parameters, such as population size and distributions
(Portner et al., 2008). Numerous studies have investigated
the thermal lethal tolerances of estuarine ectotherms, and
results suggest that species inhabiting the upper shore
have a higher thermal tolerance, but also live closer to
their thermal maxima, than those living at lower tidal
levels. In more recent studies, Madeira et al. (2012a) used
behavioral endpoints to determine the critical thermal
maximum (CTMax) and found results which were consistent with some previous studies. They confirmed lower
CTMax for subtidal, demersal, and northern species
exposed to colder temperatures. While some previous
studies have suggested that the same species from different localities might differ in thermal tolerances, this was
not found for those investigated in this study. Also, while
it was previously thought that there would be taxonomic
differences in thermal tolerances, this was not apparent
in the study, and taxa from the same habitats had similar
thermal tolerances, suggesting species have evolved similar stress responses. It is predicted that the species most
vulnerable to increased seawater temperatures are those
from thermally unstable environments. This includes
some estuarine habitats, although species that live here
are expected to have a high CTMax, have limited powers
of acclimation, and live close to their thermal maximum.
Respiratory adaptations
Dissolved oxygen is readily available in open coastal
marine environments, but in estuarine waters it is often
low, being described as hypoxic. Estuarine waters may
also exhibit hypercapnia or increased levels of carbon
dioxide. In aquatic organisms, the gills are multifunctional
organs, important not only in oxygen uptake but also in
ion transport which underlies hemolymph osmoregulation, acid–base balance, and ammonia excretion. They
are also the site of uptake of trace metals. When exposed
to air, the gills collapse and adhere together making gas
exchange difficult. Marine muds may be nutrient rich,
and the sediment becomes blackened close to the surface
where bacteria have converted nitrates and sulfates to
nitrites and sulfides. In this habitat estuarine organisms
use aerobic and anaerobic respiration and have structural
and behavioral adaptations to minimize exposure to environmental stressors.
Because diffusion alone cannot deliver sufficient
oxygen and carbon dioxide between external respiratory
surfaces, tissues, and mitochondria, animals have evolved
respiratory structures and circulatory systems to transport
respiratory gases around the body. Many taxa also use
respiratory pigments which increase their ability to pick
up and deliver oxygen to the tissues. These are similar in
structure, but hemoglobin and chlorocruorin, which are
found in many estuarine polychaetes, have a prosthetic
group called a heme, a porphyrin attached to one atom of
ferrous iron. Hemocyanin, found in mollusks and crustaceans, does not have porphyrin; instead the prosthetic
group is a polypeptide attached to copper and sulfur rather
than iron. Hemerythrin is a less common respiratory pigment, being found in three phyla (sipunculids, priapulids,
and brachiopods), and the pigment contains iron but without a heme group. The oxygen-carrying capacity of the
different pigments is affected by pH, and raised carbon
dioxide levels shift the oxygen equilibrium curve to the
right in a Bohr shift, allowing the delivery of oxygen on
the venous side of the capillary networks (Wells, 1980).
Oxygen uptake in most aquatic organisms occurs
across the gills, and numerous studies have investigated
how internal factors such as size, sex, and activity affect
the uptake. External factors also affect the rate; oxygen
tension, temperature, and salinity are the most important
(Newell, 1979). Temperature is a key variable and affects
all of the metabolic processes, with the temperature coefficient Q 10 representing the sensitivity to temperature
and the energy demands for growth. In estuaries where
temperature fluctuations may occur in a predictable way,
organisms may have a temperature insensitive region of
their rate–temperature curve where there is no increase
in oxygen uptake with increasing temperature. This adaptation minimizes energy expenditure in stressful environments where food resources may be limited. Shrimp are
usually found in high-salinity waters within estuaries,
and several authors have found that temperature rather
than salinity has the greater effect on metabolic rate.
Spanopoulos-Hernandez et al. (2005) found that the Q 10
of juvenile shrimp did not vary with salinity but increased
markedly with temperature, with a Q 10 of 1.88 between
20
C and 30
C.
In response to declining oxygen levels in water, estuarine organisms are sometimes described as either oxygen
conformers, which generally decrease their metabolic rate,
or regulators which are able to maintain a steady oxygen
uptake rate in declining oxygen tensions. While many
organisms do not fit clearly into particular categories,
many species are able to maintain some degree of regulation using a combination of mechanisms including altering the ventilation or stroke rate and using temporary
oxygen stores, blood pigments, and aerobic metabolism.
