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iLabs: Visitor Question – When Do Aquarium Snails Develop Their Shells?

July 2, 2012

A visitor to the Micro World lab on Saturday asked a number of questions about our aquarium snails, including:

— what is the snail’s body like?

— when does its shell appear?

Here is a site with a great deal of information about aquatic snails – marine, brackish and freshwater varieties. The ones we have in our lab are freshwater varieties.  The site is from Olympus and has a couple of videos of aquatic snails along with a wealth of details on how each of the three types develops, how they are similar, and how they differ. For example, marine snails go through a free-swimming larval stage, whereas freshwater snails develop into their adult form right in the eggs and emerge, ready to go.

The site offers a good description of the snail’s body, including its strong muscular foot used to move around (including climbing, creeping, swimming and burrowing). As the foot muscle ripples, it secretes a slippery substance to help the snail foot glide along any surface.

As to the question, ‘when does the shell appear?’ — at least for freshwater snails, it appears they develop right in the egg so that when the snails emerge from their egg, they already have shells and are in their adult, if very tiny-sized, form. From that same site:

As the snail offspring develop in the clear egg case, an observer can watch the progress, including the growth of miniature transparent shells and organ systems such as the two-chambered hearts of the circulatory systems.

Our freshwater snails develop in eggs, which are in a sticky, gelatinous mass that adheres to glass and rocks. Here is a mass of snail eggs stuck on the side wall of our tank. For context, the black mass below it is a small piece of gravel. So that gives an indication of how small the embryos are as they develop:

snail eggs stuck on the side of the tank

Transparent, freshwater aquarium snail eggs in a gelatinous mass stuck on the tank wall. You can see the developing snails within. For size comparison, the black mass below it is one small piece of gravel.

Here is a closer look at the eggs in the gelatinous mass, magnified

snail eggs in gelatinous mass

Snail embryos in their eggs, which are held fast to the aquarium wall in a gelatinous mass.

Here are some close-ups of the developing embryos inside their eggs. The shell is visible in all.

snail embryo in egg with tentacles visible

Here is a snail embryo still in the egg. The tentacles are visible sticking out from under the developing shell and the eyes are at the base of each tentacle.

snail embryo within egg with foot and shell visible

Here is another snail embryo in its egg. The flat foot can be seen beneath the shell.

embryo snails in their eggs

Here are three embryo snails in their eggs. The foot is on top of the embryo on the bottom left. The eye can be seen and the body of the snail is visible showing through the  slightly transparent snail shell inside the egg.

I think it is safe to say that at least for some freshwater varieties, their shell develops in the egg case, along with the rest of the body parts and as the snail grows, both its body and shell enlarge. In marine varieties, there is a lot of variation as many have a free-swimming larval stage before they become adults. Apparently even the larval stages have a thin shell of sorts that will continue to enlarge and harden as the sea snail becomes an adult. Here is a very good website with more information on sea snail life cycles including the shell development AND ESPECIALLY, how the snail shell ends up twisted! 🙂

For some additional information on how seashells (and also turtle shells) are formed here is an article from Scientific American.

In any event, I hope this helps. 🙂

Coming up soon, the answer to another visitor’s question: How big is a Scanning electron microscope? …and why did they even ask this question!

Stay tuned!

One Comment leave one →
  1. July 2, 2012 1:56 pm

    Reblogged this on NC Museum of Natural Sciences Blogs.

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