Thanks John,
I know its not a sea slug, but it's good to be able to clear up this little mystery. I wondered about Solecurtus but couldn't see any signs of the annulations on the siphons which I would have expected to see in that genus. After looking again at Willy Vanderweert's photos I can see that there are rings of white marks which must indicate the annulations normally found in species of this genus. For those of you unfamilar with Solecurtus, the 'annulations' I speak of are regular grooves around the siphons which make them look segmented. When attacked or nibbled by a predator, the bivalve is able to break off one or more of these segments as a replaceable sacrifice. [see autotomy page]. Clearly when the siphons are so inflated during swimming, the grooves disappear.  One apparent difference between this species and Solecurtus strigilatus, is that the shell is covered with mantle lobes, - something  not present in S. strigilatus.

Best wishes,
Bill Rudman

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

Dear Mirjam,
I must say when I first saw your photo I was a bit puzzled - and wished I could touch the animal. In the field, my first reaction to something puzzling - after watching it for a bit - is to gently touch it to see what it does. I am sure if I touched this it would have closed up because I think this is a bivalve of some sort - perhaps a Spondylus [= Thorny Oyster] a group which have spectacularly colourful and elaborate edges to the mantle flaps which line the inside of their shells, and in fact secrete the new shell as the animal grows.

Best wishes,
Bill Rudman