After whining last week about my tribulations with molluscan cells, my advisor, Herr Doktor Professor T. Ryan Gregory sent me two emails. One basically said "stop kicking ideas around and do something, you slacker!" and the other included contact details for a mollusc-oriented mailing list, the "molluscalist".
So I sent off my questions to the molluscalist, and the wonderful world of malacology replied with many excellent suggestions, and links (or PDFs) to relevant papers that my earlier literature searching had not found. The upshot of most of this is that menthol crystals, crushed and sprinkled on the water's surface, act as a good "narcotizing agent" for freshwater invertebrates, especially snails. The basic protocol is to sprinkle a small amount (less than one gram) of menthol crystals on the water that contains snails (e.g. in a beaker), and wait roughly 24 hours for the snails to "relax" so they can be removed from their shells without damaging the soft tissue or the taxonomically-important shell.
I tried menthol this week. I ordered some from Sigma-Aldrich* on Wednesday, and, amazingly, the menthol arrived yesterday. So I took some locally-collected 'pond snails'** from the 'fridge where I'd been slowing them down, and dropped 1g of menthol on them. Today I dissected the snails and smeared some of their cells onto glass slides. The menthol worked very well - I think I actually overdid it, given the relatively small volume (perhaps 50 mL) of water the snails were in. They were apparently dead - no response at all to mechanical stimulation (I poked them with an insect pin). A pin bent to a hook was a very useful tool for 'winkling' snails out of their shells - it worked much better than I expected.
The downside of the dissections was (as is so often the case) the smell. Dead freshwater molluscs smell unpleasant, but these had a minty-fresh overtone from the menthol that interacted to provide a truly awful stink. I may have trouble brushing my teeth this evening when I smell the toothpaste.
I also used the menthol treatment on a caddisfly larva*** I had also collected. I think it was much more susceptible to the menthol (have I mentioned how chemically tough snails are?), as it looked like it had died sometime quickly after the menthol went in, and was very floppy and difficult to dissect. I don't know how floppy they are when they're alive, though.
The dissections went fairly well, to the point I could sort-of identify some internal organs of the snails, a big surprise to me given that when the shell has to be crushed to remove a snail, the resulting blob of tissue is a homogeneous mess. I'm pretty happy with this method; one major advantage of menthol relative to other potential narcotizing agents is its total harmlessness to humans - it's a food additive, and the LD50 value for rats is so high that it suggests the toxic effects are a result of osmotic balance disruption, rather than any particular cell-physiology impact, besides being a mild anaesthetic.
* Edit June 23 2011. I received an email from a representative of Sigma-Aldrich Corporation, requesting I change my link from "Sigma" to "Sigma-Aldrich" as that more properly represents the company's name and brands. I have made the change, and I'd like to apologize to Sigma-Aldritch for the oversight. And I'd like to thank them for taking the time to read my little corner of the internet!
** My inter-library-loaned copy of Burch, 1980 and Clarke 1981 helped me to fairly-confidently identify the snails as Pseudosuccinea columella, Fossaria parva, and Stagnicola elodes.
*** I used Pennak 1978 to identify (after three tries - can you tell easily how many sclerotized plates cover the mesonotum?) the caddisfly larva as genus Heterophylax.