Tridacna clams belong to the family that is supposed to grab your foot if you're a careless diver, and hold you trapped until whatever, which is rubbish.
There are some truly enormous ones, Tridacna gigas, which might actually do that after a bad night out on the town, but the more usual species in the Philippines, and around the Indo-Pacific, is Tridacna squamosa, the Fluted Clam.
It's usually quite large, and full of good meat, which I don't really understand. I've never come across a Tridacna teenager, or any other juvenile, although, of course, there are small(-ish) ones, that perhaps I ignore.
The ones shown in the above photo come from Surigao City market, where they are sold in the 'cheap corner'. (That's why there's some seaweed on offer at the front).
But de-shelled, the reasons for eating this shellfish become very, very obvious.
It's nutritious, of course, but I suspect it's visual qualities have a lot in common with full frontals published by the likes of Larry Flynt.
You wouldn't find this shell in ancient shell middens, because it's too damned heavy to carry back home.
All you need is something to cut the joint muscle between the two halves of the shell. That could be any old bit of stone or wood that you can pick up.
Then you take home the meat, and leave the shell behind.