Larval shell chemistry of the Olympia oyster (Ostrea lurida) in Puget Sound, WA to assess population connectivity and restoration planning

Abstract

The Olympia oyster (Ostrea lurida) is the only native oyster species along the west coast of North America and is culturally and ecologically important. However, Olympia oyster populations have been severely depleted, prompting ongoing restoration efforts in Puget Sound, WA, and beyond. Understanding population connectivity is vital for successful restoration planning to ensure resilience and genetic diversity. This study examined the potential for using trace elemental "fingerprints" in Olympia oyster shells to track larval dispersal and connectivity at regional scales within Puget Sound. Brooded larvae were collected via non-lethal sampling at eight sites grouped into three geographic regions. Shell chemistry analysis showed the ability to distinguish these regions from each other with approximately 75% accuracy, demonstrating feasibility for addressing connectivity questions among sub-basins. Additionally, regional signatures were found to be temporally stable within one reproductive season, facilitating annual sampling regimes. Although settlers of unknown origin collected at two restoration sites could not yet be confidently assigned to specific source regions, nor could they be divided into groups in a cluster analysis, likely due to methodological constraints, this study provides a proof of concept and foundation for further developing this technique. With targeted improvements to analytical methods for microscopic larval shells, shell elemental fingerprinting shows promise to greatly inform ongoing restoration efforts by elucidating population connectivity patterns for this culturally and ecologically important native oyster species at ecologically relevant scales.

Fig 1. Map of Puget Sound, WA, USA, representing sites of existing Ostrea lurida populations that were sampled for brooded larvae.

Symbol color represents the regions that were defined post-hoc by their differing elemental signatures. Triangle icons mark the two sites that were sampled more frequently. Map made from Natural Earth. Free vector and raster map data @ naturalearthdata.com.

Fig 2. Canonical score plot of linear discriminant function analysis conducted on O. lurida brood shell signatures among regions.

NS - North Sound, Sequim - Sequim Bay, CS - Central Sound. Each region is represented by a different shade and symbol combination, and ellipses represent 95% confidence intervals.

Fig 3. NMDS ordination of elemental concentrations in brooded O. lurida larval shells compared for temporal variation between early (6/15/2015-7/5/2015) and late (7/5/2015-8/15/2015) reproductive season for two locations, Fidalgo Bay and Dyes Inlet.

Brooded larvae collected early in the season are depicted by triangles and those collected late in the season by plus signs.

Fig 4. Ostrea lurida settler elemental signature discriminant scores plotted in reference to linear discriminant analysis conducted on brood elemental signatures.

Of the 154 settlers analyzed, 85% (open symbols) were estimated to have a > 95% probability of originating from outside the characterized brood regions, with only 15% (closed symbols) deemed likely to be sourced to one of the characterized brood regions. Shapes represent the region of settler collection (FB - Fidalgo Bay, DI - Dyes Inlet). Confidence ellipses (95%) represent brood LDA geographic regions, and are identical to the ellipses in Fig 2.