What happened to our clam?

[doublette]

Literally overnight our crocea clam went from a thriving beautiful clam to this.

On Monday night the clam looked really good, no signs of anything being wrong. Came home Tuesday and saw this. Came home Wednesday and it was nothing more than the shell. It had been in our tank since the beginning of January. There has been no change in our water parameters, the first thing I did was check the water. No other corals are near by. Tank inhabitants include a powder blue tang, two false percs, a cleaner goby, a cleaner shrimp, a two-spot goby, a mandarin, 1 sea hare, various snails, and some scarlet and blue leg hermits. A water change of 20G was done the previous saturday. Everything else in the tank looks and is acting fine. I'm stumped as to what happened. Any ideas?