ClamsBivalvia

A clam is a mollusc that gave up its head. There is no face inside the shell: no eyes at the front, no jaws, and no radula, the toothed ribbon that nearly every other mollusc feeds with. What a bivalve has instead is a pair of gills grown far past anything breathing requires, until they became the entire feeding system.

Water is drawn in through one opening and driven across sheets of gill by millions of beating hairs, which strip out everything edible on the way. Particles are caught in mucus, sorted, and rolled toward the mouth along ciliary tracks, and the cleaned water leaves through a second opening. It works well enough that the animal can sit still for its whole life. The cost is that a clam cannot chase anything and cannot run, so the shell has to be enough, and here it is: thick, ribbed, spined, cemented into coral or buried in sand, and held shut by a muscle you will not pull open.

They are not blind. Scallops and their relatives carry rows of small eyes along the edge of the mantle, and those eyes do not work the way yours do: each one focuses with a mirror behind the retina instead of a lens in front of it. The coral scallop wedged into a Porites head watches you come in with a line of them and shuts before your hand arrives.

The giant clams turned the mantle into a field. Their tissue is packed with single celled algae, and the clam does more than house them, because cells in the mantle scatter and filter the sunlight and pipe it down into the flesh, so the crop deeper inside gets usable light rather than burning at the surface. The blues and greens are part of that machinery. Ten species are listed for this coast, from giant clams built into the reef top with the hinge down and the mantle turned up at the sun, to pen shells standing on end in seagrass with only a thin edge of shell showing.