It’s always interesting to observe a new aspect of living organisms; you can never be sure what you will discover. And so it was that a curious and unexpected interrelationship among bacteria revealed itself from the insides of a clam.

Not long ago, we were working on a new production about the Phylum Mollusca for our Branches on the Tree of Life DVD series. The script called for a close look at the finely-divided gills of the clam which are covered by tiny hair-like cilia that beat away in concert, thereby moving a mixture of trapped food and slimy mucous to the mouth of this inveterate “filter-feeder”.

We captured this fascinating process on video tape (it’s most fascinating when observed at high magnifications, as can be seen in the video/DVD) and then explored around a bit in the clam for other insights into the life of this burrowing animal before trundling it off to the kitchen to join the other clams in our dinner pot.

Photomicrographs of preparation from clam testes. Red arrows indicate clam sperm. Green arrows point to bacteria assiciated together: on the left there are two bacteria, on the right, three.

We soon discovered that this two-shelled specimen was a male, so we prepared a wet mount of tissue from the testes just to see what clam sperm looked like. Under high magnification, lots of sperm could be seen thrashing about in the drop of seawater added to the testes sample. But surprisingly there was something else--some large corkscrew shaped bacteria.

Finding spiral bacteria in tissue is not surprising in itself, but these bacteria looked different as they slowly rotated—they were doubles, one much thinner than the other. We know that bacteria divide by pinching off somewhere around their middle, not by growing a new cell along the length of the parent. Also, the partners were obviously of different diameters, again, not the expected result of bacterial fission. Thus, we ruled out fission as an explanation for the observation. A search of the science literature shed no light on these intimately intertwined prokaryotes.