Mysterious pulsating sand

[argan]

I was watching my tank just before the 10k lights went out for the night, and I noticed a small depression in the sand moving somewhat rhythmically. It moved around for about 30 seconds then seemed to stop.

I don't have any small nassarius snails or cucumbers, so maybe it was the pistol shrimp I added yesterday? Very curious.

Anyone want to add some pistol shrimp anecdotes? Or weird sand moving stories.