An evaluation of some relaxants for use with pearl oysters

Abstract

As part of a research program to improve the efficiency of round pearl culture procedures, various potential relaxants were evaluated in groups of Pinctada albino (L). The chemicals tested included propylene phenoxetol, 2-phenoxyethanol, menthol crystals, menthol liquid, clove oil, benzocaine, MS222, chloral hydrate, sodium pentobarbitone, magnesium chloride, sodium bicarbonate, carbon dioxide gas, dry ice (solid carbon dioxide) and also hypothermia. Of these, the following appeared promising: propylene phenoxetol at 2.5 ml L−1, 2-phenoxyethanol at 3.0 and 4.0 ml L−1, menthol crystals at 0.25 and 1.0 g L−1, menthol liquid at 0.25 and 0.4 ml L−1, clove oil at 1.5 ml L−1 and benzocaine at 1200 mg L−1. Propylene phenoxetol (1-phenoxy-propan-2-ol) was selected to determine the effect of changes in relaxant concentration, water temperature, duration of exposure and shell size on the time to relaxation and to recovery and on survival after 7 days, in both P. albina and P. margaritifera. Oysters unexposed to relaxants were maintained as controls. Significant responses were found in both P. albina and P. margaritifera as follows: As propylene phenoxetol concentration increased, the time to relaxation decreased while the time to recovery tended to increase. Increased duration of exposure to the relaxants increased the time to recovery. Increases in temperature decreased both the time to relaxation and the time to recovery especially in the 21–25 °C range. No association was found with oyster size within each species. No mortalities occurred within 7 days of having used propylene phenoxetol.

It would appear that propylene phenoxetol is a suitable relaxant for Pinctada pearl oysters within the concentration range of a 2 to 3 ml L−1 and that P. albina might be a suitable experimental mollusc for evaluating relaxants in other species of Pinctada.