exhibits a range of variation that is about the 

 same in both stocks. The range of relative length 

 of the petasma in shrimp with the same total 

 length in the two areas also overlaps. In both 

 stocks the third pereopod, believed to be shorter 

 in the Atlantic individuals, increases with increas- 

 ing size, and, thus, its relative length depends 

 on the size of the shrimp ; no significant difference 

 has been observed among shrimp of the same length 

 in the two populations. To be sure, the lai'gest fe- 

 males examined in this study came from the Gulf 

 of ISIexico, and their third pereopods were com- 

 paratively longer than those of the largest females 

 that were available from the Atlantic; however, 

 the Atlantic females were smaller than the Gidf 

 females. It now seems clear that the apparent dif- 

 ferences in the two populations are not real if 

 shrimp of the same size are compared. 



Burkenroad (1934) mentioned that the median 

 sulcus might be used to distinguish the two stocks. 

 The median sulcus of P. setiferus from the At- 

 lantic Coast usually consists of two shallow de- 

 pressions joined by a groove — the anterior one 

 attenuated anteriorly as a line — but this type of 

 median sulcus also occurs in some specimens from 

 several localities in the northern Gulf of Mexico 

 and in many from Campeche. White shrimp from 

 the Gulf often have a median sulcus consisting of 

 two isolated shallow concavities, the anterior one 

 blunt. The rostral tooth formula is almost the same 

 in the two populations as is indicated in figure 13. 

 Eioja (1939) described the median flagellum of 

 this species as having "16 articles bearing one long 

 central and two small lateral spines [processes]." 

 In contrast, I found that the niunber of articles 

 armed with processes varies, and that each article 

 almost always has two processes, one long and one 

 short ; only in one instance was aji article observed 

 to bear three processes. Articles bearing two proc- 

 esses are tyj^ical throughout the range of the spe- 

 cies — in the Atlantic as well as in the Gulf of 

 Mexico. 



Relationships 

 P. setiferus closely resembles P. scJwiitti, the 

 white shrimp from the Antilles, Central America, 

 and South America; the external genitalia, how- 

 ever, allow the two species to be separated read- 

 ily. The pair of fleshy, closely set, anteriorly pro- 

 duced posterior lobes and the medially turned ante- 

 rolateral ridges on sternite XIV set female P. 

 setiferus apart from females of P. schmitti. Also, 

 the posterior ridge of sternite XII has only two 



small lateral convexities. The diagonal ridge and 

 conspicuous fold on the interior surface of the 

 distal portion of the lateral lobe of the petasma 

 are typical of P. setiferus males. 



Serological tests by Leone and Pryor (1952) in- 

 dicated that, although P. setiferus is more closely 

 related to P. d. duorarum than to P. a. aztecus, the 

 latter two arc more closely allied tlian either 

 is to P. setiferus. Their results, thus, confirm con- 

 clusions reached through a study of the external 

 morphology of the three shrimps. 



Reproduction 

 SUBADULT STAGE-SEXUAL MATURITY 



Males have been found with petasmal endopods 

 joined (animals in subadult stage) at a minimum 

 size of 22 nnn. c.L, 105 mm. t.l. The joining of the 

 petasmal endopods occurs within the size range 

 22 to 27 mm. c.L, 105 to 127 mm. t.l. The minimum 

 size at which females are impregnated is not 

 known. 



According to Bui-kenroad (1934), males have 

 spermatophores fully developed, of maximum 

 dimensions they will attain throughout the life of 

 the shrimp, at a minimmn of about 35 mm. c.L, 

 about 155 nun. t.l., but ripe sperm first appear at 

 25 nnn. c.L, about 118 mm. t.l. The smallest ripe 

 females recorded (Burkenroad, 1939) had a cara- 

 pace lengtli of :>0 mm. (about 135 mm. t.l.) . 



COPULATION 



Copulation in P. setiferus is believed to take 

 place between hard-shelled individuals (Burken- 

 road, 1939). Mating in this condition is contrary 

 to that in the species of Penacus with a closed 

 thelycum, in which females are impregnated im- 

 mediately after molting. 



During copulation the male attaches the sper- 

 matophore to the female. The spermatophore re- 

 mains anchored on the thelycum by various attach- 

 ment structures and a glutinous material that 

 accompanies it when it is extruded. The coxae of 

 the fourth and fifth ^sereopods of the female are 

 produced into platelike projections bearing long 

 bristles directed medially which press the sperma- 

 tophore against the thelycum and also help it to 

 adhere to the thelycum. Despite the various devices 

 that help to hold the spermatophore anchored, it 

 seems to be rather easily dislodged because few 

 females are found with spermatophores. The 

 spermatozoa have generally been thought to be 

 released from the spermatophore simultaneously 

 with the expulsion of the ova. Harry L. Cook (per- 



480 



U.S. FISH AND WILDLIFE SERVICE 



