Really giant clams
Gigas clams can achieve over 48" (120cm) in length and weigh more than 600lbs (270kg). Clams this massive act like little complete reef structures of their own, supporting entire mini ecosystems. Isolated T.gigas have been found in the middle of otherwise barren sandy bottoms serving as "islands" for corals to anchor on their shells and recruitment sites for fish and invertebrates. A reef aquascape designed around a giant T.gigas would make an unique and lovely exhibit.
Few aquarists have seen mature T.gigas. Even the "big" specimens that sometimes enter our hobby are dwarfs compared to some of the 100+ year old behemoths found in the wild. As a point of reference, Waikiki Aquarium's famous "Gigas-77" T.gigas is only a svelte 170lbs. Or take a look at the video below of two monsters found on the Great Barrier Reef (2012). Both clams have mature, beautiful Acroporids growing on their shells.
To learn more about Tridacna gigas, read James Fatherree's Advanced Aquarist article.
And no, these monsters don't trap nor eat people.