Wednesday Weirdo: Watering Pot Shells

This tube belongs to one of the odder forms of Mollusca that this diverse phylum has thrown out.

Penicillus penis, a watering pot shell

Believe it or not (and yes, that’s its real name) this tube with what vaguely resembles the flower-attachment of a watering pot at one end, is a bivalve. A clam shell. Indeed ther bizarre group of bivalves to which it belongs are known as the watering pot shells (the proper taxonomic name for the group is the Clavagelloidea). To understand how such a major departure from the normally conservative two-part clam shell could have evolved it is important to note that many bivalve animals outgrow their shells. Just look at this geoduck.

With this in mind the tube of the clavagelloid becomes easier to understand. What needs to happen is for the shell-secreting mechanisms that secrete the ancestral shell to go crazy and extend way beyond the boundaries of the ancestral valves and coat the entire animal in a shelly tube. Indeed a closer look at the tube will reveal a pair of tiny ancestral valves embedded within it.

So in this picture the tube is upside down: the long siphonal tube that would reach up to clear water through the sediment is pointing to the bottom of the page.

Of course growing the tube isn’t  quite as simple as extending the shells, for one thing the tube is underlain by the periostracum – a flexible, non-mineralised sheet that lies on the outside of the valves (so anatomically speaking the tube lies above the valves – even though they are embedded in it – go figure). Just how the tube of watering pot shells is secreted has been a matter of debate. Surprisingly no one has ever found a half-grown watering pot, all that is known is a single tubeless juvenile specimen of just one species, all other known watering pot specimens belong to adults with fully developed tubes. So jusy how they grow is still a bit of a mystery. I spent a lot of time earlier this year learning all I could about these fascinating bivalves, so I will be returning to them several times. Eventually I can tell you why.