|Salinity; Gaping; Respiration; Mortality; Crassostrea virginica; Fischer’s pair wise test
|Oysters are ecological engineers that fabricate the infrastructure of
the estuarine habitat; their depletion creates a profound domino effect
on the ecosystem. They are considered a “keystone species” providing
many benefits to estuarine environments (SCDNR 2010). Oysters
stimulate diversity in an aquatic ecosystem by providing shelter and
habitat to other biota, creating spawning substrate for a myriad of fish
and concentrate prey species for high trophic fish. Further contributions
include stabilizing bottom sediment for benthic organisms and flora,
and manipulating a natural filtration system for clarification of the
aquatic habitat. Restoring sustainable oyster populations is about much
more than saving the marine mollusk themselves-it’s about ensuring a
healthy balance in the aquatic ecosystem [1-7]. In response, government
agencies are innovating strategies to restore and protect estuarine
habitats for relief on oyster and shellfish populations. National Oceanic
and Atmospheric Administration and the Army Corps of Engineers
lead a revitalized effort to recover native oyster reefs and establish selfsustaining
native oyster reefs sanctuaries in key tributaries by 2020 .
|A large quantity of oyster biomass can depress die-offs caused by
anthropogenically nourished eutrophication. Oysters are capable of
filtering large quantities of particulate carbon and algae that magnify
anoxic conditions . Oysters as filter feeders remove sediments and
algae from the water column to improve local water quality and clarity
while facilitating the removal of nitrogen and phosphorous from
eutrophic waters. Depressing eutrophication simultaneously decreases
acidification, while aiding in the maintenance of desirable dissolved
oxygen (DO) and pH levels [3,6,10]. Until recently, oysters have
overcome adversity through exquisite adaptations to their environment.
Oysters are a resilient species, equipped with physicochemical defense apparatus that provides a tolerance gradient against environmental
modifications. Environmental forces, both natural and anthropogenic
impinge upon the sensory organs and niche of individual oysters.
|Anthropogenic influences, including pollution by toxic industrial
chemicals, pesticide runoffs from agriculture practices, invasive fishing
methods, and habitat degradation induce environmental fluctuations
that exacerbate stress on oyster sustainability [2,3,11]. Specifically,
anthropogenic influences affect the salinity gradient of estuaries
by development and utilization of the shore line. An illustration of
anthropogenic influences altering estuarine salinity composition
is through the construction of dikes which produces hypo-saline
environments, irrigation creates hyper-saline conditions. Dredging
channels also increase transportation of salt water into the estuary .
|Over the past 200 hundred years, the increasing CO2 emissions
from fossil fuel combustion has led to an exponential increase in the net
amount of CO2 being dissolved into oceans . Excessive CO2 in the
aquatic system is dissolved to form CaCO3 (Figure 1). The conversion
of CO2 into carbonate allows acidification to prevail. Oysters are becoming smaller and less robust because they cannot compensate for
the amount of energy exerted to rid themselves of protons due to the
conversion of carbon dioxide into carbonate . Not only does this
slow down the calcification rate, moreover, carbonic acid is corrosive
to the oysters’ formation of a calcium carbonate shell; this corrosion is
more pronounced on juveniles and larva.
|Planktonic trochophore larva requires specific salinity
concentrations to develop into fully shelled veliger larva; however,
with less calcium in their shells, larva takes longer to mature.
Unable to metamorph into spat, larva remains planktonic for longer
durations, leaving them more vulnerable to predators . Indirectly,
acidification from carbon dioxide increase, affects shell formation
through physiological impacts, such as respiration rate, which can
impact energy budgets and thus alter the bivalves ability to produce
|Oysters have an open circulatory system that is continuously
exposed to physico-chemical modifications. Oysters’ haemocytes
are able to synthesize osmotic shock proteins therefore protecting
themselves from acute salinity variations . Despite this advanced physiology, rising sea temperatures may directly inhibit the oysters’
haemocyte functions involved in their defense mechanism. This defense
mechanism is critical for reducing oysters’ vulnerability to disease,
such as Dermo and MSX, which proliferate in warm temperatures and
high salinities . Osmoregulation maintains specific concentrations
of water and salt in the oysters’ body through retention and excretion.
This balance is imperative for the oysters’ cell membrane to function.
