These rhythms, which are cued by tidal variables including hydrostatic pressure, salinity, and temperature, most
likely have a molecular basis and have obvious adaptive
value in reproduction.
Circatidal rhythms are well known in blue crabs
(Callinectes sapidus) which undergo spawning migrations
to the sea, then move closer to the mouth of the coast to
release larvae. Recent research by Darnell et al. (2010)
has revealed ontogenetic changes where crabs prior to oviposition had a variety of endogenous swimming rhythms
(circadian, circatidal, or circalunadian), whereas
ovigerous crabs from estuaries with diurnal tides had pronounced circatidal or circalunadian rhythms with swimming at the time of the ambient low tide. There were,
however, differences in behavior of crabs from nontidal
situations, and the authors considered whether this could
be explained by genetic differences between populations.
Because of high levels of genetic diversity, this was
rejected, and instead the authors suggested the differences
were due to the phenotypic plasticity of the clock mechanisms which can be entrained to either semidiurnal tidal,
diurnal tidal, or 24 h light–dark cycles.
Effects of contaminants
Contaminants affect estuaries, partly because they are
often close to cities where there are often accidental spills
of sewage, oil, and industrial waste containing trace
metals, chlorinated hydrocarbons, and microbial contaminants. The organisms that live in estuaries take up contaminants from the surrounding water, pore water, sediment,
and their food (Marsden and Rainbow, 2004). There has
been considerable research into the effects of trace metals
on estuarine organisms, and, in general, crustaceans and
bivalves take up trace metals, such as copper, in proportion to the concentrations in the external media
(Rainbow, 1995; Luoma and Rainbow, 2008). There are
few consistent patterns among different crustaceans in
their responses to altered salinities, and these responses
appear to be species specific and related to body size and
differences in physiology. Salinity affects the speciation
and bioavailability of metals in the environment, influencing their uptake by aquatic organisms (Luoma and Rainbow, 2008). Generally a decrease in salinity results in an
increase in the metal content of the tissues because of an
increase in free metal ion concentration, which is a result
of decreased metal complexation at lower salinities. One
area of current interest lies in the effects that salinity has
on the physiological mechanisms in decapod crustaceans.
Because copper is a functional part of the respiratory pigment hemocyanin, we would expect to find it in the hemolymph where it would be removed to other organs to be
excreted or detoxified (Rainbow, 2007). In blue crabs,
the anterior gills are for oxygen uptake, and the posterior
gills are for ion transport. Therefore, copper, like other
metals such as silver, cadmium, zinc, and mercury, can
cause respiratory problems and osmoregulatory disturbances (Bianchini et al., 2008). Martins et al. (2011)
recently investigated the effects of salinity on the acute
toxicity of copper in the blue crab. Although there were
differences in the apparent toxicity with salinity, with an
increase in survival at 2 psu as opposed to 30 psu, these
differences were explained simply by changes in water
chemistry and not due to different physiological mechanisms at different salinities. This research showed that
the gills are the key target organ for copper accumulation,
and they also act as a barrier to high uptake by the hemolymph and further distribution of copper to the internal
organs, including the hepatopancreas. Copper is also
known to disrupt acid–base balance by restricting gas
exchange in C. maenas and in N. granulata, where it
results in an inhibition of carbonic anhydrase (Skaggs
and Henry, 2002).
Use of biomarkers
There are multiple stressors in estuaries. Chemical contaminants, physical and anthropogenic stressors, and the
effects of individual stressors can be additive, antagonistic, or synergistic (Crain et al., 2008). The search for
a way to assess the overall health of animals when exposed
to multiple stressors has led to the use of biomarkers and
bioindicators of environmental health. Biomarkers are
defined as the biological responses to environmental
stressors, which include the influences of the abiotic environment and anthropogenic pollution. They can be ecological,
behavioral,
biochemical,
physiological,
chemical, genotoxic, or molecular (Scott and Sloman,
2004; McDonagh et al., 2005; Farcy et al., 2013). Some
biomarkers are known as specific biomarkers; examples
include metallothioneins which represent trace metal concentrations in body tissues. They are salinity dependent.
Others include cholinesterase activity which is a specific
biomarker for organophosphorus and carbamate pesticides and neurotoxins (Monserrat et al., 2007). Alternatively, nonspecific biomarkers (DNA damage, protein
oxidation, antioxidant responses) reflect combinations of
stressors (Gericitano et al., 2004). Biomarkers of DNA
damage are valuable tools to assess the effects of acute
and chronic exposure of aquatic animals to genotoxic substances (Amado et al., 2006). Any changes are important
because they can be passed on to future generations and
have ecological consequences. Rodrigues et al. (2012)
assessed the effects of salinity stress on a suite of biomarkers (oxidative, energy metabolism, and neurotransmission) of Carcinus maenas from an estuary with very
little pollution compared with one where the crabs were
impacted by high chemical stress. Although crabs from
the clean site showed alteration in neurotransmission and
antioxidative defenses, those from the impacted site
exhibited responses that were induced by both high and
low salinity exposure.
