Location Matters: Passive and Active Factors Affect the Vertical Distribution of Olympia Oyster (Ostrea lurida) Larvae

Abstract

Dispersal, retention, and population connectivity are impacted by current regime and the behaviors that drive larval distribution, so understanding both is key to informing restoration of native species like the Olympia oyster (Ostrea lurida) across its range in western North America. This study explores the relationships between several factors (temperature, [chl a], larval size, tidal stage, and estimated current speed) and Olympia oyster larval vertical distributions in Fidalgo Bay (48.4828, − 122.5811), a shallow, tidally flushed bay in the Salish Sea. Olympia oyster larvae collected from four depths over the tidal cycle from July 11–14, 2017, were ~ 20% deeper near slack tide and shallower during the faster parts of both ebb and flood, with a threshold for this transition around an estimated 25 cm s−1. This pattern does not suggest tidally timed migrations as has been shown in another population of Olympia oysters, nor can this pattern be totally explained by passive processes. Larvae did not cluster at depths with specific temperatures or [chl a] but there was a difference in larval size between surface and bottom waters, with older, larger larvae more common at the bottom. Fidalgo Bay does not exhibit two-way flow or strong vertical shear, so vertical distribution of larvae likely has little effect on transport in this system but might in other similarly shallow habitat areas with higher stratification that are target restoration sites in the Salish Sea. These results add to the growing number of studies that show location-specific differences in larval vertical distribution and behavior within taxa and underscore the importance of integrating local hydrodynamics into predictions of bivalve larval transport.

Temperature and salinity, not acidification, predict near-future larval growth and larval habitat suitability of Olympia oysters in the Salish Sea

“…Not all cups reached competence, so we used 260 µm as a standard beginning competence size for modeling because this was the size above which most individuals were competent and most cultures contained 25% or more competent larvae (Fig. 3b), and is consistent with size classifications from previous work in our lab 38 .…”

“…This level is also reasonably representative of food availability in the field. McIntyre et al 38 measured chlorophyll-a in Fidalgo Bay (one field example in this study) averaging 19 ± 8.1 µg/L 38 . Using the estimation of 100,000 cell/ml ~ 10 µg/L Chl-a 22 , even the lower end of these field chlorophyll-a measurements corroborate our choice of feeding level as well within reason for larvae to experience in the natural environment.…”

“…Though published estimates of competence size for Olympia oysters vary [72][73][74] , we chose 260 µm as the a beginning competence size as this was the size at which we began seeing frequent eye spots in our experimental cultures ( Fig. 3) and is consistent with previous stage classifications from our lab 38 . We did not use these data to predict larval habitat suitability of the sites, as the low temperature values put all points in 2014 outside of the larval habitat suitability logistic regression curve.…”

“…To examine larval tolerance to conditions in the natural environment, we focused on two particular state-operated restoration sites in the Salish Sea that we know contain breeding adults of the species: Fidalgo Bay and Liberty Bay, Washington 21 , 35 . We used environmental data in Fidalgo Bay summarized from McIntyre et al 38 and Cordoba and Arellano (unpublished data); both studies measured water quality variables throughout the water column in sampling efforts during mid-July of two separate summers. We obtained data from Liberty Bay from the Western Washington University SEA Discovery Center’s long-term monitoring project, which measured water quality parameters from surface to depth twice weekly from 2017–2019, and averaged values from July 5 to Aug 23 (estimated peak larval season) 44 during each year.…”

Unlocking the secret life of blue mussels: Exploring connectivity in the Skagerrak through biophysical modeling and population genomics

“…Invertebrate larvae can control their vertical movement and thereby which water masses they are transported in (Genin et al., 2005 ; Knights et al., 2006 ; Shanks & Brink, 2005 ). However, the extent to which larval behavior interacts with vertical mixing processes in weakly swimming invertebrates is location‐specific and still relatively unknown (McIntyre et al., 2021 ; Weinstock et al., 2018 ), limiting our understanding of the ability of larvae to avoid being swept offshore (meaning certain death). Better knowledge of the actual spread of larvae and connectivity between geographic areas is important for future strategic planning of restoration projects and the identification of areas worthy of protection, where areas which contribute strongly to the export or import of larvae from larger areas, will be of a higher importance.…”