The effects of hypercapnia on the West Coast rock lobster (Jasus lalandii) through acute exposure to decreased seawater pH - Physiological and biochemical responses

The cold water palinurid Jasus lalandii ("West Coast rock lobster") is a commercially important crustacean in South Africa and Namibia and inhabits the Benguela Current Eastern Boundary System. This habitat is characterised by strong upwelling events in summer and algal blooms with their subsequent decay in autumn. Upwelling can lead to acute hypercapnia whereas the algal decay is associated with acute hypercapnic hypoxia. Both types of hypercapnic events could become more frequent and severe in the future due to ongoing climate change. The aim of the present study was, however, to study the capability and mechanisms of response in J. lalandii to hypercapnia exclusively. Accordingly, the following research questions were formulated: 1) To what extent is haemocyanin oxygen-binding affinity of adult J. lalandii pH-sensitive? 2) Can adult male J. lalandii respond swiftly to drastic changes in pH? 3) What physiological mechanisms facilitate a potential response to a drastically declining pH, i.e. acute hypercapnia? These questions were answered by analysing 1) the pH sensitivity of the haemocyanin's oxygen binding properties and 2) in vivo changes in the acid-base balance of adult J. lalandii during acute exposure to hypercapnia (pH 7.4). Results showed the following: 1) Haemocyanin displays a strong Bohr shift (whole haemolymph: ΔlogP/ΔpH=-1.17; dialysed haemolymph: ΔlogP/ΔpH=-0.84) in response to lowering of pH. 2) Acute hypercapnia leads to a decline in extracellular pH within the initial 1.5h of exposure. 3) Thereafter, active compensation becomes apparent as the bicarbonate levels start to increase, with complete compensation reached after 5h of exposure (+2.3 mmoll; +48%). 3) This bicarbonate increase is reversed when returning lobsters to normocapnia (pH 7.9). 4) Levels of molecular modulators of haemocyanin oxygen affinity (Ca, Mg and l-lactate) do not change during acute exposure to hypercapnia.Our results show the capability of adult J. lalandii to rapidly and fully compensate the experienced extracellular acidosis, protecting oxygen carrying capacity of haemocyanin and ensuring an outward gradient of CO. The West Coast rock lobster is therefore well equipped for its habitat where these hypercapnic events are known to occur frequently