Shiny, curly shells

We are staying in the mollusc family for a while, but moving on from the limpets we met in the last two posts.  Today and Wednesday are about a particularly interesting mollusc, which has been valued by humans for a very long time!

I would like to introduce you to the Cowry, or Cypraeidae.  When I first started writing this post, I was not sure how much I would have to put in it, but, as I have been researching it, I have found more and more interesting stuff!

Onyx Cowry (Erronea onyx), found in the warm waters of the Pacific and Indian Ocean, from Australia to Madagascar. Image from Wild Singapore

This amazing looking animal is Talparia talpa, and the shiny glowy thing you can see is its shell!  The black bit is its mantle (The fleshy bit from which the shell is secreted)

Talparia talpa, with fully extended mantle. Image from EOL (Encyclopedia of Life)

This animal has a very glossy shell in comparison to other molluscs because, instead of secreting from the top of the mantle (The mantle is the fleshy part protecting the internal organs), this shell is secreted downwards, other molluscs secrete their shells from the bottom up, so the inside of their shells is nice and glossy and shiny (the inside of a Conch shell for example), Cowrys start secreting from the top down, meaning the top layer is the newest, and so nice and shiny and glossy.

Most mollusca (the group which the Cowry belongs to) have a mantle as a single layer of flesh under their shell, as in this diagram:

General Mollusc Anatomy: Image from Marine Education Society of Australia

In this group of molluscs however, the mantle forms two halves, which can be clearly seen in the picture with the black mantle.  This wraps around the shell from each side.  The reason it is not fleshy coloured, as might be expected, but can come in a wide range of textures and colours is for camouflage.  Many of these animals will have mantles exactly matching the sponges and corals on which they live!

I have been digging around to find out why these animals may have their shell on the inside, rather than the outside, and have not been able to find a definitive answer, although, if I was asked to suggest a reason, I would say that the camouflage possibilities of a bit of skin are more diverse than those of a shell, which once secreted, is fixed, and that it does not really matter if the shell is exterior or interior if its function is to protect the internal organs.  Also, a shell is not really very good for sensing surroundings, and the mantle could maybe be sensitive to changes in the surroundings indicating predators, other dangers or food approaching.

Next time I will write about how these animals have played a role in human society for a very long time, and their current global status.