scribbles in the sand

If you go back to 11 arms of death (November 2000) you will see that I found Eleven-armed Sea Stars Coscinasterias muricata feeding on small bivalves in the sand at low tide on a sheltered beach.

At first I was puzzled when I found these large sea stars out of the water. However, when I gently flipped one over I saw that it was holding a small bivalve in its tube-feet. I watched as the sea star used its tube-feet to move the bivalve towards its mouth - bye-bye bivalve.

Early in December I returned to the same beach to find out more about the bivalves on which the Eleven-armed Sea Star was feeding. I must admit that I was amazed to find that in places the sand exposed by low tide was full of weird scribbles.

Despite visiting this beach often, I had not noticed these scribbles before - amazing what you can see when you open your eyes. At one end of each scribble I found a small bivalve, most likely the Narrow Wedge Shell Amesodesma angusta, ploughing through the sand.

scribbles in the sand
between high and low tide
on a sheltered beach
(click thumbnail for full image)

At one end of each scribble you will find a small bivalve, shown here in the pink circle.
(click thumbnail for full image)

What I believe to be happening is that the bivalves are moving through the sand, feeding on organic matter as they go. Something I still need to find out is whether they only do this in the sand exposed at low tide.

I suspect this may be the case because there would be too many predators about when the tide is in. What I have discovered is that the Eleven-armed Sea Stars seem to know about the bivalves' feeding habits and deliberately come ashore to eat the exposed bivalves.