In a natural environment, oysters have an acute tolerance for a wide
salinity regime. For optimum health and reproduction, oysters require
a salinity range from 10 to 30ppt (FLDEP 2010).
|The gills of oysters, by means of cilia, pump a stream of water from
which food particles are filtered and passed along definite grooves to
the labial palps, which convey them to the mouth [16-19]. It appears
that salinity may be one of the important factors influencing the feeding
of oysters . Any significant change in salinity causes an immediate
slowing or cessation in the rate of pumping .
|The purpose of this study was to investigate an acute salinity gradient
of Crassostrea virginica and to reveal the effects of anthropogenic
activity on their salinity regime. This study observed the mortality and respiratory function (gaping response) of the gills as influenced by a
salinity gradient in C. virginica retrieved from the Yuna-Samana Bay
Estuary in Sanchez, Dominican Republic.
|Material and Methods
|The Yuna-Samana Bay Estuary situated in the North-Central region
of Dominican Republic and its many tributaries provide essential
resources for the local populace; water depended sectors include:
agriculture, fisheries, mining and tourism. The Yuna River (208 km
in length) is the largest river in Dominican Republic in terms of fresh
water output, carrying quantities (>10 to 100 m3/s) of fresh water year
round . The specimen for the study were taken from the downstream
region (from Pimentel to the Bay of Samana), characterized by low plain
and opened valleys with clayed soils (Figure 2). The nutrient rich water
outflow and ideal temperatures (between 25-32°C), pH (between 7.7-
8.5), and salinity (0-25%) Tobey  has supported sustainable oyster
population densities in the estuary. However, increased anthropogenic
activity such as deforestation of mangroves, increased biological and
chemical contaminations and cultivation with limited management
contribute to amplify runoff impacts on degradation and sediment
|Eastern oysters (Crassostrea virginica) were retrieved from Yuna
River. Before the experiment, the oysters were acclimated (using water
from the Yuna-Samana Bay water source) in a bucket containing the
estuarine water. In order to determine the short-term and acute response
to salinity, the oysters were transferred directly from the acclimated
condition into the experimental medium according to Shumway and
Koehn . Three adult oysters (>75 mm) were housed per a 2.5 gallon
tank, providing a total of thirty-six oysters for observation (Figure 3).
Experiments were conducted for seven days.
|Water samples were collected at two different locations: Yuna
River in Sanchez and Samana Bay at La Tambora Beach Resort. Both
sample locations were in a place where the water was flowing. The
sample containers were rinsed three times with the sampling water
prior to collection. Samples were brought back to the lab station at
La Tambora Beach Resort and diluted with distilled water, using one
liter and twenty-five ml graduated cylinders, to make the appropriate
test salinities. A hydrometer was used to measure the specific gravity
of all the samples as percent salinity and then calculated to parts
per thousand. The hydrometer was gently lowered into the sample
containers and allowed to stabilize. The water sample was able to
flow freely, and the reading was recorded when the surface of sample
reached the stem of the hydrometer. This protocol was replicated for
six different salinity gradients 3.5 ppt(10%); 11ppt(control)(30%); 18ppt(50%);
25ppt(70%); 32.5ppt(90%); and 36ppt(100%)). The salinity of the bay the
oysters were collected in this study was recorded about 9-11ppt.
|Oyster gaping was depicted as a cumulative gaping per day in a
graph due to the labels’ coming off in the tanks during seven days
experimental period. The mortality of oysters with respect to different
salinity gradients was analyzed using Fisher’s exact test/pair wise test
using Graph Pad software. P values were calculated using one tailed
test. Our first alternate hypothesis (Ha1) was: oyster mortality would
be high in lower salinity treatment. Our second alternate hypothesis
(Ha2) was: oyster respiration (gaping) would be low in lower salinity treatment. We used 90% confidence interval where P<0.10 are
considered as significant, the values in bold are significant P values.
|Oysters were exposed to varying salinity gradient to monitor their
tolerance levels and respiratory functions. As shown in table 1, we
found significant difference (P<0.10) between oysters survived in 11ppt
(control) treatment vs. oysters in the rest of other treatments.
|Pairwise comparisons display a P value of 0.001 for 11ppt vs.
36ppt; 11ppt vs. 32.5ppt and 11ppt vs. 25ppt. These results are highly
significant and it rejects our first alternate hypothesis. This means
that 11ppt (control) salinity treatment is the optimal concentration
for oysters in our study. For 11ppt vs. 18ppt the P value is 0.03 which
also rejects our hypothesis that 11ppt salinity gradient is more optimal
than 18ppt salinity treatment in our study. Mortality was absent in
11ppt whereas mortality was less in 18ppt when compared to other
treatments. Specifically, no mortality was recorded during the first
two days of experimentation for 3.5ppt, 11ppt and 32.5ppt salinity
treatments, which means that oysters tolerated high and low salinity
concentrations for a short period.
|Gaping was prescribed as a measurement of respiratory functions.
Figure 4 shows the oysters’ gaping response under different salinity
|On day 1, oysters gaped in all salinity treatments except oysters in
3.5ppt treatment where our second alternate hypothesis was rejected.
