bay 

 (also one postlarva about 96 km. off Fort Myers). 

 This subspecies attains its maximum density along 

 the coast of Texas but is also relatively abundant 

 off the northeast coast of Mexico south to Vera- 

 cruz, and a dense concentration exists in Campeche 

 in the area west of Carmen. 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



Highest densities of brown shrimp are between 

 15 and 30 fm., but they are commercially abundant 

 down to 60 fm. At depths less than 10 f m. they are 

 immature and relatively small and move to deeper 

 waters as they grow. Although most individuals 

 stay in water shallower than 30 fm., some go three 

 times as deep. At Pelican Sta. 114-4 (lat. 26°56.5' 

 N., long. 96°27' W.) off Armstrong, Tex., two fe- 

 males were caught in 90 fm. These specimens were 

 identified by Lipke B. Holthuis and are now at 

 Tokyo University of Fisheries. P. a. aztecus was 

 also found at Pelican Sta. 11 (lat. 29°11' N., long. 

 88°30' W.) off Mississippi in 88I/2 fm., and at Sta. 

 115-1 (lat. 26°55' N., long. 96°27' W.) off Arm- 

 strong, Tex., in 90 fm. The specimens taken at these 

 latter stations were tentatively identified by Mil- 

 ton J. Lindner (personal communication). 



P. a. aztecus shows no significant morijhological 

 variations within its range. 



Relationships 



P. a. aztecus differs rather strikingly from the 

 southern subspecies P. a. svhtilis in having a long 

 and deep median sulcus and a long and broad 

 adrostral sulcus. Figure 59 shows that, despite 

 some overlapping in the length of the adrostral 

 sulcus in the two subspecies, the adrostral sulcus 

 reaches 1/12 to Vio c-1- from the posterior margin of 

 carapace in the higher percentage of specimens of 

 P. a. aztecus, whereas in the higher percentage of 

 P. a. suhtilis it reaches from % to 142 c.l. Further- 

 more, P. a. aztecus usually has a broader dorsolat- 

 eral sulcus and its K/S has a modal value of only 

 1.25 (fig. 44), whereas in P. a. subtilis K/S has a 

 modal value of 3.5. 



Adult P. a. aztecus can be separated readily 

 from the closely related P. d. duommm and P. 

 irasiliensis. Females may be distinguished by the 

 anteriorly bifurcate median carina on the pos- 

 terior process and tlie widely divergent antero- 

 median corners of the lateral plates. Males of P. a 

 aztecus differ from those of P. d. duorarum by 

 the shape and armature of the ventral costa of 

 the petasma (turning proximally in an arc, lack- 

 ing spines on free margin, and with a patch of 

 small, closely set teeth on the distal portion of 

 the attached margin) and the relatively elon- 

 gated — rather than broad — appendix masculina. 

 The compact patch of teeth on the attached mar- 

 gin of the ventral costa also distinguishes male 

 brown shrimp from male P. brasiliensis, which 



537 



