Increasing Tolerance for Perkinsus marinus Among Natural Crassostrea virginica Populations

from Virginia Waters

Ryan B. Carnegie and Eugene M. Burreson

Virginia Institute of Marine Science

Perkinsus marinus (“Dermo”)

• Protistan parasite, directly transmissible among oysters in a population

• Enzootic from Maine to Texas

• Present in Chesapeake Bay since at least the 1940s

• Activity intensified in 1980s during droughts

• Currently the most destructive pathogen of Crassostrea virginica in mid-Atlantic waters

Perception

• Resistance to dermo disease is documented in selected aquaculture lines (Ragone Calvo et al. 2003), but not in nature in the mid-Atlantic

• Bay populations sustained by reproductive contributions of

Susceptible oysters living in low salinity refuges

Young, doomed but pre-P. marinus individuals in enzootic waters (larger, older oysters dermo-ridden, effectively non-reproductive)

“Naturally occurring wild oysters which are resistant to the disease are not available in Chesapeake Bay . . .”

J.D. Andrews, 1984

2005

Perception

• Resistance to dermo disease is documented in selected aquaculture lines (Ragone Calvo et al. 2003), but not in nature in the mid-Atlantic

• Bay populations sustained by reproductive contributions of

Susceptible oysters living in low salinity refuges

Young, doomed but pre-P. marinus individuals in enzootic waters (larger, older oysters dermo-ridden, effectively non-reproductive)

“Naturally occurring wild oysters which are resistant to the disease are not available in Chesapeake Bay . . .”

J.D. Andrews, 1984

2005

The perception that natural oysters are hopelessly susceptible to dermo disease (they “just die”) underlies regional oyster management strategies today . . .

Reality?

Have natural oyster populations in P. marinus-enzootic waters not begun to “catch up with” hatchery-selected strains, especially following the dermo-intense 1999-2002 years?

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Peak-season York-Mobjack samples, 2001-2005

Observations I: Shell Bar Reef, Great Wicomico River

• September 2005: analysis of P. marinus in samples (each n = 25) of deployed DEBYs and naturally recruited C. virginica

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Observations II: Shell Bar Reef, Great Wicomico River

• June-September 2006: biweekly analysis of P. marinus in samples (each n = 25) of deployed DEBYs and naturally recruited C. virginica

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Observations III: York River

• Four oyster groups each deployed in triplicate to bags (~ 200/bag) on VIMS beach between 20 April-2 May 2006

DEBYs (resistant; 64.6 ± 5.4 mm (mean ± SD)) Ross Rock native (susceptible; 55.2 ± 4.5 mm) Wreck Shoal native (48.8 ± 8.0 mm) Aberdeen Rock native (56.9 ± 5.3 mm)

Observations III: York River

• Monthly mortality estimates, sampling for histology & RFTM

Observations III: York River

• Weighted prevalence of P. marinus higher in naïve Ross Rock oysters than in domesticated DEBYs

Observations III: York River

• Weighted prevalence of P. marinus higher in naïve Ross Rock oysters than in domesticated DEBYs

• P. marinus profile of native Aberdeen Rock, Wreck Shoal oysters more similar to selected strain (DEBYs) than to naïve Ross Rocks

Observations III: York River

• Cumulative mortality higher in Ross Rocks -- approaching 100% by September -- than in DEBYs (63% in October)

Mortality, York River, 2006

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Observations III: York River

Mortality, York River, 2006

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• Cumulative mortality higher in Ross Rocks -- approaching 100% by September -- than in DEBYs (63% in October)

• Cumulative mortality in Aberdeen Rocks (58% by October) similar to DEBYs; Wreck Shoals slightly higher (72%; MSX disease?)

Disease performance of natural stocks is similar to selected strains

How Might Dermo Tolerance Arise in Chesapeake Bay?

• Reliance of populations on reproduction of oysters in low-salinity refuges, and by young pre-dermo individuals in enzootic waters, should tend to maintain a high inherent susceptibility

How Might Dermo Tolerance Arise in Chesapeake Bay?

• Reliance of populations on reproduction of oysters in low-salinity refuges, and by young pre-dermo individuals in enzootic waters, should tend to maintain a high inherent susceptibility

• Size-specific dermo disease impacts may hold a key P. marinus parasitism is more intense in older individuals . . . Right?

