Dear Jan,

Thanks for the great photos - and the humour

Best wishes,
Bill Rudman

Dear Lily & Erika,

I have a Fact Sheet on swimming but I am afraid at present it just contains a list of references - although it does have a composite photo of Sagaminopteron ornatum swimming. Haefelfinger & Kress (1967) analysed the swimming behaviour of the Mediterranean Gasteropteron rubrum by meticulously studying a film of it swimming, frame by frame. Wes Farmer's summary swimming in gastropods (1970) may be more accessible to you as it was published in The Veliger.

The name Gastropteron, which basically means 'stomach on wings' is quite appropriate. Quite a few unrelated opisthobranchs have evolved wing-like extensions on the sides of their foot - which we call parapodia - and quite a few of them have adapted them for swimming, including many species of Aplysia [see A. fasciata, A. brasiliana], Akera [see Akera bullata] as well as species of the Gastropteridae. We don't know what the function of swimming is in various species. Perhaps it is just an escape mechanism when they are disturbed, or perhaps in some species it is an important means of locomotion.

If by chance you have photos of this species swimming, they would be a nice addition to the Forum

• Farmer, W. M. (1970) Swimming gastropods (Opisthobranchia and Prosobranchia). The Veliger 13(1): 73-89.
• Haefelfinger, H-R. & Kress, A. (1967) Der Schwimmvorgang bei Gasteropteron rubrum (Rafinesque 1814) (Gastropoda, Opisthobranchiata). Revue Suisse de Zoologie, 74: 547-554.

Best wishes,
Bill Rudman

Dear Jackie,
Thanks for this interesting photo. Sitting like this, the Gastropteron looks quite like a little bivalve with an anterior inhalent siphon drawing water into the mantle cavity and a posterior exhalant siphon expelling 'used' water. Functionally, that is exactly what is happening, but anatomically it is a bit different.

If you look at photos of Gastropteron chacmol [see #12363] you will see that gastropterids have a head shield with a pointed crest in the posterior midline which is folded into a tube, or siphon.  The many photos of Siphopteron flavum on the Forum also give a good idea of the general shape of gastropterids. The back half of the gastropterid body contains the body organs, and the remnants of the shell, mantle cavity and gills. In many ways it is similar in layout to the sea hares [see Fact Sheet ]. In both groups the large parapodia, which can act as swimming organs, can also wrap around the body enclosing the shell, gills etc in a large water-filled cavity. The 'oral groove like structure' you ask about is the siphon which I mentioned above, on the back of the head shield. It acts as an inhalent siphon directing water down into the water-filled cavity formed by the parapodia. You will note that I do not call this cavity a 'mantle cavity' because it is not. All that remains of the true mantle cavity in both these animals, and sea hares, is the small crowded space beneath the remnant shell in which the gill is hidden. [For more on mantle cavities see Torsion Fact Sheet].

Your Gastropteron is one of the largest species in the family. At present we have no idea what any species feeds on, so this species might be easier than many to catch feeding. Can you put it on your 'to do' list? It would be very interesting to know. I know of no identifiable stomach contents being reported so I suspect they may feed on small flatworms or other invertebrate with no hard parts - but that is just a guess.

Best wishes,
Bill Rudman

Thanks Marli,
Quite a spectacular photo of it swimming. For anyone unfamiliar with members of the Gastropteridae, have a look at the photos of Sagaminopteron ornatum. Gastropterids have very large lateral extensions of the foot, called parapodia, which can flap vigorously, enabling the animal to swim, a bit like an out-of-control butterfly. This photo is looking at the animal head on.

A literal translation of the name 'gastropteron' would be 'winged stomach', which I am glad to say has not become a widely used common name.
Bill Rudman