Climate change futures: health, ecological and economic dimensions

• Terminating practices that destroy or extract reef
frame for ornamental aquarium trade, construction
materials, navigational clearance, and for other
commercial applications (for example, coral calcium
health supplements).
• Develop and expand regulated, sustainable and
environmentally friendly ecotourist activities in tropical
settings.
Together, these local measures can make restoration
and recovery of residual reefs possible. But for reefs,
even more so than other ecosystems, their survival is
intimately tied to stabilization of the global climate.
CASE 2. MARINE SHELLFISH
Eileen Hofmann
Figure 2.33 Oysters
Oysters feed by filtering gallons of water each day to extract nutrients
and plankton. This filtering activity helps keep bays clear and clean.
Image: Lyn Boxter/Dreamstime
BACKGROUND
The Eastern oyster (Crassostrea virginica) has been a
major component of the biology and ecology of
Chesapeake Bay for the past 10,000 years (Mann
2000). This species supported a strong commercial
fishery for the first part of the twentieth century. During
the past four decades, Eastern oyster populations in
Chesapeake Bay have been greatly reduced by the
effects of two protozoan (one-cell animal) diseases:
Dermo, caused by Perkinsus marinus, and
Multinucleated Spore Unknown (MSX), caused by
Haplosporidium nelsoni. The addition of overfishing
and habitat deterioration has caused a long-term
decrease in Eastern oyster populations in Chesapeake
Bay. The result is that native oyster populations are
now only a small percentage of the populations that
existed until the middle of the last century.
Benthic (bottom-dwelling) filter feeders like oysters are
an important component of estuarine and coastal food
webs. Loss or reduction of these organisms can have a
large effect on ecosystem structure and function as well
as economic impacts, as many are commercially harvested
shellfish. The current condition of Chesapeake
Bay oysters illustrates the imports of losing any important,
keystone component of the marine food web.
Newell (1988) estimated that the pre-1870 oyster
stocks in the Maryland waters of Chesapeake Bay
would have been capable of removing 77% of the
1982 daily carbon production in waters less than nine
meters deep. Newell further concluded that oysters
were once abundant enough to have been the dominant
species filtering carbon from the water column in
Chesapeake Bay.
It should be noted that the Eastern oyster is only one of
many shellfish species that are being impacted by diseases
that are thought to be gaining in prevalence
and intensity due to a changing climate. For example,
Brown Ring Disease, caused by Vibrio tapetis, in
Manila clams (Ruditapes philippinarum) has had a significant
impact in Europe.
DERMO AND MSX
Dermo and MSX are parasitic diseases that affect oysters.
They render these bivalves uneatable, but the parasite
itself is not known to harm humans.
Dermo is an intracellular parasite (2 to 4 um) called
Perkinsus marinus, that infects the hemocytes of the
eastern oyster, Crassostrea virginica. Dermo is transmitted
from oyster to oyster. Natural infections are most
often caused by parasites released from the disintegration
of dead oysters. Waterborne stages of the parasite
may spread the disease over long distances.
Transmission may also occur by vectors such as scavengers
feeding on infected dead oysters or by parasitic
snails. Alternate molluscan hosts can serve as
important reservoirs for Dermo.
Since Dermo is considered a warm water pathogen
that proliferates most rapidly at temperatures above
25°C (77°F), ocean warming influences its activity and
range climate. In the northeast US, the disease may
become endemic as a result of a series of warm winters.
However, Dermo can also survive freezing. It is
suppressed by low salinities, less than 8 to 10 parts per
thousand, but the parasite proliferates rapidly when oysters
are transplanted into higher salinity waters (Ford
and Tripp 1996).
83 | NATURAL AND MANAGED SYSTEMS
CASE STUDIES