Anaesthetisation with isotonic magnesium chloride

Many meiofaunal organisms deploy methods to maintain their position in the sediment or algae which makes removing live meiofauna from the substrate difficult. The simple decantation methods used with fixed samples will only yield a small subset of the fauna in the sample. The key is to get them to let go! One method is to soak the sample in an anaesthetic. The most widely used anaesthetic is isotonic magnesium chloride (75g MgCl2L-1) in freshwater (preferably distilled). The sample is mixed with the anaesthetic and left for a period of around 15min. Then the decantation method is used to remove the fauna from the substrate. The anaesthetic should be collected for reuse, though it will become dilluted and thus less effective over time. Once extracted the fauna can be returned to seawater, where they will usually recover, for live observation. Note the anaesthetic can also be used to slow down fast organisms, such as some species of turbellarians, in order to make live observation easier. The anaesthetic can also be used to relax specimens prior to fixation.