brown shrimp. Of about 47,000 juvenile shriinj) 

 examined from Galveston Bay bait landings between 

 January 1900 and December 19(13, only 17 (less than 

 0.04 percent) were pink shrimp. In earlier work, 

 the second author (1958-59) found no pink shrimp 

 among more than 10,000 juvenile penaeid shrimj) 

 taken from upper Galveston liay. Although post- 

 larval pink shrimp obviously occur in the Galveston 

 area they evidently are scarce; all postlarvae we 

 caught were classified as brown or white shrimp. 



MORPHOLOGY 



No single criterion is sufficient to distinguish 

 brown and white shrimp postlarvae, but they can be 

 separated by taking into account various morpho- 

 mctric characters, relative size, and seasonal occur- 

 rence as juveniles in the estuary. Mori)hological 

 and morphometric differences between postlarval 

 brown and white shrimp provided by Pearson (1939) 

 and Williams (1959) are sufficient to separate these 

 species during most seasons. Williams, working 

 with shrimp from North Carolina, developed a pro- 

 visional key to early postlarvae. He stated that the 

 tip of the rostrum and the extended third pereiopod 

 on postlarval white shrimp do not e.xtend to the dis- 

 tal edge of the eye. Conversely, in the brown 

 shrimp, both the tip of the rostrum and extended 

 third pereiopod reach to or beyond the edge of the 

 eye. In postlarvae from the Galveston area, these 

 characteristics suffice only to separate postlarval 

 white and brown shrimp with a total length of 10 

 mm. or less, whereas Williams was able to use them 

 in North Carolina for separating postlarvae up to 

 12 mm. total length. 



OCCURRENCE ON GALVESTON BAY 

 NURSERY GROUNDS 



According to our records, brown shrimp are the 

 only postlarval Pcnacus that enter Galveston Bay 

 during the first 4 months of the year. This observa- 

 tion agrees with findings from several previous 

 studies conducted in the bay. Rcnfro (1959) found 

 only brown shrimp postlarvae and juveniles (17 mm. 

 and above) in upper Galveston Bay during April and 

 May 1959. Guntcr (1960) also found brown shrimp 

 to be the only species at the juvenile stage present in 

 Galveston Bay during April and May ]9(>0. I.ater 

 reports by biologists of the Texas Game and Fish 

 Commission corroborate the observations of Renfro 

 and Gunter (Pullen, 1962). 



By June, advanced postlar\al and early juvenile 



white shrimp (18-28 mm.) become abundant in 

 Galveston Baj% and both brown and white shrimp 

 are present throughout the summer (Gunter, 1960). 

 Additional evidence regarding the identity of the 

 winter and early spring postlarvae was provided in 

 1960 when 1,200 postlarvae, taken on April 12 at 

 Galveston Entrance, were brought into the labora- 

 tory to be reared. Ail that grew to identifiable size 

 (150) were brown shrimp. 



SEASONAL SIZE DLSTRIBUTION 



The size of postlarvae caught at the entrance to 

 Galveston Bay provides a strong clue to species 

 identity during .some seasons (fig. 3). During the 

 winter, the total length of brown shrimp postlarvae 

 ranged from 10 to 14 mm. and averaged 12 mm. 

 (fig. 3). lieginning in May of each year, a second 

 group of much smaller (6.0 to 8.0 mm.) postlarvae 

 appeared in the samples. These shrimp possessed 

 the external morphological characteristics of post- 

 larval white shrimp described by Pearson (1939) and 

 Williams (1959). By late June the length distribu- 

 tions of the two groups of postlarvae began to over- 

 lap. The modes of the length distribution of brown 

 postlarvae decreased, possibly because adult brown 

 shrimp were spawning near shore in spring and sum- 

 mer, or because warm water temperatures increased 

 the developmental rates of larvae. During the same 

 period, some white shrimp postlarvae as long as 10.5 

 mm. entered the estuary. Most of the larger post- 

 larvae, however, exhibited the characteristics as- 

 cribed to brown shrimj) by Williams (1959). The 

 overlap in length distributions persisted throughout 

 the summer, but the mean length of brown shrimp 

 postlarvae always exceeded that of white shrimp in 

 the same samples (fig. 3). In the latter part of each 

 year, the modal length of brown shrimp postlarvae 

 increased, and b}- October in some years the overlap 

 in length distributions had ended. 



Postlarvae of brown and white shrimj) caught at 

 beach stations and at Galveston I^nt ranee were of 

 similar sizes (table 2). The total length of po.st- 

 larval brown shrimp ranged from 8.5 to 12.0 mm. 

 (mean, 11.5 mm.). White shrimp ranged from 5.0 

 to 9.5 mm. (mean, 7.0 mm.). No significant differ- 

 ence existed among the mean lengths of postlarvae 

 taken at the various beach stations on the same day. 



SUMMARY 



Collections of penaeid postlarvae were obtained 

 semiweeklv at Galveston I^ntrance over a 4-year 



152 



U.S. FISH AND WILDLIFE SERVICE 



