Wednesday, February 22, 2012

Boxes in the sand

I must go down to the seas again,
To the lonely sea and the sky,
And all I want is more diatoms
And a 'scope to see them by.
(With apologies to John Masefield)

I've looked at every grain of sand in my pill bottles, with the naked eye, the camera, a magnifying glass, and three microscopes. Tonight, the worms were still active, but the heat of the lamps has done for the rest. The diatoms are still there, but none are moving. I saw one roundish, lumpy beast tumbling around, as usual, and a large copepod was struggling feebly. This is the first time I've seen a copepod moving slowly enough to appreciate its beauty. And how that one red eye glows!

But I promised you diatoms.

As my little camera sees diatoms (Navicula) and sand grains.

Beach sand is largely composed of silica, in the form of quartz and feldspar. Quartz makes a white beach; feldspar darkens it, or gives a slightly orange tint. Our local beaches are a composite. Under the scope, in a good light, much of the sand is glassy white; the other major colours are orange or dark grey, accented with the occasional green and brick-red stones.

In between these grains, one-celled algae float or swim encased in their own silica shell, as transparent as glass. The shells, called frustules or tests, are unique to each species, but usually consist of two similar sections, fitted together along a center line. (The name, "diatom", comes from the Greek "dia" and "tome", meaning "cut in two".)

We can think of the frustules as a little box with a lid slightly bigger than the base. Like the borrowed shells of hermit crabs, these boxes do not grow with the diatom; they are mostly mineral. So when the time comes to reproduce, several times daily, the cell divides down the middle and each new cell gets half of the frustule. It counts this as the bigger "lid", and builds a new one, a smaller version to fit. So half of each generation of the diatoms, the half that started out with the bottom of the "box", is a bit smaller than the previous one.

(Which explains the many different sizes I saw in my sample.)

Of course, if this continued, the diatoms would dwindle away to nothing, so when they're too tiny, they switch reproductive tactics, and become male or female. Their young build new boxes from scratch, back to full size again, and the process of shrinking starts over.

Ma Nature is endlessly inventive.

Some of the marine diatoms. Photo from Wikipedia.

Most diatoms do not move on their own, but float or are carried about. But some, like the Navicula lanceolata, have a groove (raphe) along the center line, where a moving band of cytoplasm serves as a motor. Some I saw in my sand samples moved fairly quickly; some diatoms can race along at a speedy 25 micrometers per second. (25/1000 mm.)


  1. This is all so fascinating! I don't think I knew or even thought anything about the tiny things between grains of sand.

  2. Trillions upon trillions of critters underfoot. It's hard to imagine.


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