Giant Clams - a genus called Tridacna - has several species.  The flesh inside the two shells has two holes - one to take sea water in where it's filtered for any food like plankton or organic matter - and one where the "cleaned"  sea water is pumped back out as waste (along with the clam's waste).  Often spectacular colours, they have symbiotic algae in their tissues that also contribute to the clam's nutrition.  This design features the intricate patterns revealed by the burrowing clam (Tridacna crocea) of Low Isles, Port Douglas.  The are one of the many nutrient recyclers on a reef - ensuring that the precious organic matter in the reef's seawater is recycled within the reef and not lost out to the relatively barren blue ocean beyond the reef's edge. Some can be 1 metre long and live for a century, while others burrow into the coral and remain relatively small.