Thornback Ray on a Zero Tide

The warm weather and low tide brought us back to the beach today. It was a zero tide, which, a tide-pool docent explained is calculated as the point between the highest and lowest tides of the year. This zero tide brought us a very rare treat - a Thornback Ray. 

 According to an article on California Diving News, "the thornback's genus name, Platyrhinoidis, comes from the Greek platys, meaning broad, and rhinos, meaning snout. The species name, triseriata, is Latin for three rows and refers to the parallel rows of "thorns" that run from the middle of the creature's back to just before its tail."

Here's an unanswered question: What causes these sand pile holes? My guess is some kind of worm. I've observed them as they are made, but the animal can not be seen. I have also tried to dig up the hole to find the creature, and it evades me. I have read that lug worms create a crater with a hole in it, but this is a pile with a hole in it. Maybe a similar species?

You can just catch a glimpse of one of the holes being cleared out in this video