Hydromedusae
Fixation and preservation

Hydromedusae should be anaesthetised before fixation. The animals are allowed to extend in a vessel of sea water where the anaesthetic substance is added slowly, crystal by crystal or drop by drop. The commonest anaesthetics for marine medusae are menthol crystals, propylene phenoxetol, and magnesium chloride (about 7.5% MgCl2 6H20 in fresh water), the last being the most recommended. For general taxonomic purposes hydromedusae can be fixed in 10% buffered "formalin" in sea water (40% commercially available formaldehyde being considered as 100% "formalin"), and preserved shortly in 5% formaldehyde. Buffering the preserving fluid with borax or calcium carbonate should be avoided, since the medusae may adhere to any precipitate formed by these chemicals on the bottom of the containers. Hexamine should also be avoided for this purpose because it destroys the mesoglea. The best buffer seems to be sodium glycerophosphate. Alcohol is to be avoided as a fixative because it causes specimens to shrink. Nevertheless, for long term preservation (for instance in museum collections) formaldehyde, which causes auto-maceration of tissues, is inadequate. It should be replaced gradually by 70% alcohol, going from formaldehyde to a very dilute alcohol solution (less than 10%) and then, step by step (10% by 10%) over several days, to the final 70% solution. Polythene containers should be avoided also because chemical precipitates may damage the specimens. For histological studies the best fixative (after anaesthesia) is cold (5-8°C) acidic Bouin's fixative (3 parts of saturated aqueous solution of picric acid and 1 part of formaldehyde, just before use 5% glacial acetic acid should be added to this solution). Fixation time from 2 to 6 hours. This procedure minimises tissue maceration in organisms subsequently kept for long periods of time in 5% formaldehyde.

Another long-term storage method has been developed by Van Impe (1992), in which the medusae are suspended in a solid agar-agar gel coloured with serva-blue, and from which extraction is easy when required. This method is particularly useful for transportation and long term preservation. All holotypes should be stored in such a gel, which to a certain degree also prevents drying and nicely stains the proteinic medusae tissues blue.

In most museum collections the majority of hydromedusae specimens, including the holotypes, have been destroyed because label cards were placed in the storage jars. After a few examinations or a single trip to a specialist, only the label remains. Cotton or paper materials in the container caps and lids, whose fibres adhere to and damage specimens, should also be avoid.