Newbury, Massachusetts The Great Marsh
I had to close my eyes to the spray of ice as I slammed the claw hammer into the 3-inch ice that armored the marsh. I use the claw to peel back ice chunks and hunt for life. Ten minutes later I find two winter denizens of the Great Marsh. I de-glove and carefully remove a thin-legged wolf spider (Pardosa littoralis) and a coffee-bean snail (Melampus bidentatus) from the grass stems. They are not dead, but possibly as near as you can get. Their organs are slowed, their metabolism at a glacial crawl. They are, for now, frozen in time. One of them quite literally (and I mean ‘literally’, not ‘figuratively’). All overwintering animals, including you and me and your little brother, have two choices when it comes to surviving the winter. Freeze avoidance or Freeze tolerance. Freeze avoidance usually means finding a shelter for the winter (called a hibernaculum) such as a burrow below the frost line or a shelter with a thermostat cranked to 72 F and stocked Rolos and oatmeal chocolate chip cookies (no. not raisins. raisins belong in only two places: bran and the trash). Freeze tolerance usually means that you’ve got to freeze some-to-most of your body. Some frogs and salamanders and many intertidal molluscs (snails, clams) do this.
But what’s wrong with freezing? The biggest issue with freezing has to do with ice crystals that form and ice that expands. Ice crystals are assassins that form inside the cell and use their stiletto-sharp crystals to rupture and kill cells. So whether you’re a freeze avoider or a freeze tolerator, your imperative is to protect your cells. Too many dead cells equals one dead you. Both the snail and spider are freeze avoiders in terms of their behavior: in the winter they are ‘snuggled’ together under grass clumps that are slightly warmer than the surrounding air. In terms of their internal physiology, the snail is a freeze tolerator and the spider is a freeze avoider.
Avoiding the freeze
Many insects and spiders (not all) produce anti-freeze compounds (some insects actually produce the same anti-freeze we use in our cars, ethelyene glycol), which lowers the freezing point of their body fluids. At the same time, they will dump sugars into their body fluids which also lowers the freezing point (anytime you dump sugars or salts into water, it lowers it’s freezing point). So when it’s 18 F, the spider still won’t freeze though freshwater will. If I hold this spider in my hand long enough or bring him inside, he will eventually start crawling around (though he’ll be really confused and very hungry).
Tolerating the freeze
The snail actually freezes part of the body to save the rest. Part of the fluid outside of the cell (extracellular fluid) freezes which forces body salts into the liquid part of the extracellular fluid. This creates an osmotic gradient with higher salt content outside of the cell versus inside the cell. Because Nature seeks balance, the water inside the cell moves out of the cell until the salt levels are equal in and out of the cell. This cell dehydration then increases the salt content of the cell, which in turn lowers the freezing point of the cell. Again, saltier or sugarier solutions have lower freezing points than freshwater solutions.
Wood frogs (Rana sylvatica) perform a similar feat and people have claimed to have found ‘frog-sicles’ under logs. The take-home is that the frog, just like the snail, is not completely frozen. It can’t be or it would die. It may, however, be mostly frozen (up to 65% of it’s body liquid) to protect the cells. Most of the cells inside are not frozen despite that fact that the soft bodies of snail and the frog are now hard as rocks.
In both freezing strategies, animals are trying to protect their cells and do so by lowering the freezing point of some portion of their body.
We humans do something similar so we can salt icy roads. The salt melts the ice because it’s lowering the freezing point of water. The first guy to do this probably thought he was pretty clever. The coffee-bean snail would shake his head and say, “no, no you’re not. i’ve been doing it for millenia.” Then the clever guy would say, “Holy crap! A talking snail!”
Thanks to Duley for spending more time than either of us should have trying to craft the perfect snail jokes. And perfect they are!