Gaping was seen in 3.5ppt treatment on day 3 and 5 but the number of
oysters responded was very less and on days 6 and 7 gaping was absent in all oysters. For other treatments (11 to 36ppt), most of the oysters
gaped on day 1 and no gaping was recorded in oysters in hyper-saline
treatments resulting decrease respiration function and then mortality
of those oysters. Gaping of oysters decreased gradually from day 2 for
most of the treatments including control oysters. On day 2, gaping was
absent only in 32.5 and 3.5ppt treatments. We need to count the fact
that some of these oysters take more time to acclimatize to different
salinity gradients resulting those oysters gaped later in our study such
as oysters in 3.5ppt treatment. Although, dissolved oxygen is more
readily available in lower salinity concentrations than in hyper-saline
concentrations, oysters in 3.5ppt hypo-saline treatment stressed as
much as hyper-saline treated oysters in our study.
|Figure 4 show that oysters under hypo-saline (3.5ppt/10%)
conditions did not experience significant respiratory or feeding
functions throughout the entire study. Oysters held in (11ppt/30%)
saline conditions revealed relatively constant respiratory and feeding
functions and no mortality throughout the experiment (Table 1)
suggesting how vulnerable those oysters to acute change in salinity
from anthropogenic disturbance.
|Oyster hemocytes are able to synthesize osmotic shock proteins
therefore protecting themselves from acute salinity variations .
Therefore, they are tolerant to acute salinity gradients. The purpose
of this experiment was to check the tolerance gradient of Eastern
oysters (Crassostrea virginica). When optimum saline concentration is
accomplished, oysters respire and feed better by opening their valves.
When oysters are exposed to hyper-saline concentrations for duration
of time, they have tendency to shut down and mortality is expected
from increased acute osmotic shock. Temperature range (25-32°C)
under which this experiment was performed was optimum and any
fluctuation in temperature would have added further complication and
negative effect on the biological function of oysters such as gaping and
filtration (Ozbay-pers. comm.).
|Tolerance of oysters to salinity gradient is well depicted in our
study because gaping was present in all salinity treatments in the first
day except 3.5ppt salinity treatment. Gradually oyster gaping decreased
in all hyper-saline treatments this may be due to this acute osmotic
pressure. Our study also shows that 30% or 11ppt salinity concentration
is optimum for oysters which corresponds with the literature (FLDEP
2010). At 10% or 3.5ppt salinity concentration, oyster mortality was
recorded which explains why hypo-saline concentrations are lethal to
oysters’ survival and respiration. Although the oysters were exposed to
both acute hypo- and hyper-saline conditions in this experiment, all
oysters survived and gaped in Day 1 supports their tolerance for the
acute salinity changes for short duration as indicated by Tirard et al.
|Oyster mortalities may be contributed to anthropogenic
disturbance as well as natural. Climate variability is of imminent
concern because changes in average climate patterns could subject
oysters to fluctuations and agitation from unfavorable environmental
conditions. A combination of stressors, specifically rapidly changing
salinity levels, may contribute to oyster mortalities as long as the
conditions persist . Additionally, intense storm surges and frequent
hurricanes will increase freshwater inputs that recede into estuaries;
causing prolonged periods of reduced salinity concentrations.
|Climate variability and stochastic events have a profound
influence on the Yuna River flow regime . Given its geographic orientation, Dominican Republic is vulnerable to climate variability
and natural hazards such as hurricanes, storm surges, and earthquakes.
Anthropogenic activity and one or more natural phenomena can have
a deleterious impact on the river’s physical parameters, including
land use practices, water diversion, and construction of dams .
Operation of dams exerts some influence on the river’s flow dynamics,
such as freshwater inputs; however, adequate research has not been
conducted to examine the dynamic effects on the ecosystem. The
Yuna River is the largest tributary that flows into Samana Bay-one of
the largest estuaries in the Caribbean. The Samana Bay is of special
interest because it supports winter populations of humpback whales in
densities second only to those found on Silver and Navidad Banks .
Endangered species of sea turtles also call the estuary home.
|Estuaries are kinetic ecosystems with natural environmental
fluctuations associated to processes such as tidal flow and day-to-day
temperature patterns. Superimposed upon these natural processes
of fluctuating patterns are stochastic events such as weather related
phenomena, which can create significant alterations in the estuarine
environment. Salinity dynamics appear highly vulnerable to climatic
pressures. Salinity can have a direct influence on the physical and biotic
components of estuaries . Numerous species of estuarine fauna,
including oysters, cannot exist below particular salinity thresholds
. Salinity has an important association with estuarine equilibrium.
Current climate change due to increases in atmospheric CO2 , may
affect estuarine salinity equilibrium in a myriad of ways. Sea-level
rise is speculated to have the most imminent impact because as sealevel
rises, salt water intrudes the delicate estuarine water dynamics.
It is apparent from these observations that further research is critical
to fully understand the correlation between sea-level rise and estuary
salinity dynamics. The timing and magnitude of deleterious effects of
climate change on aquatic systems have yet to be determined. However,
precautions to mitigate repercussions must be undertaken now if we
are to have any hope of protecting this valuable natural resource.
|We would like to acknowledge Marion McClary, Hector Ramirez, Anthony
Stampul, Jennilee Jankowski, and Kenneth Hannum for their constant support and
contribution. We would like to thank Dr. Karuna Chintapenta for her assistance in
editing the manuscript. This project is partially funded by USDA-NIFA and USDAEvans-
Allen Grant Program awarded to Dr. Gulnihal Ozbay.
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