Friday, March 14, 2014

Snail's Lunch

Mark Bower's upcoming exhibition of mushroom photographs at Springfield's Nature Center features a snail caught in the act of vandalizing one of our precious morels.  This led me off into the study of the snail's diet.  It was appropriately slow going.

I started by asking Dr. Chris Barnhart for the background on the perp.  He identified the snail as one of several possible species in the Polygyridae family.  These are air breathing land snails that make up a significant proportion of the land snail fauna of eastern North America.  They have eyes and  the absence of a "love dart,"  a detail that only a dedicated malacologist  (mollusk expert) or possibly Hugh Heffner would notice.

Land_snails  are the ultimate omnivore with a diet of live and dead vegetation, fungi, fruit, leaves, flowers, bark, algae, limestone, chalk, occasional carrion and even damp paper and cardboard on occasion.  Polygyrid specie's diet is said to be comprised mostly of microfungi associated with leaf litter.  
"Land snails and slugs may eat herbaceous plant leaves or stems; rotting herbaceous plants, leaves, wood or bark, including the fungi that live within these items; fungal fruiting bodies such as mushrooms or conchs; and coatings of fungi or algae on rock or bark.  Snails and slugs are also found eating animal scats and carcasses; nematodes; old shells of other snails; or snail eggs, shells, and flesh. In the Pennsylvania woods large snails such as the toothed globe (Mesodon zaletus) might be found upon white-tailed deer scats, while the gray-foot lancetooth (Haplotrema concavum) hunts and consumes live snails and slugs. 
Organic and inorganic soil and rock particles are also ingested by snails. Consumption of calcium-bearing minerals provides the nutrient that snails need to build their shells, which are mostly calcium carbonate with a protein outer coating, the periostracum."  Pennsylvania Land Snails
The wide range of foods probably reflects the relatively small home range of a snail.  Although a few species have been noted to "migrate," that is to seasonally move from a damp off-season home in the humus to a stump that has been rotting over several years, the majority live in a neighborhood of a few feet.  Under that circumstance, you can't be too choosy about your food.

Surprisingly little is known about the dietary preferences of most snail species.  As you can imagine, recording a snail's diet over weeks is research that moves at, dare I say it, a snail's pace.  Recent research however has emphasized the importance of at least one species eating its fresh vegetables.  In a study by Rebekah Martin and colleagues last year, they were able to show that a species of  Polygyrid land snails (Patera appressa) were stunted when consuming only leaf litter but caught up to normal size when fed fresh greens in the form of Romaine lettuce.  Their mother could have told them that, but who listens to mom?

Click to Enlarge
Click to enlarge

If you are fortunate enough to find a snail on a mushroom, look carefully behind it and you may be able to see a trail of slime.  Land snails, a classification that includes shell-less slugs, travel along on a lubricating layer of slime.  Slime is unique in that it is hygroscopic, meaning that it absorbs water rather than losing it.  Moisture is critical to these creatures which initially evolved in the water, so they seek moisture.  Many only come out in the night and most prefer damp areas.
Snail on a false morel- Mark Bower
Also remember that as with all wildlife, the fact that a snail can eat a mushroom does not mean it is safe or non-toxic to humans.  If you have trouble remembering this rule, just remember all the things your dog eats on a walk that you wouldn't touch.  Fido and snails are much more omnivorous than humans.
Video of a snail eating a mushroom.
Overviews of land snails are at The Living World of Mollusks and  Pennsylvania Land Snails.

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