The effect of low salinity on survival and growth of the Pacific white shrimp Litopenaeus vannamei was examined in the laboratory due to the interest of raising shrimp inland at low salinities. In three separate experiments, individual L. vannamei postlarvae (∼ 0.1 g) were cultured at salinities of either 0.5, 1, 1.5, 2, or 3 ppt (N= 5 or 10/treatment) for 18 to 40 d at 30 C in individual 360‐mL containers. In each experiment controls of 0 and 30 ppt were run. There was no postlarval survival at salinities < 2 ppt. Survival was significantly different (P < 0.01) at 2 ppt (20%) compared to 30 ppt (80%). Growth was also significantly different (P < 0.01) at 2 and 3 ppt compared to 30 ppt (416%, 475%, and 670%, respectively). A fourth experiment compared juveniles (∼ 8 g) and postlarvae (∼ 0.05 and 0.35 g). Shrimp were cultured at salinities of 0, 2, 4, and 30 ppt for 40 d at 25 C, in individual 360‐mL and 6‐L containers (N= 7/treatment). There was no postlarval survival at < 2 ppt. Postlarval survival at 4 ppt (86%) was not significantly different (P > 0.05) from 30 ppt (100%). Juveniles exhibited better survival at lower salinities (100% at 2 ppt) than 0.05 and 0.35 g postlarvae (29% and 14% respectively, at 2 ppt). The effects of salinity on growth varied with sizdage. Final growth of 0.05 g postlarvae at 2 ppt (693%) was significantly less (P < 0.01) than at 4 ppt (1085%) and 30 ppt (1064%). Growth of 0.35 g postlarvae was significantly less (P < 0.01) for 4 ppt (175%) than for 30 ppt (264%). There was no growth data for juveniles (8 g). It appears from these experiments that the culture of L. vannamei poses risks when performed in salinities less than 2 ppt.

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We examined the role of local adaptation in structuring the stable genetic step-cline of the eastern oyster Crassostrea virginica along an environmental gradient in the lagoon system of eastern Florida, USA. Reciprocally transplanted progeny, produced by a 10 × 10 genetic cross of wild brood stock from northern and southern genetic lineages yielded significant evidence of local adaptation (interaction of genes and environment) in variables related to fitness, including survival, wet meat weight, and reproductive maturation. The strength of local adaptation was asymmetric, with greater effects on the northern compared to the southern genetic lineage. To a lesser extent, we found evidence of both the role of environment (in particular, adverse effects on both genetic crosses in the southern region), and the role of genetic differences between the 2 crosses independent of environment, with higher initial growth of the southern genetic lineage and higher condition of the northern lineage. These differences suggest that maintenance of the genetic step-cline involves natural selection. We discuss the potential role of temperature and phytoplankton community composition between the northern and southern regions. Our study is the first to determine the genetic basis for fitness-related phenotypes, and to relate this to local adaptation of the eastern oyster. Understanding the role of the environment in structuring the eastern oyster throughout its range is critical for effective management, and the results of this study also suggest that small environmental changes may have significant effects on conservation of the eastern oyster, particularly in the northern genetic lineage.

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