12 May 2009

Gastropods at 100% humidity

In this post, I wrote about the detailed drawings of experimental setups that they used to include in the old scientific papers. Here is another such drawing, this time from a 1934 paper*.


Howes & Wells, the authors of the study, noticed that there were irregular changes in the body weight of the snail Helix pomatia over many days. They determined that the weight changes were mostly due to changes in the water contents of the snails. Then they wanted to see if the snails kept in an atmosphere saturated with water vapor would still undergo similar weight changes**. The figure above shows the setup they used to create a container of air at 100% humidity.

Of course, the simplest thing to do would be to seal the snails in a jar partially filled with water. But they intended to keep the snails in there for many days, so they needed to supply the animals with fresh air also saturated with water vapor. That’s what the tank labeled A did: the water inside the tank was heated and then the air saturated with steam was pumped, after it was cooled down, to the container labeled D in which were the snails. There was also water at the bottom of that container and even a “wooden bridge for the animals to climb up and down on” (that must be that stick dipping into the water). Finally, the tubes in part F contained a manometer to regulate the rate of air flow.

Last weekend I did an experiment with slugs to determine if they could absorb water vapor from air saturated with water. Basically, I needed a setup similar to the one Howes & Wells used, but because I was going to keep the slugs in the saturated air for not more than 6 hours, I went the simpler way. Here was my setup.


The slugs (Megapallifera mutabilis) that had been dehydrated previously (using Drierite) were suspended from a mesh sealed around the cap of a large plastic jar (that once held chocolate biscotti). There was a shallow layer of water on the bottom. The mesh prevented the slugs from contacting the water; they were exposed only to the water vapor in the air.

And here is an electronic hygrometer recording the humidity and the temperature inside the jar thru a hole in the lid. (The hole was otherwise sealed with tape.) The humidity was at 99.9%, the end of the scale of the hygrometer.


If I repeat the experiment, I may consider supplying the slugs with fresh air. But it would take some thinking and tinkering before I can come up with a modified setup that would do the trick without being not so complicated as that used by Howes & Wells.

*N. H. HOWES & G. P. WELLS. The Water Relations of Snails and Slugs: I. Weight Rhythms in Helix pomatia L. J. Exp. Biol. 1934 11: 327-343. (pdf from this page.)
**Yes, the snails' weights fluctuated even in saturated air, but the amplitudes were smaller.


xoggoth said...

Those little aeration pumps and blocks from aquarium shops are cheap. Smaller bubbles through water must saturate pretty well without heating as in that jar A.

PS Saturated air prevents drying of course but how would any animal absorb water from saturated air unless it had some special hygroscopic component? What was the result? I bet they don't.


No, the slugs didn't absorb water from the air. But Machin (J. Exp. Biol. 41:759-769, 1964) claimed that mucus isolated from snails absorbed a small amount of water when exposed to saturated air for a week. Any aqueous solution would absorb water from the air; it's a thermodynamic requirement.

xoggoth said...

Did thermodynamics a few decades back as qualified as a Chemical Engineer but very rusty now. Raoult's law? However, I would geuss that very little of the non aqueous part of a slug is solute, it is mostly colloidal proteins/fats, so water absorbtion would be very small.