By Dr. John Nickum, Retired Biologist, U.S. Fish and Wildlife Service
Questions: What is “Didymo”? Is it the same as “rock snot”? Should we be worried about it or is this just more scare talk? Is there anything that anglers can do if it really is a problem?
The Old Professor replies: “Didymo” is one of the more interesting “problems” to come along in years. The easy answers to your question are: 1) “Didymo” is a diatom, a form of algae and the name “didymo” is simply a short name for its long scientific name, Didymosphenia geminata. With a scientific name like that, is it any wonder that people shorten it and that the folks in New Zealand simply call it “rock snot”. “Didymo” and “rock snot” are the same thing.
Diatoms belong to a scientific category called Bacillariophyta and are frequently called golden algae. Diatoms are extremely abundant in both fresh and salt waters, with some scientists estimating that they carry on as much as 25 per cent of the total photosynthesis that occurs on earth. They are a major source of atmospheric oxygen. The structure of diatoms is unusual in the fact that they have the appearance of little pill boxes and consist of two, overlapping valves. Viewed through a microscope they have intricate patterns and can even be described as beautiful. They can exist as single cells that float in the water as part of the plankton community, but they frequently form encrusting colonies that cover rocks or any other structure in the water. They have even formed dense colonies on the skin of whales. Diatoms tend to be rich in oils and the planktonic forms often provide the first food consumed by newly hatched fish in lakes. The encrusting forms commonly found in streams often serve as the foundation for the food web of these streams.
In the case of “Didymo” these colonies sometimes develop into blobs of gelatinous material (a.k.a. “rock snot”) or even massive sheets. When I first saw pictures of these large, trailing sheets, I thought that I was looking at Sphaerotilus natans, a cylindrical bacterium that forms colonies commonly called “sewage fungus”. However, in contrast to Sphaerotilus, which is an indicator of serious organic pollution, “didymo” typically is found in clean, unpolluted streams. Other than general appearance and the fact that they both live in water, “didymo” and Sphaerotilus have little in common.
But the primary question now is: where did “didymo” come from and why has it become such a nuisance in some places? Is it a non-native invader? “Didymo” has been in North American waters, including Colorado waters for a long time, perhaps for as long the lakes and streams of North America have existed. It is thought to be native to North America and Europe and was first described in scientific literature in 1819 as, Echinella geminata. We really do not know how widely it was distributed historically, but it is not a non-native and apparently was not a nuisance until the last 20 -25 years. At this point in time, the reasons that it has become a nuisance are not known. Perhaps habitat conditions changed, or perhaps there was a slight mutation in “didymo”, but it can now be classified as an aquatic nuisance species, at least under certain conditions, even though we do not know exactly what those conditions are.
So what is an angler to do? My answer is that anglers should be cautious. Assume that “didymo” is not present in all waters and take actions to avoid spreading it. Check your boots, your gear, and all equipment carefully. Remove all obvious materials that might be “didymo”. After checking for the obvious, clean everything that has been in the water with bleach and dry everything for at least two days before entering other waters. Yes, this is a bit of a pain, but not as much of a pain as having your favorite stream carpeted with “didymo”.
If you want more scientific details and photographs, the Government of New Zealand has several excellent websites that can be reached through search engines such as Google, or Yahoo by typing in the scientific name (Didymosphenia geminata), or just the term “didymo”.