Summary

The effects of the muscle relaxant magnesium chloride (MgCl2) were monitored in the Sydney rock oyster, Saccostrea glomerata, under laboratory conditions. The relaxant was first tested for efficiency and signs that treatment led to deleterious effects on spawning and immunological activity. A range of immunological parameters were tested including, total haemocyte frequencies, acid phosphatase, superoxide and phenoloxidase activities in the haemolymph as well total haemolymph protein levels.  All of the oysters exposed to MgCl2 demonstrated total anaesthesia within 6 hours of the initial exposure. Resultant spawning events were sporadic, showing no correlation with relaxant exposure times. Total haemocyte counts and acid phosphatase and superoxide activities all increased significantly during the 48 hours after exposure to the anaesthetic. Conversely, phenoloxidase activity increased immediately after relaxation but then decreased consistently until 48 hours after exposure. Overall the data indicate that MgCl2 is an effective relaxant for use on Sydney rock oysters, and that any potentially deleterious effects of the relaxant do not persist for more than 96 hours.