2017 February 11

By June 26, 2016, a lot of the slow-moving water down at the Pilgrim River wildlife area had been covered by mats of duckweed, like this:


The mat is not continuous, it is made up of hundreds of thousands of tiny individual plants.


The duckweed plants consist of nothing much beyond tiny, plump little leaves that are filled with gas bubbles so that they will float. You can see by comparing these with the width of my fingerprints that each plant is only about 5 millimeters across, or about the size of some of the smaller insects that I’ve posted over the years.


While duckweed has kind of a vague resemblance to the green mats of algae that form in the summer, and has a similar lifestyle, these are not actually algae at all. The duckweeds are in the subfamily Lemnoideae, and are members of the Arum family, the Araceae. So, these are actually flowering plants!

The duckweeds are rootless aside from a couple of tiny strands that hang down into the water. They mostly reproduce by budding, but they will occasionally have what are widely regarded as the world’s tiniest flowers, and produce seeds. Amusingly, some of their relatives in the Arum family are practically the opposite of the duckweeds in every respect[1], tending to have large, starchy roots and large, prominent flowers (in particular, the Titan Arum is in the running for having the largest flowers in the world!)

Duckweeds overwinter either as seeds, or with a somewhat denser winter form that sinks to the bottom to avoid the ice. So, there is very little duckweed on the surface of water early in the spring, but once they get started they rapidly spread over the surface. The roots are sticky, allowing them to stick to the feet of waterfowl (like ducks), which spread the plants between bodies of water.

Duckweeds are edible, even nutritious, although there are a couple of problems with growing them for food. One is that some species contain enough oxalate to be toxic, so one has to make sure of having the correct species. The other big problem is that the mats tend to grow in “nutrient rich water” (a euphemism for water that has lots of possibly unappetizing things dissolved in it), and they trap a lot of floating trash, making it hard to cultivate in a way that is really sanitary. Ducks will eat it, although I see a lot of sites claiming that only domestic ducks eat it in quantity. The wild ducks will eat it, but evidently prefer to spend their time looking for more high-energy foods first.

[1] A plausible way for these to have evolved would be that their ancestor was an aquatic arum that broke off leaves that continued to float in the water until they could re-root somewhere, and some of them just never got around to re-rooting again, floating away and basically reverting to the simple lifestyle of their distant algal forebears. Leading to a very simplified organism whose closest relatives are large, complex plants. In a way, it is vaguely similar to something that is in the process of happening with humans. See, back in 1951, George Gey cultured some cells from a cervical cancer biopsy that he collected from Henrietta Lacks, and these proved to be the first human cell cultures that could be grown indefinitely, without the cells eventually self-limiting their reproduction and dying off. These became extremely popular for medical research, used for so many applications that, even though any given researcher might only have a few grams of them at a time, the total quantity grown to date is estimated at around 20 tons. The thing is, in the over half a century that these cells have been cultured, it is now questionable whether they should be regarded as “human” cells. They have between 76 and 80 chromosomes, compared to the normal 42 for humans, and have incorporated the genome for the Human Papilloma Virus, along with other significant genetic differences. And, they have become a common contaminant of other cell lines, to the point where they can be considered a species that has opportunistically moved into the “laboratory research cell culture” environment as their ecological niche. Which leads us to the unusual situation where we have a single-celled organism whose closest relative is humans.

5 Responses
  1. February 13, 2017

    Totally off-topic. In the office, a discussion arose: How would you kill a crab so you could dissect it? I suggested reading Finnegans Wake to it until it died of boredom, but there’s got to be something more humane. Your suggestion?

  2. February 14, 2017

    According to this site, they can be anesthetized with clove oil (which is effective on most aquatic animals, and in large doses will kill within about 20 minutes). There is also a device called a “crustastun” that electrocutes them, but it’s about the size of a microwave oven and is more suited to restaurants than to killing a single crab. Freezing is effective, although it can take quite a while for larger creatures, and there is some question whether it is particularly humane.

  3. February 14, 2017

    Clove oil, eh? Hmm. How much do you think you’d need if you had a 30,000 gallon swimming pool crawling with about 500 crabs?

    Asking for a friend.

  4. February 14, 2017

    Well, in that case, I’d probably just go buy a jug of Aqui-S 20E Fish Anaesthetic, and dose according to the manufacturer’s recommendations.

  5. February 16, 2017


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