Did you notice how the abdomen looks "fuzzy"? This spider is carrying its young, so those are tiny little spiderlings clinging to their mother.
Some of the young spiders let go and started running (and I mean running!) among the sand grains.
One stopped briefly in an empty mussel shell:
Here's an even closer view of this spiderling that's smaller than a sand grain (an admittedly large sand grain):
I'm not sure about the identity of this spider, but I'm guessing it's a species of wolf spider (Family Lycosidae). If you're familiar with it, I'd love to learn which species it is.