These guys are usually frustratingly fast, closing the minute you even think about taking their picture.  I gave up trying a long time ago.  Then I started using the 200, and on this trip I returned to the oysters.  They're still too quick for me almost all the time (I want a rebreather!) but this one let me get fairly close.  I tried once and was too far away, but I saw that this wasn't the fastest oyster on the planet.  I took a lap around the bommie and tried again.  Bingo.  I took another lap, but it clamped shut when I came around the corner.  Even an oyster can learn.

Nikon F4, 200mm f/4D Auto Micro Nikkor,  Aquatica Housing, 2 Ikelite 200 strobes, f/16, Kodak Royal Gold 100.