The primary aquaculture regions in Maryland are situated in the upper and middle Chesapeake Bay, characterized by low-salinity waters with an average annual salinity of around 12 ppt. This environmental factor has led to a monoculture in Maryland’s aquaculture - the eastern oyster, as most other popular seafood species require higher salinity levels. While oysters can survive in low-salinity conditions, their growth rate is significantly reduced compared to those in higher-salinity environments. Notably, during years of high rainfall, acute low salinity events have caused substantial mortality in both wild and farmed oysters. Consequently, enhancing growth and survival rates of oysters in low-salinity waters has been a key breeding goal in Maryland.
The Patuxent Environmental and Aquatic Research Laboratory initiated an oyster genetic breeding program in 2019 , in response to significant losses in Maryland’s oyster aquaculture due to two consecutive years of abnormally low salinity. This program consists of two objectives, one is to develop superior oyster lines, including diploids, triploids, and tetraploids, better adapted to Maryland’s low-salinity waters, with improved survival and growth rates. Given their growth advantage , triploid oysters have been widely adopted by Maryland growers to mitigate the slow growth caused by low salinity. Enhancing the low-salinity tolerance of diploids and tetraploids that are fundamental in producing triploid seeds for commercial use, is anticipated to improve overall performance of the commercial triploid seed . The other objective is to develop genomic selection (GS) or marker-assisted selection (MAS) method, which expected to facilitate rapid genetic enhancements in wild populations, significantly shortening the breeding cycle.
For the first objective, three low-salinity diploid lines were developed from 2020 to 2023 . All t he parents of these lines were survivors from laboratory acute low-salinity challenge experiments (2 - 3 ppt), among which one line’s parents also incorporated a disease-resistant line . Three triploid lines were produced from Maryland wild stock using chemical induction in 2020, which will be used for a tetraploid creation by crossing with the established low-salinity diploids. The selected low-salinity diploid lines exhibited higher survival rates compared to both wild oysters and triploids in laboratory tests. In a three-year field test, one of the three triploid lines demonstrated accelerated growth compared to the control triploids (from industry) and the other two triploid lines. For the second objective, a multi-trait genomic-best-linear-unbiased-prediction model was identified for shell height, and a Bayes B model was established for survival, both selected for their higher accuracy in cross-validation compared to other models . A genome-wide association analysis identified 30 significant SNPs across the genome for the survival trait, suggesting the potential for marker-assisted selection in this trait. Future efforts will focus on tetraploid production, continuous genetic improvement, and validation of GS models or genetic markers.