Summary
Estuaries are short-lived geological features which have
been subjected to changing conditions as a result of
PHYSIOLOGICAL MECHANISMS
489
likely have a molecular basis and have obvious adaptive
value in reproduction.
Circatidal rhythms are well known in blue crabs
(Callinectes sapidus) which undergo spawning migrations
to the sea, then move closer to the mouth of the coast to
release larvae. Recent research by Darnell et al. (2010)
has revealed ontogenetic changes where crabs prior to oviposition had a variety of endogenous swimming rhythms
(circadian, circatidal, or circalunadian), whereas
ovigerous crabs from estuaries with diurnal tides had pronounced circatidal or circalunadian rhythms with swimming at the time of the ambient low tide. There were,
however, differences in behavior of crabs from nontidal
situations, and the authors considered whether this could
be explained by genetic differences between populations.
Because of high levels of genetic diversity, this was
rejected, and instead the authors suggested the differences
were due to the phenotypic plasticity of the clock mechanisms which can be entrained to either semidiurnal tidal,
diurnal tidal, or 24 h light–dark cycles.
Effects of contaminants
Contaminants affect estuaries, partly because they are
often close to cities where there are often accidental spills
of sewage, oil, and industrial waste containing trace
metals, chlorinated hydrocarbons, and microbial contaminants. The organisms that live in estuaries take up contaminants from the surrounding water, pore water, sediment,
and their food (Marsden and Rainbow, 2004). There has
been considerable research into the effects of trace metals
on estuarine organisms, and, in general, crustaceans and
bivalves take up trace metals, such as copper, in proportion to the concentrations in the external media
(Rainbow, 1995; Luoma and Rainbow, 2008). There are
few consistent patterns among different crustaceans in
their responses to altered salinities, and these responses
appear to be species specific and related to body size and
differences in physiology. Salinity affects the speciation
and bioavailability of metals in the environment, influencing their uptake by aquatic organisms (Luoma and Rainbow, 2008). Generally a decrease in salinity results in an
increase in the metal content of the tissues because of an
increase in free metal ion concentration, which is a result
of decreased metal complexation at lower salinities. One
area of current interest lies in the effects that salinity has
on the physiological mechanisms in decapod crustaceans.
Because copper is a functional part of the respiratory pigment hemocyanin, we would expect to find it in the hemolymph where it would be removed to other organs to be
excreted or detoxified (Rainbow, 2007). In blue crabs,
the anterior gills are for oxygen uptake, and the posterior
gills are for ion transport. Therefore, copper, like other
metals such as silver, cadmium, zinc, and mercury, can
cause respiratory problems and osmoregulatory disturbances (Bianchini et al., 2008). Martins et al. (2011)
recently investigated the effects of salinity on the acute
toxicity of copper in the blue crab. Although there were
differences in the apparent toxicity with salinity, with an
increase in survival at 2 psu as opposed to 30 psu, these
differences were explained simply by changes in water
chemistry and not due to different physiological mechanisms at different salinities. This research showed that
the gills are the key target organ for copper accumulation,
and they also act as a barrier to high uptake by the hemolymph and further distribution of copper to the internal
organs, including the hepatopancreas. Copper is also
known to disrupt acid–base balance by restricting gas
exchange in C. maenas and in N. granulata, where it
results in an inhibition of carbonic anhydrase (Skaggs
and Henry, 2002).
Use of biomarkers
There are multiple stressors in estuaries. Chemical contaminants, physical and anthropogenic stressors, and the
effects of individual stressors can be additive, antagonistic, or synergistic (Crain et al., 2008). The search for
a way to assess the overall health of animals when exposed
to multiple stressors has led to the use of biomarkers and
bioindicators of environmental health. Biomarkers are
defined as the biological responses to environmental
stressors, which include the influences of the abiotic environment and anthropogenic pollution. They can be ecological,
behavioral,
biochemical,
physiological,
chemical, genotoxic, or molecular (Scott and Sloman,
2004; McDonagh et al., 2005; Farcy et al., 2013). Some
biomarkers are known as specific biomarkers; examples
include metallothioneins which represent trace metal concentrations in body tissues. They are salinity dependent.
Others include cholinesterase activity which is a specific
biomarker for organophosphorus and carbamate pesticides and neurotoxins (Monserrat et al., 2007). Alternatively, nonspecific biomarkers (DNA damage, protein
oxidation, antioxidant responses) reflect combinations of
stressors (Gericitano et al., 2004). Biomarkers of DNA
damage are valuable tools to assess the effects of acute
and chronic exposure of aquatic animals to genotoxic substances (Amado et al., 2006). Any changes are important
because they can be passed on to future generations and
have ecological consequences. Rodrigues et al. (2012)
assessed the effects of salinity stress on a suite of biomarkers (oxidative, energy metabolism, and neurotransmission) of Carcinus maenas from an estuary with very
little pollution compared with one where the crabs were
impacted by high chemical stress. Although crabs from
the clean site showed alteration in neurotransmission and
antioxidative defenses, those from the impacted site
exhibited responses that were induced by both high and
low salinity exposure.
Summary
Estuaries are short-lived geological features which have
been subjected to changing conditions as a result of
PHYSIOLOGICAL MECHANISMS
489