De la Rosa-Velez et al,; Genetic structure of Penaeus californiensis and P sty/irostns 



675 



when a resource is desired to be characterized genetically: 

 1) What amount of genetic variation is present across its 

 populations? 2) Is the variation homogeneously distrib- 

 uted? This line of research has impelled a number of stud- 

 ies on populations of shrimp species to ensure adequate 

 exploitation and optimal rearing (for a review see Rodri- 

 guez-Romero and Rosa-Velez, 1993). 



Our study aimed to characterize the genetic variability 

 and structure of two wild populations of shrimp species, to 

 recognize the actual genetic pool currently segi'egating in 

 them, and to render information to design rearing strate- 

 gies based on existing genetic variability. 



Penaeus (Farfantepenaeus) ealifornicusifi Holmes is an 

 oceanic species distributed in the eastern Pacific, San 

 Francisco Bay, U.S.A., to Sachura Bay, Peru and Galapa- 

 gos Islands, Ecuador (Rodriguez de la Cruz, 1976). Adults 

 are found up to 220 m deep, although the peak of abun- 

 dance occurs at 55 m on silt-clay or sand-silt bottoms 

 (Rodriguez de la Cruz and Resales, 1970). Penaeus (Lito- 

 penaeus) stylirostris Stimpson is a more coastal species 

 distributed from the upper Gulf of California, Mexico, to 

 Tumbes. Peru. Adults are found in shallow waters around 

 the mouth of coastal lagoons, up to 40 m deep (CICTUS, 

 1985), where they are more abundant. 



Life cycles are similar for both species but there is one 

 very distinctive difference: P. californiensis can complete 

 its whole life cycle in the marine environment, whereas 

 P. stylirostiis must spend part of its life cycle (postlarval 

 and juvenile stages) as an inhabitant of coastal lagoons. 

 In brief for both species, females deposit eggs in demer- 

 sal zones where they undergo total segmentation in 12-15 

 hours. The outcome is a planktonic lai-va that metamor- 

 phoses through five naupliar, three protozoean, and three 

 mysis stages, before reaching the semibenthic postlarval 

 stage. Once the rostral form is accomplished, the animal 

 is considered a juvenile and is totally benthic. The com- 

 plete cycle may takel2-17 days (Rodriguez de la Cruz and 

 Resales, 1970). 



In our study we ascertained different levels of genetic 

 variation between species, a clinelike pattern of genetic 

 variability in the more coastal species, and significant 

 genetic structure among subpopulations of both species. 



Materials and methods 



Samples o^Penaeus califor-niensis were obtained in Novem- 

 ber 1995 by means of bottom trawling nets operated 

 from commercial shrimp fishing vessels performing stan- 

 dard catching efforts (average trawling time: 1 hour; trawl- 

 ing speed: 2 knots), in the following locations: south of 

 Santa Clara Port (31°34'N, 114°19'W); west of Guaymas 

 Port (27°50'N, 111°05'W); and southwest of Mazatlan Port 

 (22°27'N, 106°44'W). Samples ofP stylirostris were caught 

 in September 1995 with a cast net thrown from small 

 boats in the following shallow coastal areas: off Santa 

 Clara Port (31°44'N, 114°19"W); off Guaymas Port (27°54'N. 

 110°55'W); and offMazatlan Port, (23°12'N,106°30'W). The 

 samples covered three out of four distinctive areas (called 

 here "upper Gulf middle Gulf lower Gulf and mouth of 



Figure 1 



Map of the Gulf of California showing its subdivision (after 

 Round, 1967): 1 = upper Gulf; 2 = middle Gulf; 3 = lower Gulf; and 

 4 = mouth of the Gulf; and the sampling locations: ^ = Penaeus 

 stylirostris, and • = Penaeus californiensis. 



the Gulf") according to Round ( 1967; Fig. 1 ). Samples were 

 frozen (-20°C) and shipped to the laboratory, where they 

 were stored at -75°C until dissection. 



Soft tissues from head and abdominal muscle from adult 

 specimens were separated and homogenized in 1 to 2 vol- 

 umes of cold buffer prepared with 0.1 M Tris-HCl, pH 7.00 

 + NAD+ + NADP+ + PVP (10:1:1:100; v:p:p:p). Homoge- 

 nates were spun ( 12,000 x g) at 4°C, for 20 min. Superna- 

 tants were stored at -75°C until needed. 



Electrophoretic assays were performed on 12.5"f starch 

 gels (Sigma Chemical Co.. St. Louis, MO) in a horizontal 

 apparatus that was placed in the refrigerator (4°C). Three 



