Biogeographic Gradients

Host-Parasite Biogeography

Pea crabs at various stages of development

Outplanted oyster spat

Parasite prevalence can vary greatly across space, and the drivers of this variability are not always well understood. Spatial patterns in parasite prevalence can be revealed through observational studies, but to understand the mechanisms that underlie these patterns, standardized and controlled comparisons are needed. A serendipitous result from our collaborative, large-scale experiment on oyster reef trophic ecology allowed us to examine spatial variability in infection of the eastern oyster (Crassostrea virginica) by its pea crab parasite (Zaops ostreus) across 700 km of the southeastern U.S. coastline. Oyster pea crabs sit on the gills of oysters and steal food from their hosts.

Juvenile oysters from a homogeneous source stock (minimizing the influence of host genetics on infection patterns) were outplanted and raised in the field for 3 months at multiple sites with similar environmental characteristics. We found that prevalence of pea crab infection varied between 24 and 73% across sites, but not systematically across latitude. Of all measured environmental variables, oyster recruitment correlated most strongly (and positively) with pea crab infection, explaining 92% of the variability in infection across sites. These data suggest that regional processes driving variation in oyster recruitment similarly affect the recruitment of one of its common parasites.

Byers, J. E., T. L. Rogers, J. H. Grabowski, A. R. Hughes, M. F. Piehler, D. L. Kimbro. 2014. Host and parasite recruitment correlated at a regional scale. Oecologia 174:731–738.

Coastal Geomorpholgy Gradient Causes Biogeographic Variance in Oyster Reef Morphology

The physical factors that structure aquatic communties can vary widely across large, biogeographic scales. With collaborators at the University of North Carolina, the University of Georgia, and Northeastern University, we investigated linkages between physical factors and oyster reef characteristics across the U.S. South Atlantic Bight. The eastern oyster (Crassostrea virginica) is an important ecosystem engineer that creates intertidal reefs, filters water, promotes denitrification, stabilizes shorelines, and provides habitat throughout estuaries in the southeastern U.S.

We quantified physical variables (temperature, salinity, duration and depth of water inundation), oyster reef properties (slope, vertical relief), and oyster recruitment, density, and biomass at 10 sites from Florida to North Carolina (1500 km range) for one year. Many oyster and oyster reef traits showed unimodal patterns with latitude, with the highest abundances of oysters occurring in Georgia and South Carolina estuaries. Of the physical variables, salinity and duration of water inundation were similar across all sites, and temperature declined linearly with increasing latitude, except during summer when it had no relationship with latitude. Depth of water inundation over reefs (tidal magnitude) was the only physical variable with a prominent unimodal distribution that may explain the oyster’s peak abundance at mid-latitudes. Higher water depths at the mid-latitude sites indicate that these sites experience higher flow velocity, energy, and net water volume delivery per unit time, resulting in higher accumulation of oyster biomass and heightened reef structure. These results reveal how physical forces (specifically gradients in tidal energy) affect the biology of the eastern oyster, including its reef properties, with potential implications for community structure and ecosystem service delivery at a biogeographic scale.

Byers, J.E., J. H. Grabowski, M. F. Piehler, A. R. Hughes, H. W. Weiskel, J. C. Malek, D. L. Kimbro. 2015 Geographic variation in intertidal oyster reef properties and the influence of tidal prism. Limnology and Oceanography 60 (3), 1051-1063.