Benthic behaviour Gastropteron pacificum

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