Saturday, December 27, 2014

Water Boatmen in Ice

Yesterday when I was out walking along the river I came across an unusual sight. Thousands of water boatmen inside the ice! They must have been between layers of the ice. The air temperature was -20 and ice was forming on the river so the water temperature must have been close to freezing. Some of the beetles were frozen into the ice, but others were swimming around quite vigorously.

Where did they come from? Why was there such a concentration of them in this one spot? (I didn't see them anywhere else along the river.) Did they hatch recently or do they overwinter in the mud and something brought them out? The weather has been relatively warm recently, getting up around freezing, so there isn't much ice on the river (at least in the city). And the storm sewers probably discharge warmer water into the river. Maybe a rise or fall in water level flushed them out of wherever they were? Regardless of where they came from, it's amazing that they can stay active and swim around at just above freezing.

I think these are Corixidae, supposedly easy to identify because they swim near the bottom and right side up as opposed to Notonectidae (backswimmers). But they weren't near the bottom because they were stuck between layers of ice, and it was hard to tell through the ice which way up they were swimming, and many of the dead ones were upside down.

I didn't get very good photos. I couldn't get too close because the ice obviously wasn't very solid where they were. And of course the ice obscured the view. But they'll give you an idea.

No comments:

Post a Comment