Thanks for the comments everyone!
Scott - That certainly tallies with my my observations. As Mike says - seeds. In the case of this particular rock, it won't be there for long as the next decent swell will reconstruct the outlet of the river.
Mike - Thanks for the link and the the story. It's always a smile to celebrate our own foolishness.
And now a bit of additional natural history. The rock in the image is in the outflow of a fresh water pond trapped behind a cobble bar that separates the pond from the bay. In Atlantic Canada these ponds are known as barachois which apparently derives from the Portuguese (small bar). As I indicated, the bars are very dynamic and every time we have a decent swell the bar and outlet change significantly. We live about 1/2 km from the water and when the swell is on you can hear the large cobbles rolling in and out with the swell.