The functioning of the haemocyanin of the terrestrial christmas island red crab gecarcoidea natalis and roles for organic modulators Available to Purchase

Gecarcoidea natalis is a land crab that migrates annually several kilometres to breed. The O₂-binding properties of haemocyanin in G. natalis were investigated in vitro to test the idea that the O₂-binding properties of the haemocyanin of land crabs are not dependent on circulating modulators and to provide a model of haemocyanin functioning during exercise. The affinity of the haemocyanin for O₂ decreased with increasing temperature (change in the heat of oxygenation; ΔH=−59 kJ mol⁻¹ ). The haemocyanin of G. natalis apparently differs from that of other terrestrial crabs in showing haemocyanin O₂ modulation by both organic and inorganic molecules. Haemocyanin O₂-affinity was not affected by Mg²⁺ but was sensitive to changes in Ca²⁺ concentration (ΔlogP₅₀/Δlog[Ca]=−0.61, where P₅₀ is the partial pressure of O₂ required for half-maximal O₂ binding). The Bohr factor was modest (ϕ=−0.26±0.03, N=4, in whole haemolymph at 25 °C) and there was no specific effect of CO₂ on the O₂-binding properties of the haemocyanin. An increase in urate concentration increased haemocyanin O₂-affinity, but the effect was linear (ΔlogP₅₀/Δ[urate]=−0.06) and not logarithmic as is the case in other species. The effect of L-lactate on the haemocyanin O₂-affinity in G. natalis was unique among the crustaceans, because an increase in L-lactate concentration decreased the haemocyanin O₂-affinity. The effect of L-lactate on haemocyanin O₂-affinity (ΔlogP₅₀/Δlog[lactate]) was time- dependent and decreased from a maximum of 0.044 on day 1 to 0.001 after 4 days of storage at 4 °C. The presence of an unknown dialysable and unstable factor in the haemolymph is postulated to explain the time-dependent effect of L-lactate on haemocyanin O₂-binding properties. Model oxygen equilibrium curves constructed for in vivo conditions showed that the reverse effect of L-lactate was advantageous by decreasing the O₂-affinity of the haemocyanin beyond that predicted by the Bohr shift alone and assisted in O₂ off-loading at the tissues. This effect of lactate can only provide an advantage if the gas-exchange organs maintain arterial O₂ loading and thus is dependent on lung function in land crabs and must have occurred coincident with the evolution of these other features.