Tuesday, September 18, 2012

Hunger is the best spice

Another Campbell River post

On a beach at the south end of the Campbell River Rotary seawalk, there is a tidewater pool, which looks artificial; a square dredged out of the stony beach, with not much to say for itself as yet. We walked out to the far side of this at low tide. The scenery, looking out across the water to Quadra Island, was beautiful, as always. But the beach was mostly clean rocks, with nothing much living underneath, not even seaweed.

Dry rocks and tiny barnacle shells

Periwinkle snails gather just above the water line.

It was difficult walking on the loose stones, and Laurie's injured leg was bothering him by the time we got to the water's edge, so he sat on a rock and I entertained myself looking for interesting stones. At the edge of a small tide pool - grey rocks, grey stones, grey rocks - I noticed a half-dried, rotting kelp stipe, balanced on the stones. The tide was coming in, and as it reached the kelp, purple shore crabs began to crawl out from between the rocks to attack the kelp, which they began eating hungrily.

Not much else to eat on this beach.

As the water rose, more and more crabs appeared. The first arrivals fought off the newcomers, often knocking them back down into the water. Occasionally, three or more would be struggling together, legs and pincers intertwined.

I've seen crab fights before, usually over some choice bit of fish, but never over dry kelp.

I count 14 in this photo, of a small section of the kelp.

The green crab at the right is also a purple shore crab.

Green shore crabs (aka,  Oregon shore crabs) may be many colours, usually green, white, or grey. The purples are usually purple, sometimes green. I have a white one in the aquarium. They are distinguishable, sometimes, by the polka-dotted pincers (see the one near the left on the photo above) and by their hairless legs; the green shore crabs are hairy. (Here's a green and white green shore crab; notice the hairy legs.)

4 comments:

  1. Ah, yes, Hemigrapsus oregonensis (green) vs. Hemigrapsus nudus (purple/no leg hair)! I studied H. oregonensis in college during an REU internship; the grad student in my lab often talked about taking a picture of a bunch of purple shore crabs, making a poster of it, and labeling it "Nudus Colony."

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  2. I didn't appreciate for a long time how uncommon beach areas were with LARGE, STABLE, very hard rocks. Since my first biological monitoring was of said rocks, and the plethora of life they sport/enable, I thought it was normal.

    Then I noticed how very common it is if rocks are just a bit smaller, they get flung around by the waves, and crush any tiny barnacle etc. trying to grow upon it. And there is precious little out there for snacking.

    I love the yellow in those 1st rocks--beautiful!

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  3. Seems to have more living than Bellingham Bay already. - Margy

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  4. Olivia, "Nudus colony"! I'm going to start calling them this from now on.

    biobabbler, On the White Rock beach in the winter, when the wind is up, the upper beach is constantly rattling with the stones rolling in and out. It's a difficult type of environment, even for mobile species like crabs. I wonder how far we'd have to dig down to find live critters.

    Margy, only in spots. But maybe, six inches beneath those rocks, we would have found more life.

    I'll have to do a bit of real digging next time I'm in a stony area.

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