Tuesday, March 18, 2008

Ongoing intertidal angst.

Purple Shore Crab, Hemigrapsus nudus.

Three years ago:
I took my five-year-old son to the beach at Spanish Banks, on English Bay near Vancouver. As we came up behind a couple perched on a log, he announced, “Something smells stinky!” The couple turned and laughed.

“That, my boy, is the ocean. It’s a good kind of stinky. Let’s go find some creatures.” The tide was out, and there were acres of mud and barnacle-encrusted rocks. Each rock was the hiding place of countless invertebrates, hunkered down in the dark and damp, awaiting the tide’s return.

The first rock turned resulted in the usual scurrying of shrimp-like amphipods, and, thrill of thrills, a purple shore crab the size of a nickel. Crabs are great – large or tiny, they’re always feisty. I offered my finger and it pinched for all it was worth.
I tried to explain barnacles to my son, but I don’t think it made sense. Barnacles are crustaceans that early in their development cement themselves to a solid object, form a plated skeleton, and spend the rest of their lives waving their feathery legs around, snatching plankton from the water. When the tide is out, they are sealed shut. If you place a barnacled rock in a pail of seawater, the barnacles will eventually open and start kicking. It’s a fun and relatively innocuous way to show sea life to a child, or anyone, so long as you return the rocks below the high tide level, the barnacles on the up side.

The challenge in studying intertidal animals is the same as studying any organism anywhere — doing it in a non-damaging way. No matter how careful you are, every footfall may mean the death of something, and every rock turned can lead to a very local catastrophe as it is placed back down.

I worked at an ocean park where groups of school children came to learn about intertidal life, and, after stern instructions on mudflat etiquette, would be released to find the same animals I was showing my son at Spanish Banks. I remember one child standing petrified among the rocks. She had taken the instructions so dearly to heart that she was afraid to move, in fear of hurting anything.

It’s a dilemma: How to investigate and share the splendor around us without harming it?
When you’re with a single five-year old, you pick him up and carry him back to shore, knowing only one of you is stepping on other lives, which is a bad kind of stinky.


Back to the intertidal, this time to Belcarra Park (49 18 45.55 N, 122 55 37.32 W), an over-visited, somewhat trampled muddy bay strewn with barnacled rocks. Unfortunately the tide was high, so we didn’t have the opportunity to step on as many barnacles as we otherwise would have. We found several purple shore crabs, one white one and the rest a muddy brown-green that made them difficult to see in the turbid water.

We found a lot of clams, mostly Japanese Littleneck Clams, Tapes philippiarum, an introduced species, a tag-along from the commercial oyster industry. Many were freshly broken and eaten by gulls and crows, who drop them on the rocks to break the valves. This mudflat also holds Native Littleneck clams, Protothaca stamina, in smaller numbers.

Japanese Littleneck Clam valves.

This mudflat has been a source of food, specifically bivalves, for humans for thousands of years. The land rising behind the mudflat is in part a midden, a heap of millions of discarded clam shells. In places the soil is made up of crushed shells, grinding underfoot.

Modern times, the mudflat is closed to shellfish harvesting. Apart from being within a park, the bivalves are prohibited from being harvested because of high fecal coliform counts. Oh, and then there was that massive oil spill from the refinery across the inlet last summer.

And we were worried about stepping on barnacles. Well we were, and will continue to be.


pookie said...

What do these little crabbies eat? (or even big ones, for that matter)?

Hugh said...

I suppose they eat algae, worms, dead stuff. (Catholic crustaceans.)