Co-infection is linked to infection prevalence and intensity in oysters amidst high environmental and spatial variation.

Co-infecting parasites modify infection outcomes in the wild. However, it is unclear how multiple environmental factors influence co-infection. The Chesapeake Bay metapopulation of the eastern oyster, Crassostrea virginica, provides an opportunity to test the importance of co-infection across heterogeneous environments because multiple parasites infect oysters across a broad salinity gradient. This study leverages Maryland and Virginia oyster monitoring for a large-scale survey of four co-infecting organisms, including two tissue parasites and two shell bio-eroding parasites. We diagnosed infection in 440 oysters across 16 paired harvested and unharvested reefs and tested the importance of co-infecting organisms for each parasite relative to environmental conditions, host traits, and marine spatial management. Microscopic visual methods were used to diagnose prevalence and intensity of tissue infections with Perkinsus marinus (the causative agent of dermo disease) and Haplosporidium nelsoni (the causative agent of MSX disease). Macroscopic visual methods were used to diagnose prevalence and intensity of shell infections with Cliona boring sponges and blister-inducing Polydora worms. For the three oyster parasites that were detected [H. nelsoni infections were absent in all oysters], salinity was the overall strongest predictor, corresponding to bay-wide patterns of parasite prevalence and/or intensity. Despite high environmental and spatial variation, co-infections corresponded to altered prevalence and/or intensity for all three oyster parasites. The correlational patterns suggest that P. marinus acts as a lynchpin in co-infection, as its intensity increased with Cliona sponge prevalence and P. marinus co-infection predicted higher Polydora blister intensity. Oyster shell height, reef habitat, and harvest status also predicted parasite prevalence and intensity, further reflecting the multivariate drivers of infections in this system. Unharvested reefs had greater vertical habitat structure and higher intensities of Cliona sponge infections, but no differences in the prevalence of any of the three parasites. Spatial patterns unexpectedly show that reef-level predictors of parasite patterns were more important than differences between tributaries. This correlational survey provides novel insights through the statistical relationships between the three oyster parasites, environmental conditions, host traits, and human resource management. New and more detailed scenarios are needed to expand disease ecological theory to encompass co-infection in anthropogenically impacted wildlife populations.

Diseases of oysters Crassostrea ariakensis and C. virginica reared in ambient waters from the Choptank River, Maryland and the Indian River Lagoon, Florida.

To assess potential benefits and liabilities from a proposed introduction of Asian Suminoe oysters, susceptibilities of exotic Crassostrea ariakensis and native C. virginica oysters were compared during exposures to pathogens endemic in temperate, mesohaline waters of Chesapeake Bay and sub-tropical, polyhaline Atlantic waters of southern Florida, USA. Cohorts of diploid, sibling oysters of both species were periodically tested for diseases while reared in mesocosms receiving ambient waters from the Choptank River, Maryland (>3 yr) or the Indian River Lagoon, Florida (10 to 11 mo). Haplosporidium sp. infections (e.g. MSX disease) were not detected in oysters from either site. Perkinsus sp. infections (dermo disease) occurred among members of both oyster species at both sites, but infections were generally of low or moderate intensities. A Bonamia sp. was detected by PCR of DNAs from tissues of both oyster species following exposure to Florida waters, with maximum PCR prevalences of 44 and 15% among C. ariakensis and C. virginica oysters respectively during June 2007. Among C. ariakensis oysters sampled during April to July 2007, a Bonamia sp. was detected in 31% of oysters by PCR (range 11 to 35%) and confirmed histologically in 10% (range 0 to 15%). Among simultaneously sampled C. virginica oysters, a Bonamia sp. was detected in 7% by PCR (range 0 to 15%), but histological lesions were absent. Although this is the first report of a Bonamia sp. from Florida waters, sequences of small subunit (SSU) rDNA and in situ hybridization (ISH) assays both identified the Florida pathogen as Bonamia exitiosa, which also infects oysters in the proximate waters of North Carolina, USA.

Two epizootic diseases in Chesapeake Bay commercial clams, Mya arenaria and Tagelus plebeius.

Declining Chesapeake Bay harvests of softshell clams, together with historical and emerging reports of epizootic diseases in Mya arenaria, prompted a survey in summer 2000 of the health status of selected commercial clam populations. All sampled populations (8 M arenaria softshell clam, 2 Tagelus plebeius razor clam) were infected by Perkinsus sp. protozoans at prevalences ranging from 30 to 100% of sampled clams. Nucleotide sequences for the internal transcribed spacer (ITS) region of the rRNA gene complex were determined for clonal in vitro Perkinsus sp. isolates propagated from both M. arenaria and T plebeius. Multiple polymorphic sequences were amplified from each isolate, but phylogenetic analysis placed all sequences into 2 clades of a monophyletic group, which included both recently described clam parasites P. chesapeaki and P. andrewsi. Sequences amplified from each clonal isolate were found in both sister clades, one containing P. andrewsi and the other P. chesapeaki. Most (7 of 8) M. arenaria samples were also affected with disseminated neoplasia (DN), at prevalences of 3 to 37%, but neither T. plebeius sample showed DN disease. Disease mortalities projected for sampled clam populations, especially those affected by both diseases, may further deplete subtidal commercial clam populations in mesohaline portions of Chesapeake Bay.