Preserving jellyfish for morphological analyses
Scyphozoans are typically preserved for morphological analyses in a solution of 4% formalin in seawater (i.e. 4 parts formalin [37% w/v] and 96 parts seawater). Excess 4% formalin solution is used, and it can be renewed after two weeks to ensure successful fixation. If a specimen is also to be used in DNA analyses, then tissues for DNA analyses must be subsampled prior to fixation in formalin because formalin rapidly degrades DNA making it unsuitable for most molecular studies.
Some scyphozoans are very fragile and so utmost care must be taken in handling them both before and after fixation to prevent damage. In general, this means keeping them suspended in liquid at all times, never handling them out of the water. One modification to the method above which provides some protection to specimens after fixation is to preserve them in 4% formalin in seawater with 2% agarose. The agarose should be added to the seawater, microwaved until fully dissolved, and then allowed to cool until it can be handled comfortably with bare hands (but is not yet solid). The formalin is added (to 4%), the liquids mixed thoroughly, and then poured over the specimen. The preservative solidifies around the specimen, providing physical support. The method may not work with very fragile species, and may not be necessary with robust species.
Specimens preserved in formalin should be stored in sealed containers that are resistant to (or protected from) breakage, in a cool, shaded, well ventilated area. Ideally, they should be deposited at the nearest major natural history museum.
See also http://www.medusozoa.com/curating.html
Note, though, as stated above, that it is not always necessary "to make a choice, whether to have a reference specimen or a DNA sample, as the two are not yet complimentary." Most scyphozoans, even those just a couple of centimetres long, are sufficiently large to take a small tissue sample for DNA analyses and fix the rest in formalin for morphological studies. Having both samples from the same specimen is ideal.
WARNING! Formalin is highly toxic and should not be used without adequate protection, including overalls, gloves, and eye-glasses in a well ventilated space e.g. fume cupboard). Please consult the Materials Safety Data Sheet (MSDS) prior to use.