Exploration of enigmatic pelagic larval oysters (Crassostrea virginica) fostering estuarine restoration of oyster fisheries

Dramatic declines in oyster reefs in estuarine ecosystems have drawn considerable attention to oyster restoration; however, most efforts so far have primarily targeted their benthic phase leaving the pelagic oyster larvae largely elusive. In this study, the density, distribution, and size-structure of pelagic larval oysters (Crassostrea virginica) were studied in a subtropical estuary (Galveston Bay, Texas, USA) during the oyster spawning season (May–October) in 2023 and 2024 to assess the impact of environmental conditions on larval dynamics and recruitment. Except for relatively similar thermal conditions, salinity and chlorophyll a (Chl-a) were significantly different between the two years with extremely low salinities observed in May and June of 2024. Larval density was significantly related to sites and months, with relatively lower densities observed at 9 of 11 sites and 4 of 6 months in 2024 compared to 2023. Size structures of pelagic larvae exhibited significant differences between the early-middle (May–August) and late (September–October) spawning periods, with abundant small-sized individuals (70–200 μm) gradually shifting to less abundant large-sized individuals (>250 μm). A breakdown of the life stages showed higher percentages of Stages 1&2 and lower percentages of Stage 5 over the two spawning seasons. Salinity was the most important environmental factor influencing the larval density followed by its interaction with water temperature, water temperature and Chl-a. Geostatistical co-kriging analysis disclosed relatively high larval density in the upper bay, East Bay, and West Bay in 2023, while in 2024 higher density was found along the western shoreline down to West Bay. Early-stage larvae appeared denser in the lower bay compared to the middle bay and upper bay in 2023, whereas abundant late-stage larvae emerged in the lower bay in 2023. In the northern inshore zone, denser early-stage larvae occurred in 2024, meanwhile late-stage larvae appeared relatively abundant, especially in the northwest and southwest regions. Finding areas with higher densities of early- and late-stage pelagic oyster larvae may help identify larval sources and sinks. These insights can better inform the strategic placement of oyster sanctuaries and enhance the effectiveness of restoration and management efforts for this economically and ecologically important marine living resource.