Although water temperature and salinity may
change hypoxic thresholds in aquatic organisms most aquatic species can maintain adequate oxygen uptakes at
dissolved oxygen concentrations (DO) above 5 mg l−1. Below their specific optimum DO level, aquatic species display physiological
and behavioural adaptations to maintain satisfactory oxygen uptake
rates. Tolerance levels are generally higher for less active organisms
(physiological adaptations), whereas mobile organisms may
show behavioural responses such as avoiding hypoxic areas. Animals
from chronically or periodically hypoxic habitats are generally more tolerant
to hypoxia than animals residing in well oxygenated waters.
However, in all cases, when the activities necessary for maintaining
homeostasis are performed at a higher metabolic cost due to low DO
conditions, less of the organism's energy budget is available for growth
and reproduction. Moreover, the respiratory rates of the organisms become
more dependent on the external DO below a hypoxic threshold,
which is usually higher in organisms with higher resting respiratory
rates. That is, if
an organism displays similar oxygen consumption irrespectively of DO
(high oxygen independence) in a wide range of abiotic conditions it
could be considered to be better adapted to unpredictable and/or
oxygen-depleted environments.Therefore, evaluating respiration rates
and their dependence on DO could be an integrative way of determining
the potential effects of increased hypoxia on different species.
According to the current available information, 50% of crustacean species
show sublethal effects at aDObelow 3.2 mg l−1, suggesting that they
are among the most sensitive aquatic invertebrates to hypoxia. Among crustaceans, caridean shrimp
are important components of thewater column and bottom fauna of temperate
estuaries and associated habitats. Salinity plays a dominant
role in determining the spatial and, to a lesser extent, temporal distribution
of shrimp inhabiting these aquatic systems, whereas temperature is
more important in determining their seasonal density patterns. Concerning the respiration rate, the oxygen uptake of shrimp generally
increases with an increase in temperature, although they may show partial
metabolic suppression above an upper thermal threshold. However, the shrimp
respiratory response to changes in salinity is more complex. In addition, salinity and temperature have been found to have an
interactive effect on the respiration rates of some shrimp species.
Keeping the above considerations in mind and based on the distribution
of decapod crustacean species within estuaries and associated habitats
in the Gulf of Cádiz , six species
of shrimp fromdifferent habitats were selected to examine their respiratory
responses to increasing temperature and salinity (warming
effects) and progressive hypoxia (eutrophication effects). The atyidae
Atyaephyra desmarestii (Millet, 1831) mainly inhabits well oxygenated
freshwater habitats in the upstream of estuaries, where shrimps usually
display epibenthic behaviour strongly associated with submerged plants. However, the species is tolerant
to moderate changes in temperature and salinity and is sometimes
collected in the less saline estuarine reaches. The
palaemonids Palaemon longirostris (A. Milne Edwards, 1837), Palaemon
macrodactylus Rathbun, 1902 and Palaemonetes varians (Leach, 1814)
are shrimp species which live in the water column (hyperbenthos),
they complete their entire life cycles within the estuarine realmand, despite
having a very high osmoregulatory capacity, show partial spatial
segregation by species: P. longirostris is more abundant in the outer and
more saline estuarine area; the introduced P. macrodactylus occurs
most commonly in the inner and less saline estuarine reaches; and P.
varians is the dominant shrimp species in marches, ponds and channels
that are periodically connected to the estuary. Finally, the crangonids Crangon crangon (Linnaeus, 1758) and
Philocheras monacanthus (Holthuis, 1961) are marine species that both
live in benthic habits. However, due to their different tolerances to salinity
changes, the juveniles and adults of C. crangon seasonally migrate into
the estuary to use it as nursery and feeding areas, whereas P. monacanthus inhabits the more saline coastal habitat and is
only found within the estuarine realm accidentally.
Although the spatial distribution of species cannot be attributed solely
to inter-specific differences in their tolerance to a single environmental
factor, the inherently integrative nature of metabolism responses to
environmental changes makes them useful for correlating the environmental
conditions of a particular habitat with the tolerance levels of its
inhabitants. Under the generally
accepted assumption that climate changes and eutrophication
have led to aquatic systems having warmer,more saline and less oxygenated
waters, in this study we hypothesized that assays on
the oxygen consumption an