It only took us 15 years SCUBA diving around Koh Phangan before we finally managed to identify Oceanapia sagittaria! This strange adaptation is definitely not typical of members of the phylum Porifera…
The main body of the sponge is below the surface – what we see is the unique “capitate structure” where the water comes out (and from whence baby sponges are distributed).
We have now found a scientific paper written specifically about it, from the Gulf of Thailand – read here.