It was not the first time I encountered a cooked but closed halaan clam. I was sure it was well done. Other clams were widely opened with a well cooked meat. Mom told me not to open it. Just set it aside and get another one. Cooked but closed clams are bad, has a bad taste and foul odor. I have not tried opening once ever since.
It is a good thing that bad clams are tightly closed and hard enough to resist breakage during cooking and serving. It does not impart any bad flavor to the dish. Hope no one is curious enough to break it intentionally.
Follow the following tips in case you want to spot and discard bad clams before cooking.
In public markets, halaan are usually sold fresh and alive. They are place in tub of water or on table. If on table, the person in charge is sprinkling water on them occasionally to keep them hydrated.
Live halaan opens after few minutes when submerged in water and keep undisturbed. Check for this feature if you want to be sure.