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| White-lip globe snail |
We encountered these snails clinging onto a sycamore
leaf in the midst of mating last week. Chris Barnhart identified them
as white
-lip globe snails,
Mesodon thyroidus, members of the
Polygyridae family
. They
are land snails, species that left the water and lost their gills long
ago when they evolved into air breathers. They are equipped with lungs
and are therefore called pulmonate. In case you have forgotten some of
your snail anatomy I've attached a reminder below.
When we first saw this pair there was a bright blue band connecting them, their intertwined copulatory
organs filled with their blue blood. By the time I had my camera out
they had separated, leaving their pale white organs dangling, but Alan
Cresslor caught a pair like ours
in flagrante delicto in the photo below. The whole subject of
snail reproduction gets very complex and I won't get into their
love darts which are not fodder for a family blog like ours.
Most land snails are hermaphrodites, equipped with both male and female
organs. Some species may reproduce without mating but other species like our
M. thyroidus transfer
their sperm to the vagina of another snail via the penis seen above.
We
kept these snails long enough to get them to the WOLF School. I
followed one attempted escape as it climbed the wall of a small plastic
aquarium. In the midst of its journey it deposited a small green pile,
then crawled over it. The opening on the left (the snail’s right) is
the breathing pore (pneumostome). Its anus is located in the
pneumostome!
We are talking bad breath! Chris Barnhart tells me that "they often stand on the feces like that. I’m not sure why, but it
might have something to do with recovering salts. They can absorb ions
through the foot."
When you move as slowly as a snail, you
can't be too particular about what you eat. Most tend to be omnivores
eating plant material, fungi, and anything in their path. Some snails
are even
predatory! Our little friends have been rasping away at the tops trimmed off of our sweet peppers. They use their
radula,
a tongue equivalent with rasp-like teeth built for scraping the surface
of food into the esophagus, but even here there is tremendous
differences between species.
"The radula is a membrane covered with series of tiny teeth made of
chitin, so it is coarse like sandpaper. The shapes of these small teeth
are used to help identify some snail species. However, the teeth of the
radula should not be confused with denticles in the aperture of a
snail’s shell, which are often referred to as “teeth” as well.
The radula is drawn over a ridge of cartilage (the odontophore),
somewhat like a chainsaw chain slides around its bar - though it moves
back and forth rather than in a circular motion. Bits of food are broken
off and drawn into the snail’s esophagus for digestion. Several bouts
of crawling and feeding can occur during an outing.
If you allow a larger snail to crawl on your hand, you may be able to
feel it “taste,” or rasp, your skin – the sensation is painless, but
feels like a cat’s licking." Carnegie Museum of Natural History
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| Don't put snails on the table cloth. |
And now the
slime. It was best demonstrated on a colored tablecloth which one of our snails escaped to after climbing out of the aquarium.
Note to self: Close top of aquarium to maintain domestic harmony.
Since leaving its aquatic origins, preservation of its water has become
critical. A snail's slime is essentially mucous composed mostly of
water protecting it against evaporation. The
slime can serve as a lubricant or an adhesive. When one of our snails
tried to escape across the table it left a trail of adhesive slime that
lifted the tablecloth when I tried to pick it up. The chemistry can be
far more complicated as in the common garden snail,
Cornu aspersum.
In the case of Cornu aspersa,
the discharge is composed of synthesized products from various types of
secretory glands. These are all single cell glands found in connective
tissue and secrete their products via pores that pass between the
epidermal cells. They are of various shapes and usually have a long
excretory duct. There are eight different types of secreting glands.
Four of these different types of mucus secreted protein, calcium,
pigments, and lipids. Wikipedia
There is a sinister side to little snails like this. Brainworm,
Parelaphostrongylus tenuis, is
a parasitic nematode commonly affecting the brain of white-tail deer.
The worm lays eggs in the meningeal tissue of the deer which pass into
the lungs where they hatch. The larvae then are coughed up and
swallowed, and eventually passed out in the feces. Slugs and snails
pick up the larvae which live in them as intermediate hosts until the
mollusks are eaten by deer,
starting the cycle again. They pose no risks to the snail or humans.
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| The End |
Our snails are now living under a large rotting oak
log, hopefully rasping away at fungal hyphae and sharing the story of
their visit with the 5th grade class. You can watch the snails move at a snail's pace x4
here.
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Chris Barnhart shared
this extensive set of photographs of
Sonorelix sp. desert snails mating.
Resources
The Living World of Mollusks
More on the White-lipped Globe Snail is in this
University of Florida link.
Gastropod reproduction
Snail anatomy, the source of the diagram above.
