The blood parameters tested for here cannot be considered separately, there are many studies showing that there are links between them and that alterations in one parameter, both organic and inorganic can lead to changes in another. For instance, carbonic anhydrase is an enzyme in aquatic invertebrates that facilitates rapid equilibration between molecular CO2 and HCO3
-
and serves in gas exchange and acid–base balance regulation (reflected in pH and lactate values). In the crabs Chasmagnathus granulata and Callinectes sapidus this enzyme is sensitive to copper (Vitale et al. 1999; Skaggs and Henry 2002) suggesting that it may play a role in copper induced disturbances to the acid–base balance. HCO3- and H+ not only affect acid–base equilibria but also
act as counterions in the transfer of Cl- and Na+ across plasma membranes via electroneutral ion transporters between the extracellular space and either the ambient water or the intracellular compartment (Whiteley et al. 2001). Brown and Terwilliger (1992) hypothesised that in Cancer magister, Ca and Mg may be involved in modulating the oxygen binding properties of haemocyanin as these elements have already been implicated in affecting the oxygen affinity of haemocyanin from a number of crustacean species (Larimer and Riggs 1964; Truchot 1975). In Carcinus maenas both ions decrease the O2 partial pressure at 50% saturation, and Mg
2+
increases the Bohr factor (Truchot 1975). In osmoregulating crabs, transfer from marine to low salinity results in a metabolic alkalosis in the haemolymph that can be transient or persistent (Whiteley et al. 2001). For some animals living in hypersaline waters, what appears to be the upper limit of salinity tolerance may in fact be the lower limit of dissolved oxygen tolerance as the solubility of oxygen decreases with increasing salinity (Bayley 1972).
The differences found in C. pagurus may be due to high intraspecific variability in this species, or rather the fact that there are few significant differences in C. pagurus (and none in the N. puber acute trial) may also suggest that there is a strong degree of regulation happening as nothing changes significantly with salinity. This is likely to be the case for C. pagurus as it has a high 96h LC50 (salinity 55.5) despite being the acute trial, however in the case of the acute N. puber trial, the
96h LC50 (salinity 41.9) was low and the reason there are no significant changes in the
haemolymph parameters may be due to death occurring so quickly that the changes could not be seen. It may be therefore that is not the changes in the blood chemistry in this case that led to the death of the organism (at least those blood chemistry changes that were recorded) but rather
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something else happening e.g. the cessation of respiration/cardioventilatory behaviour, although it would be expected that this would prompt some sort of change to the haemolymph.
The salinities where 100% mortality occurred (as evidenced in chapter 4) indicate that at this point, regardless of what is happening in the body, the animals are unable to handle hypersalinities. This is especially true for the late-postmoult lobsters which could not survive hypersalinity at all past salinity 40. It is envisaged from these results and those of the intermoult adult lobsters (which could tolerate higher salinities than the soft) that early-postmoult lobsters would be even less tolerant of changes to environmental salinity, due to having no hard shell to act as a barrier and the implications of this finding for population survival in areas of brine discharge is a concern.
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5.7 Conclusions
Elevated salinity is toxic, both lethal and sublethal, to the species studied. When given sufficient time to acclimate, significant changes in protein, pH, glucose, ammonia and haemocyanin levels occur in Homarus gammarus and Necora puber which are characteristic of those found during periods of hypoxia in other decapod species. In general, the responses seen here indicate that when in higher than normal salinities, H. gammarus is a weak regulator, with the ionic composition of the blood increasing with the external increases in ion concentration, although maintaining the haemolymph at a slightly lower level than externally. Lobsters in the late-postmoult stage have been shown to be less tolerant of salinity change than their intermoult counterparts, and even when the carapace is approaching full hardness the animals cannot tolerate salinities over 40. Cancer pagurus has the highest tolerance of the three species studied at salinity 55.5 (96h LC50). The
fewer indicators of haemolymph change in this species when compared with the other test species suggests a stronger degree of osmo and iono regulation in C. pagurus which is supported by the highest mortality point of the species tested. When exposed to a hypersaline environment this study has shown that N. puber is able to strictly regulate the haemolymph variables within the salinity range 35 – 50 units. Subsequent changes require a more prolonged acclimation period otherwise they are lethal. The inability of N. puber to survive at salinities of 55 and 60 in the acute trial indicates that they cannot cope with the sudden changes that this induces.
The changes shown in haemolymph variables may be eventually lethal or, given additional time may be restored to normal (normoxic values/normal salinity values). Ultimately for the purposes of keeping commercial fisheries sustainable, brine discharges should be limited at tolerance level of the species with the lowest tolerance, in this case N. puber, hence by limiting the discharge to keep this species alive in the affected area, it therefore keeps the other commercially important species which have higher tolerance levels alive too.