I was looking at those on Canada Corals too.
It's never good, generally the acclimate pretty quickly if they are introduced to the system slowly but they should only be gapping if unhappy for some reason.
Have you had a clam before?
How do you have it mounted?
Anything protecting the foot?
Can you see any obvious pinching or damage to the mantle?
from
http://animal-world.com/Aquarium-Cor...ridacnid-Clams
" Recently imported or transported clams usually exhibit a behavior called gaping. A gaping clam will appear as follows: shell fully open, mantle poorly extended, and inhalant siphon widely stretched. This eventually passes.
Gaping will continue if the clam is kept under insufficient lighting, is damaged, or unhealthy. The mantle will begin to pull inwards, shriveling and tearing between the siphons. A healthy clam's inhalant siphon can open wide sometimes, but gaping leaves a very wide opening.
The clam will stay like this for as long as the clam is unhealthy."