Monday, July 20, 2009

An eelgrass mystery.

Yesterday there was a very low, mid-morning low tide at Boundary Bay. The weather was warm and sunny, so off we went to walk on the intertidal mud flat. You can walk a great distance out, and eventually will come to large, shallow pools of standing water that contain thick patches of Zostera (eelgrass). Eelgrass is where the action is.

Within and beneath the Zostera are small fish, crustaceans galore, mollusks a-plenty, and my favourite intertidal beast, the Western Sand Dollar, Dendraster excentricus.

As you near the Zostera beds you start finding the skeletal remains, the tests, of sand dollars. And then, once at the beds, if you poke around a bit, you should find live ones, which look as though they're carpeted with purple-brown velvet--a covering made of thousands of tiny spines. I have written more on their morphology previously.

Yesterday, for whatever reason, my son and I couldn't find a single live one. We did, however, find dead ones.

Thousands of dead ones. These are in about six inches of water.

I don't know if this is something to be alarmed about. For all I know there may have been thousands of live ones buried beneath the silty sand. I was carrying a camera and keeping my hands dry, so didn't want to dig through the sediment to look.

Most of dead tests were close to full-sized. Perhaps this is normal die-off---although tests of smaller individuals are more fragile, and more likely to disintegrate, so may have disappeared. Were these all stranded by an extremely low tide earlier this summer? Sand dollars exposed to air and sun don't last long.

I envy those who live in the houses that line the bay. There's not just an ocean view, there's a hobby out there, an endless series of questions to find, perhaps even to answer.

Sand dollars, dead or alive, are pretty things.


Mary said...

What an interesting place to explore. I love the title of your blog :-)

spinyurchin said...

I always think that too; houses right on the shoreline...all that potential...