The flashing is reflected light off silica spheres in the mantle, not bioluminescence.
A file clam to about 5 cm across the shell, almost always wedged back into a crevice so that only the mantle shows: a dense fringe of long orange-red tentacles and, along the mantle edge, a line that appears to flash white two or three times a second. Nothing else in the region does this. Other file clams — Limaria and the other Ctenoides — show the same red tentacle fringe from the same sort of crevice without the flashing, and that is the whole of the identification.
The mechanism was settled in 2014. The inner face of the mantle lip is packed with silica spheres about 340 nanometres across, which are efficient broadband reflectors; the outer face has none and absorbs blue light. The clam furls and unfurls the lip, alternating mirror and dark, and what is seen is ambient light thrown back rather than light the animal makes. It is not bioluminescent, and the display is therefore brightest under a lamp and absent in true darkness. What it is for remains open — attracting prey, signalling to other clams and deterring predators have all been proposed and none demonstrated.
Recorded from about 3 to 45 m, usually in twos and threes in the same crevice, on rubble slopes, ledges and wall cracks rather than open sediment. The animal withdraws when a light is held on it for long, so the display is best watched from a little back and with the beam off to one side.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.