Life-History Trade-Offs in the Pearl Oyster Pinctada radiata: Growth Limitation and Reproductive Persistence in a Hypersaline Semi-Enclosed Sea

Hypersaline semi-enclosed seas impose physiological constraints on marine bivalves by increasing osmotic regulation costs and limiting somatic growth. This study examined life-history variation in natural populations of the pearl oyster Pinctada radiata across contrasting salinity regimes along the eastern and western coasts of Bahrain in the Arabian Gulf. Environmental conditions, shell morphometrics, growth modeling, oocyte abundance, physiological condition indicators, and the production-to-biomass ratio (P:B) were assessed to determine whether high salinity constrains somatic growth while reproductive persistence is maintained. Monthly surveys were conducted from June 2021 to May 2022 at one pearl oyster bed on each coast, comprising 12 surveys per coast. The two coasts formed a salinity gradient, averaging approximately 42‰ in the east and 55‰ in the west. Oysters from the western coast were smaller, with a mean shell height decreasing from 63.12 ± 6.47 mm in the east to 45.88 ± 3.41 mm in the west. The Gompertz model indicated a lower asymptotic shell height in the more saline population. The population-level P:B ratio was 0.23 yr−1 on the eastern coast and 7.46 yr−1 on the western coast. Despite reduced shell size, western oysters maintained strong seasonal oocyte abundance. The higher biomass turnover in the west partly reflected greater oyster density. This pattern is consistent with osmoregulatory costs and constrained shell biomineralization under chronic hypersalinity. Reproductive activity followed a bimodal pattern on both coasts. These findings are consistent with a life-history trade-off in which pearl oysters exposed to chronic salinity stress maintain reproductive activity despite constrained growth. Overall, this study links environmental forcing, growth limitation, population-level biomass turnover, and reproductive activity, providing field-based evidence that chronic hypersalinity constrains somatic growth while reproductive persistence is maintained in a benthic bivalve inhabiting hypersaline coastal systems under climate-related stress.