Influence of aqueous aluminium on the immune system of the freshwater crayfish Pacifasticus leniusculus.

Little is known of the effects of aluminium (Al) on invertebrate immunity despite the ubiquitous nature of the metal and its toxicity to aquatic organisms. Here we examine the effect of Al at neutral pH on the immune system of the freshwater crayfish Pacifasticus leniusculus. Heat-killed bacteria were injected at intervals into the haemolymph of crayfish continuously exposed to a sub-lethal concentration (500 microg l(-1)) of Al over 40 days. Circulating haemocyte and bacterial numbers were monitored for 16 days post-injection at each time interval. In the absence of bacterial challenge, the number of circulating haemocytes increased in Al-exposed crayfish compared to unexposed controls. Aluminium exposure initially reduced the crayfish's ability to clear bacteria from the circulation and decreased the rate of recovery in haemocyte numbers following bacterial challenge. These effects on bacterial and haemocyte numbers were abolished after prolonged exposure (>10 days) to Al, indicating adaptation to the metal. Aqueous Al impairs gill function in P. leniusculus by inducing hypersecretion of mucus and we suggest that the decrease in immunocompetence is due to haemolymph hypoxia. We conclude that exposure to episodic pulses of aqueous Al over the short term (<10 days) increases the risk of infection in the crayfish by impairing the ability of haemocytes to recognise and/or remove bacteria from the circulation.