So… ever since our first deep dive here in Pohnpei (which seems like ages ago), we’ve consistently seen these strange flakes on the sandy slope below a depth of about 270 feet (80 meters). Being the fish and gorgonian nerds that we are, we initially only had a vague and passing interest in them. However, as we continued our deep diving here, it became increasingly difficult to ignore them.
We’ve brought some up to the surface for closer inspection, assuming that we’d be able to figure out at least generally what they are. But our team of biologists remains stumped. They look and feel (but don’t taste) like potato chips. They are extremely abundant within their habitat, which is sand and rubble slopes below a depth of about 270 feet (80 meters). On one recent dive along the outer reef slope on the northeast side of Pohnpei, Rob and Brian saw an area where the entire bottom was literally completely covered by them in all directions between about 270-315 feet (80-95 meters); they said it was an eerie sight.
We see them on the bottom in sizes ranging from about 3 millimeters (1/8 inch) in diameter to approximately 5 centimeters (2 inches). In areas where they are not completely covering the bottom, they appear to occur in clusters of several dozen, analogous to the way that mushroom corals (Fungia) are often encountered on the reef. They are not attached to the substrate; they are just sitting out in the open forming part of the loose material on the bottom.
We’ve brought some up for closer inspection. They are radial in structure, with concentric rings (presumably growth rings). They are usually white in the center and brown over most of the rest of the disk, with a thin white outer edge. Both sides seem similar (i.e., there does not seem to be a “top” and a “bottom”). The brown fades a bit after we dry them out, but we are confident they were alive at the time of capture, based on the fact that we sometimes find them covered in encrusting organisms (suggesting that when they are not encrusted, they are still alive).
We do not have proper microscopes with us, but when we break them, they appear in cross-section to consist of two thin “halves” with a dark brown layer in-between. They are not always flat; sometimes they form extrusions off the plane of the disk.
What are these things? Coral? Algae? Forams? Something else? We welcome any suggestions about what manner of life-forms these things might be.
UPDATE: There is a growing consensus that this is, in fact, a foram (Foraminifera). In fact, that was the most popular guess among our group before I made the post. But many thanks to several individuals (particularly my old friend Stan Jazwinski and also Valéry Malécot, who pointed us to Marginopora vertebralis as a close match).