Identification of Postlarval and Juvenile Shrimp 



Williana C. Renfro 



Postlarvae of the three species of Penaeus common in the Louisiana- 

 Texas area are difficult to separate when less than 20 mm. long. White shrimp, 

 P. setiferus, 20 mm. or more in total length, are easily identified and hence 

 distinguished from the others by the absence of grooves flanking the postrostral 

 ridge. On the other hand, separation of the "grooved" shrimps, P. aztecus 

 and P. duorarum, is not easily accomplished until they reach a size at which 

 their external genitalia are well developed. In an attempt to identify post- 

 larvae systematically collected inside Galveston Entrance and along the Gal- 

 veston Island beach front, specimens were brought to the laboratory for rear- 

 ing individually or en masse to sizes at which non-grooved (white) and grooved 

 species could be distinguished. 



Specimens reared individually were isolated in small covered dishes 

 and fed newly hatched brine shrimp nauplii. Although mortality was low, they 

 grew slowly, their monthly length increments averaging less than 9 mm. over 

 1^- to 2-|-month periods. The rearing containers were inspected daily and any 

 casts removed and preserved for study. In this manner a series of exoskele- 

 tons was obtained from shrimp during metamorphosis through postlarval to 

 separable juvenile sizes. Examinations of casts obtained from non-grooved 

 (white) and grooved shrimp as length increased from 12 to 17 mm. have not 

 yet revealed morphological characters sufficiently diagnostic to separate the 

 two groups within this size range. It was noted, however, that "grooved" 

 shrimp (postlarvae) as short as 15 mm. could be identified as such through 

 faint grooves alongside the postrostral ridge, which become readily discern- 

 ible when the partially dried cast of the cephalothorax is viewed under strong 

 incident light. 



Groups of postlarvae reared en masse in aquaria of up to 300 gallons 

 capacity were fed fine pieces of shrimp or fish in addition to brine shrimp nau- 

 plii. Mortality due to cannibalism was high, and the highest rate of growth 

 within any group was a rather low 13. 5 mm. per month. Weekly samples of 

 shrimp were measured and preserved for morphological studies. 



The results of six rearing trials showed that postlarvae of grooved 

 shrimp, most likely the brown shrimp, P. aztecus, were present in Galveston 

 Entrance from April through mid-December. Only one group of postlarvae sub- 

 sequently proved to be white shrimp, these having been collected in late Novem- 

 ber I960. 



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