Ability to survive aerial exposure and desiccation
The ability to survive aerial exposure is a requirement for
estuarine organisms living in the intertidal zone. Some
invertebrates such as polychaetes become inactive on the
outgoing tide, while barnacles and bivalves can open and
close irregularly to expose respiratory surfaces to the air,
which has higher oxygen content than seawater. Crustaceans and mollusks from upper tidal levels are able to
maintain oxygen uptake in air, while those from lower
tidal levels tend to have reduced rates of respiration when
exposed to air (Newell, 1979). Because they are regularly
exposed to air, many estuarine organisms have a high tolerance of water loss, being able to survive up to 70 % total
body loss. Shell closure and reduced metabolic rates also
minimize energetic costs. Avoidance also is a good strategy for mobile invertebrates, snails and crabs, which form
PHYSIOLOGICAL MECHANISMS
487
(Portner et al., 2008). Numerous studies have investigated
the thermal lethal tolerances of estuarine ectotherms, and
results suggest that species inhabiting the upper shore
have a higher thermal tolerance, but also live closer to
their thermal maxima, than those living at lower tidal
levels. In more recent studies, Madeira et al. (2012a) used
behavioral endpoints to determine the critical thermal
maximum (CTMax) and found results which were consistent with some previous studies. They confirmed lower
CTMax for subtidal, demersal, and northern species
exposed to colder temperatures. While some previous
studies have suggested that the same species from different localities might differ in thermal tolerances, this was
not found for those investigated in this study. Also, while
it was previously thought that there would be taxonomic
differences in thermal tolerances, this was not apparent
in the study, and taxa from the same habitats had similar
thermal tolerances, suggesting species have evolved similar stress responses. It is predicted that the species most
vulnerable to increased seawater temperatures are those
from thermally unstable environments. This includes
some estuarine habitats, although species that live here
are expected to have a high CTMax, have limited powers
of acclimation, and live close to their thermal maximum.
Respiratory adaptations
Dissolved oxygen is readily available in open coastal
marine environments, but in estuarine waters it is often
low, being described as hypoxic. Estuarine waters may
also exhibit hypercapnia or increased levels of carbon
dioxide. In aquatic organisms, the gills are multifunctional
organs, important not only in oxygen uptake but also in
ion transport which underlies hemolymph osmoregulation, acid–base balance, and ammonia excretion. They
are also the site of uptake of trace metals. When exposed
to air, the gills collapse and adhere together making gas
exchange difficult. Marine muds may be nutrient rich,
and the sediment becomes blackened close to the surface
where bacteria have converted nitrates and sulfates to
nitrites and sulfides. In this habitat estuarine organisms
use aerobic and anaerobic respiration and have structural
and behavioral adaptations to minimize exposure to environmental stressors.
Because diffusion alone cannot deliver sufficient
oxygen and carbon dioxide between external respiratory
surfaces, tissues, and mitochondria, animals have evolved
respiratory structures and circulatory systems to transport
respiratory gases around the body. Many taxa also use
respiratory pigments which increase their ability to pick
up and deliver oxygen to the tissues. These are similar in
structure, but hemoglobin and chlorocruorin, which are
found in many estuarine polychaetes, have a prosthetic
group called a heme, a porphyrin attached to one atom of
ferrous iron. Hemocyanin, found in mollusks and crustaceans, does not have porphyrin; instead the prosthetic
group is a polypeptide attached to copper and sulfur rather
than iron. Hemerythrin is a less common respiratory pigment, being found in three phyla (sipunculids, priapulids,
and brachiopods), and the pigment contains iron but without a heme group. The oxygen-carrying capacity of the
different pigments is affected by pH, and raised carbon
dioxide levels shift the oxygen equilibrium curve to the
right in a Bohr shift, allowing the delivery of oxygen on
the venous side of the capillary networks (Wells, 1980).
Oxygen uptake in most aquatic organisms occurs
across the gills, and numerous studies have investigated
how internal factors such as size, sex, and activity affect
the uptake. External factors also affect the rate; oxygen
tension, temperature, and salinity are the most important
(Newell, 1979). Temperature is a key variable and affects
all of the metabolic processes, with the temperature coefficient Q 10 representing the sensitivity to temperature
and the energy demands for growth. In estuaries where
temperature fluctuations may occur in a predictable way,
organisms may have a temperature insensitive region of
their rate–temperature curve where there is no increase
in oxygen uptake with increasing temperature. This adaptation minimizes energy expenditure in stressful environments where food resources may be limited. Shrimp are
usually found in high-salinity waters within estuaries,
and several authors have found that temperature rather
than salinity has the greater effect on metabolic rate.
Spanopoulos-Hernandez et al. (2005) found that the Q 10
of juvenile shrimp did not vary with salinity but increased
markedly with temperature, with a Q 10 of 1.88 between
20
C and 30
C.
In response to declining oxygen levels in water, estuarine organisms are sometimes described as either oxygen
conformers, which generally decrease their metabolic rate,
or regulators which are able to maintain a steady oxygen
uptake rate in declining oxygen tensions. While many
organisms do not fit clearly into particular categories,
many species are able to maintain some degree of regulation using a combination of mechanisms including altering the ventilation or stroke rate and using temporary
oxygen stores, blood pigments, and aerobic metabolism.
Ability to survive aerial exposure and desiccation
The ability to survive aerial exposure is a requirement for
estuarine organisms living in the intertidal zone. Some
invertebrates such as polychaetes become inactive on the
outgoing tide, while barnacles and bivalves can open and
close irregularly to expose respiratory surfaces to the air,
which has higher oxygen content than seawater. Crustaceans and mollusks from upper tidal levels are able to
maintain oxygen uptake in air, while those from lower
tidal levels tend to have reduced rates of respiration when
exposed to air (Newell, 1979). Because they are regularly
exposed to air, many estuarine organisms have a high tolerance of water loss, being able to survive up to 70 % total
body loss. Shell closure and reduced metabolic rates also
minimize energetic costs. Avoidance also is a good strategy for mobile invertebrates, snails and crabs, which form
PHYSIOLOGICAL MECHANISMS
487