What is this?

Dear Willy,

I have never seem anything like it, but I am pretty sure it is a strange bivalve, with an internal shell. Most bivalves, like oysters, cockles etc., have the shell on the outside, protecting their internal body. They often have two tubes [siphons] which funnel water in and out of the shell and mantle cavity, which houses their gills and feeding apparatus. In your animal it looks like the siphons have become the largest part of the body and are able to close off the opening of their tube and blow up like a balloon, and somehow swim.

If you look at the close-up alongside, you will see that the end of one of the siphons is partly open [compare it with the two siphons in the middle photo where their ends are tightly closed]. Also in the photo alongside you can see the whitish shell through the layer of skin covering it. The whitish lines on the shell are almost certainly growth-lines, deposited as the shell grew.

If it is a bivalve, I have no idea what it is but have emailed a few colleagues to see if they do. It is possibly something no one has yet seen alive.

Best wishes,
Bill Rudman