Leidy's legacy

Natural History, Oct, 2005 by Donald A. Windsor, Michael McMahan, Lynn Margulis

In "Jointed Threads" [6/05], Lynn Margulis implies that Bacillus anthracis--the bacterium that causes anthrax--violates some law of symbiosis because it is a pathogen. She calls it % freak of nature"; I disagree.

The ecological role of B. anthracis is to prevent overgrazing by animals. Its spores lie dormant in the soil of a field until patches of bare ground appear between the surviving tufts of grasses and forbs. Then, during dry periods, grazers may ingest or inhale the anthrax spores. As the grazers die off, the field can recover. Later, when it is overgrazed again, the anthrax spores will defend it once more.

Donald A. Windsor

Norwich, New York

In Lynn Margulis's article, the caption for Joseph Leidy's teaching chart states that two nematodes are depicted. But the uppermost wormlike organism in the drawing actually appears to be an oligochaete--most likely of the species Stylaria lacustris. The specimen in the drawing is segmented, which is a diagnostic characteristic of an oligochaete, and it has a long proboscis, an important feature that occurs in members of the genus Stylaria. S. lacustris commonly lives in freshwater in North America, but as far as I know, it does not occur as a symbiont.

Michael McMahan

Union University

Jackson, Tennessee

LYNN MARGULIS REPLIES: I implied nothing. I wrote of the need to condemn people who would arm a bacillus to become a human pathogen. I'm grateful to Donald A. Windsor for his elegant ecological explanation of grazer control by anthrax spores. The dead body of a sheep or calf on a drought-ridden, dusty field not only supports generations of bacilli over a long winter, but also releases nitrates, phosphates, sulfur, calcium, sodium, and potassium that plants can use when spring rains arrive.

I looked again today at Leidy's teaching chart on my office wall. Its remarkable fidelity enabled Michael McMahan to recognize that I was wrong: the segmented worm with a proboscis probably is not a nematode, but an annelid. I hope Mr. McMahan finds it, as Leidy did, inside the intestines of Julus marginatus or another millipede, so that, if warranted, he can describe a new symbiotic oligochaete. If he uncovers a new species, perhaps he will name it Stylaria leidyii, in Leidy's honor. Or maybe he will find that the worm Leidy depicted so accurately in his chart is indeed Stylaria lacustra from a nearby lake, taking refuge and food inside the body of an arthropod.