How Might Dermo Tolerance Arise in Chesapeake Bay?

• Reliance of populations on reproduction of oysters in low-salinity refuges, and by young pre-dermo individuals in enzootic waters, should tend to maintain a high inherent susceptibility

• Size-specific dermo disease impacts may hold a key P. marinus parasitism is more intense in older individuals . . . Right?

How Might Dermo Tolerance Arise in Chesapeake Bay?

• Reliance of populations on reproduction of oysters in low-salinity refuges, and by young pre-dermo individuals in enzootic waters, should tend to maintain a high inherent susceptibility

• Size-specific dermo disease impacts may hold a key P. marinus parasitism is more intense in older individuals . . . Right?

A Closer Look: Lynnhaven River, October 2006

• Oysters collected from dermo-intense Pleasure House Creek, assigned to four size “bins” for evaluation of size-specific P. marinus parasitism: Small (45.7 ± 8.3 mm) Submarket (66.7 ± 9.7 mm) Market (90.7 ± 8.6 mm) Large (109.8 ± 11.5 mm)

• Processed for histology, RFTM

A Closer Look: Lynnhaven River, October 2006

• P. marinus weighted prevalence highest in the “submarkets”, lowest in the “small” and “large” groups

• Heaviest infections restricted to intermediate size classes--the most significant source of parasite cells

Many larger (older) oysters were only lightly infected--presumably healthy and fecund

Contribution of Older Oysters

• Larger, older and still healthy and very fecund oysters do exist in natural populations from dermo-enzootic Chesapeake Bay waters

Contribution of Older Oysters

• Larger, older and still healthy and very fecund oysters do exist in natural populations from dermo-enzootic Chesapeake Bay waters

• It is possible that these oysters have survived repeated dermo disease challenges by virtue of some heritable resistance or tolerance -- whatever its nature

Contribution of Older Oysters

• Larger, older and still healthy and very fecund oysters do exist in natural populations from dermo-enzootic Chesapeake Bay waters

• It is possible that these oysters have survived repeated dermo disease challenges by virtue of some heritable resistance or tolerance -- whatever its nature

• A large reproductive contribution by these very fecund oysters may drive the evolution of dermo tolerance in natural populations

Contribution of Older Oysters

• Larger, older and still healthy and very fecund oysters do exist in natural populations from dermo-enzootic Chesapeake Bay waters

• It is possible that these oysters have survived repeated dermo disease challenges by virtue of some heritable resistance or tolerance -- whatever its nature

• A large reproductive contribution by these very fecund oysters may drive the evolution of dermo tolerance in natural populations

• Still working against the evolution of this tolerance may be the pre-dermo contribution of numerous small but susceptible and doomed individuals

Contribution of Older Oysters

• Larger, older and still healthy and very fecund oysters do exist in natural populations from dermo-enzootic Chesapeake Bay waters

• It is possible that these oysters have survived repeated dermo disease challenges by virtue of some heritable resistance or tolerance -- whatever its nature

• A large reproductive contribution by these very fecund oysters may drive the evolution of dermo tolerance in natural populations

• Still working against the evolution of this tolerance may be the pre-dermo contribution of numerous small but susceptible and doomed individuals

Is this scenario valid? Does it also characterize the more recruitment-strong environments and oyster populations of

the Southeast and Gulf?

Conclusions

Data point to evolution of dermo tolerance in Chesapeake Bay oyster populations, and a possible mechanism

Suggest that the assumption that individuals in natural oyster populations have no value as broodstock -- which underlies “genetic rehabilitation”/oyster eugenics models (Allen et al.

2003), is not valid

Preservation of some natural stocks in sanctuaries, rather than complete elimination through intensive harvest with

establishment instead of reefs of domesticated broodstocks, may have merit as a component of regional restoration

strategies

Acknowledgments

• Rita Crockett and Susan Denny (VIMS Shellfish Pathology Laboratory)

• Melissa Southworth

• P.G. Ross

• Brian Barnes

• Jessica Moss

• Paul Oliver

• Jim Wesson, VMRC