Siphopteron flavum feeding?

Dear Ayumi,
Thanks for these photos. I am afraid I can't tell whether the pale whitish lumps are small sponge colonies or not. They could be, or they could be the tubes of small polychaete worms encrusted with sand grains, or even small bryozoan colonies. All these animals can form lumps like this on branching algae.

We don't know what any gastropterid [Gastropteron, Siphopteron and Sagaminopteron ] feed on, but I think they are closely related to the carnivorous aglajids, philinids and scaphandrids, which all feed on particular prey, including polychaete worms, flatworms, sea slugs, and small bivalves. The shape of the teeth in gastropterids is very similar to that of philinids and scaphandrids so I am pretty sure they feed on something similar and are not sponge feeders.

Perhaps they are feeding on polychaete worms living in burrows or tubes? There certainly seem to be a couple of holes visible in your close-up photo. These Siphopteron are quite small so it is difficult to see precisely what they are doing in the field. Unfortunately though, I think that is the only way we are going to discover what they eat. All I can say is good luck and please keep looking.

Best wishes,
Bill Rudman