Sunday, June 2, 2013

Bounding about a bay

A few weeks ago, a friend convinced me that we should go to the beach.  As I hadn't done that in a while, and low tide was conveniently in the afternoon, we decided to head to Boundary Bay.

The low tide measured about 1 metre, which in the tidal flats of Boundary Bay means about a kilometre to walk out from the beach out to the water's edge.  That meant plenty of tidal pools to explore.

Eel grass provided habitat for many animals, including nudibranchs (the first time I've seen them in tide pools)

Anemone (1cm wide)

A mass of eggs of something
As we made our way out to the water's edge, we saw more and more sand dollars.  Most of you have probably seen them either at the beach or a souvenir shop - a white, chalky disk.

A sand dollar test on the North Coast Trail
This is merely the "test", or skeleton, of a sand dollar.  Sand dollars are relatives of sea urchins, and like urchins, are equipped with spines when alive.  By the time most of them wash up on the shores of beaches, the abrasive sand scrubs off most of the spines.

Live sand dollars will live where the water is fresher, in the lower portion of the intertidal zone.  It was here that we found living sand dollars.

With the spines, one can easily see the resemblance to sea urchins.  However, the spines are tiny and much more densely packed than that of an urchin, and they are used more for digging than protection.  With a coordination similar to that of a sea star (sea stars, sea urchins, and sand dollars  are all in the echinoderms phylum), the sand dollar coordinates its spines and tube feet to position itself nearly vertically in the sand, half buried. It catches small particles of food as it floats by, and moves them towards its central mouth.

Other critters were much more difficult to observe without actually catching them.

As we walked through the tide pools in our bare feet, we could easily see the fish darting away from us.  With their camouflage, they became invisible as soon as they stopped moving.  However, we took advantage of their escape behaviour to capture them.  Some would squirm into the shade of our feet each time we took a step.  After we got over our delight of them tickling our feet, we could trap them against our feet and our cupped hands.

Only a flounder as small my fingernail could stick onto a vertical surface

The sculpins were likely tide-pool sculpins, a common species that grows to a maximum size of a couple of inches.  As for the flounders, I knew that they start off life free-swimming in the plankton, whose eyes migrate to the same side of their head as they settle on a life on the sea floor.  I didn't know how large they are when this eye migration happens.  Internet research says that in one species, it migrates at around 1/2 of an inch in size, which matches the size of this flounder, so it's possible this is really a baby flounder and not an adult (I don't know of flounders whose adult sizes are this small).

The largest sculpin we caught was maybe an inch long

My friends and I quite thoroughly enjoyed our day exploring at the beach.  The abundance of life in what one might think is a barren mud flat will definitely bring us back.


Susannah Anderson said...

Great photos! I loved the green nudibranch. I've never seen one of those.
I'll have to watch those tide tables, and get out there in time to get to the edge before it turns.

Konrad said...

yay! sand dollars!

Hugh said...

Me too. I've not seen those cool green guys, but have found the brown ones before. There's probably a lot of magic going on in those eelgrass beds.