p. a. asfecns have passed their peak of abundance. 

 In the northern Gulf of Mexico the second peak 

 of brown shrimp spawning occurs later in the fall 

 than those of the white and pink shrimps. 



Burkenroad (1939) indicated that because fe- 

 males first spawn at sizes only a little above 30 mm. 

 C.I., the largest females may sj^awn more than once. 

 He also observed that the ovaries of large females 

 contained ripe ova together with young ova, which 

 suggests preparation of the ovary for a second 

 spawning. Studies by Kenfro and Temple (per- 

 sonal communication) indicated that recovery and 

 redevelopment are fairly rapid at least during 

 the warmer months. They found in recently spent 

 ovaries that the immature ova present at spawning 

 were developing rapidly wliile remnants of ripe 

 ova from a previous spawning were being re- 

 absorbed. This finding is taken also as evidence of 

 more than one spawning by one individual. 

 SEX RATIO 



The sex ratio in inshore as well as offshore 

 waters is about 1: 1 (Renfro and Brusher, 1963; 

 Joyce, 1965). 



Postembryonic Development 



LARVAE, POSTLARVAE, AND JUVENILES 



Wheeler (1060) reared P. a. aztecus from eggs 

 spawned in the laboratoiy through five nauplial 

 stages to the first protozoea. Later Cook (1965) 

 succeeded in rearing brown shrimp to postlarvae. 

 The entire larval cycle was completed in a mini- 

 mum of 12 days at about 29° C. Harry L. Cook 

 (personal communication) found that the larval 

 cycle of brown shrimp is similar to that of the 

 white and pink shrimps, and includes, in addition 

 to five nauplial stages, thi'ee protozoeal and three 

 mysis stages. To date, no distinctive characters 

 have been observed which will allow specific identi- 

 fication of the different lan^al stages of the various 

 grooved Penaeits from the western Atlantic and 

 the Gulf of Mexico. 



Various investigations have been conducted on 

 the morphology of the postlarvae of P. a. astecus, 

 P. d. duorarum^ and P. setifenis. 



Pearson (1939) described various postlarvae of 

 P. hrasiliensis {P. a. azteciis and P. d. duorantm) 

 and distinguished them from those of P. setiferus. 

 Williams (1959) separated the postlarvae under 

 12 mm. t.l. of P. a. aztecus, P. d. duorarum, and 

 P. setiferus on the basis of two of the diagnostic 

 characters pointed out by Pearson (length of 



rostrum and third pereopod) plus body size and 

 shai>e of distal portion of antennal scale. Baxter 

 and Renfro (1967) found that those morphological 

 and morphometric characters, combined, allowed 

 the identification of brown and white shrimp post- 

 larvae below 10 mm. t.l. in the Galveston Bay area. 

 Christinas et al. (1966), however, as a result of the 

 examination of extensive collections from Missis- 

 sippi, concluded that intraspecific variation among 

 the postlai-vae of the brown, white, and pink 

 shrimps is wide. Tliey fovmd that during spring 

 postlai-vae of brown shrimp have a larger size than 

 those of the pink and white shrimps, but that the 

 differences disappear in summer when postlarval 

 brown shrimp have more nearly the same size as 

 the other two. Baxter and Renfro (1967) also re- 

 ported an overlap in length distribution of the 

 postlarvae of brown and white shrimps during the 

 summer, but tlie mean length of the brown shrimp 

 always exceeded that of the white shrimp. 



Juvenile P. a. astecu.s 18 mm. t.l. have very shal- 

 low but distinctly long adrostral sulci and at 20 

 mm. t.l., the sulci are well developed. Brown 

 shrimp of this size and larger can, therefore, be 

 readily separated from P. setifoits, as well as from 

 P. schtnifti, the only other nongi'ooved Penaeus in 

 the western Atlantic. 



In juveniles sex can be determined when they 

 reach about 20 mm. t.l. Males can be distinguished 

 by the shape and position of the endopods of the 

 first pair of pleopods, which are lower on the bases 

 and longer than in females. Small males can also 

 be recognized by having a low rib on the midline 

 of sternite XIV, whereas in females the stemite 

 XIV is produced ventrally, often bearing a mi- 

 nute knob at its extremity. (See also under Rela- 

 tionships.) 

 GROWTH 



There are various estimates of the rate of growth 

 of P. a. aztecus at different sizes and under differ- 

 ent environmental conditions. Pearson (1939) re- 

 ported that postlarvae of "P. hraMliensis,^'' which 

 most probably were P. a. aztecus, held in the lab- 

 oratory grew at a maximum rate of 0.56 mm. per 

 day. Zein-Eldin and Aldrich (1965) studied in the 

 laboratory the growth of postlarvae 12.1 mm. t.l. 

 at temperatures ranging from 7° C. to 35° C. 

 through a 28-day period. They concluded that 

 growth increased with temperature, with signifi- 

 cant growth beginning at some temperature above 

 11° C. but below 18° C. The most marked increase 

 in growth rate occurred in the temperature region 



WESTERN ATLANTIC SHRIMPS OF GENUS PENAEUS 



541 



