Preserving sea slugs

There are many reasons why sea slugs, especially those without shells, are difficult to preserve in a way that makes them useful for future research. In most mollusc groups, the shape and colour of the shell is a very useful, and often all that is needed to identify a species. In slugs however, it is necessary to know the shape and colour of the living animal, two characters often lost on preservation. Slugs are without an internal or external skeleton. If they are not narcotised (put to sleep) before being killed and preserved, they tend to retract into a shapeless ball. When dissected, the internal organs of such animals are usually so misplaced and jammed together, that an understanding of their anatomy is very difficult. Here are some basic instructions on how to preserve specimens so that they are useful for future research..

Preserving and fixing specimens.
The first step is to narcotise the animals so they don't contract into an unrecognisable ball when placed in preservative. Two commonly used techniques are menthol crystals and magnesium salts.
Menthol crystals Put slugs in a shallow dish and float crystals on the surface of the sea water. It may take some hours for this method to work.
Magnesium salts Make a saturated solution of Magnesium chloride or Magnesium sulfate, and gradually add to seawater. This is a much quicker method and if you add carefully can give a good result. [Magnesium sulfate = Epsom Salts]

I'll just mention the two common methods for general anatomy. For histology and DNA work more specialised techniques are required.
Formalin. I fix my material in 10% formalin overnight then transfer to 5% formalin for longterm storage. Formalin fumes can be unpleasant and it can sting if you have cuts on your hands. During dissecting I transfer the animal to water so that I don't breathe in the formalin fumes.
Alcohol. Many workers store their material in 70% alcohol (usually ethanol). Two disadavantages I find, are that it hardens reproductive structures so they are difficult to untangle, and if the specimen is dissected in water, mucus glands when broken or cut begin to rapidly swell and can completely obscure the dissection. For some reason the hardening does not seem to happen with land snail specimens.

I can't say whether alcohol or formalin is best, both work quite well.

The most important point is to photograph specimens alive, and if possible draw them and make colour notes.

Authorship details
Rudman, W.B., 2000 (July 17) Preserving sea slugs. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Related messages

Preservation of colour

January 30, 2002
From: Hakan Kabasakal

Is there any effective method is available to preserving the colours of nudibranchs under fixed conditions (e.g. in formalin or ethanolic solutions).
Best regards,
Hakan Kabasakal, MSc.
Private collector

Kabasakal,H., 2002 (Jan 30) Preservation of colour. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Hakan,
The only reference I know is to a paper by Gordon Robilliard who reported successfully preserving the color and pattern in some opisthobranchs with the anti-oxidant ionol (The Veliger, 11:288-291, 1969).

I am afraid there is no good way to preserve their colour other than by taking a good photograph. There are a number of techniques which have been suggested but none of them seems to produce consistent results. One problem is that the colour pigments are from different sources. Some pigments are made by the slugs, but others are taken from their food, and every type of pigment reacts differently to the preservatives. You describe yourself as a collector. My advice, if you are interested in nudibranchs, is to collect photos and new information about these animals. There is no way you can capture their beauty by preserving them in a bottle.
Best wishes,
Bill Rudman.

Rudman,W.B., 2002 (Jan 30). Comment on Preservation of colour by Hakan Kabasakal. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Re: Slug anesthesia

January 30, 2002
From: Brian Penney

Re: Kathleen's message.
I have had reasonable success taking damp weight of non-anesthetized individuals for my ecology work. Survival seems quite high for most species- few deaths over several weeks in captivity. My rationale is that, if the slugs need to be dried off a little bit, there is no need to compound the trauma by using anesthesia. However, I have worked mostly with large species that spend some time intertidally.

Another possibility is measuring volume, which could be done in salt water. I haven't tried it myself, so you might want to test the reproducibility of these measurements.

Best of luck!

Penney, B., 2002 (Jan 30) Re: Slug anesthesia. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Slug anesthesia

January 22, 2002
From: Kathleen Archer

Hi Everyone,
I am looking for a way to measure size and weight of my sea slugs over time. I have read in old papers about using "menthol crystals" to anesthetize slugs. This appears to cause them to relax so reasonably accurate length and width measurements are possible. Has any one tried menthol crystals? Can anyone suggest other methods that will not harm the animals?
Thanks for your help,
Kathleen Archer

Archer, K., 2002 (Jan 22) Slug anesthesia. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Kathleen,
There is some information on the Forum about anaesthetising slugs and I have had animals recover from treatment with both menthol and magnesium salts but I am afraid it doesn't work every time and there is a very fine line between sleep and death. As long as you can live with a degree of inaccuracy measuring their live crawling length is quite simple. I usually use a pair of old forceps or dividers to make quick 'head to tail' measurements as the animals are crawling along in a dish. Other people have the animals crawling in a glass dish hich is sitting on a page of graph paper so again you can measure the slugs as they are crawling along. Today with digital cameras I guess you could standardise a method so you could record your measurements at leisure. Obviously you wouldn't need to anaesthetise them to weight them.

My advice would be to avoid anesthetising them if possible as you don't know what effect doing this a number of times would have on the animal's physiology, and it would seem to be an unnecesary complication if you are trying to study the algal component as well. Plants may not like the increased magnesium levels or the menthol.
Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Jan 22). Comment on Slug anesthesia by Kathleen Archer. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from