Euglenoids are enclosed in a unique pellicle that's often helically wrapped, which gives them their characteristic movement, which biologists term "metaboly" but which cartoon animators will recognize as "stretch-and-bounce." Watch 'em go at the Euglenoid Project site, in videos of euglenoids in action. The site offers a scientific research and teaching tool that will introduce you to the general features of these protists.

Euglenoids and algae don't manufacture chlorophyll all throughout their cells. The process of photosynthesis takes place only within chloroplasts. Chloroplasts and other plastids are the photosynthetic organelles in nucleated cells. A couple of billion years ago, when their ancient forebears, routinely eaten, resisted digestion inside their bacterial prokaryotic hosts, they found themselves in a protected environment safe from oxygen, which was detoxified to their benefit, and in an environment rich with the metabolic wastes of the host cell, notably carbon dioxide. When the plastids combined that CO₂ with the energy of light and water, they made foods, and their metabolic byproduct, oxygen, was immediately taken up by the host cell. A symbiosis was born that evolved into an irrevocable link. But plastids remain conspicuously similar to bacteria. They even retain their own DNA and messenger RNA, and in a further remainder of their independent origin, remain enclosed in a triple membrane that separates them from the cell's cytoplasm.

"Green Water." Euglenoids and planktonic single-celled green algae can become dense enough under the right circumstances to turn the water impenetrably soupy green. Sometimes green water is desirable, in which guise it figures as a live food. It's certainly always gone just when you need it. "The sure cure for green water is day-old gouramis," I have said ruefully. Drifting single-cell algae and simple algal colonies are part of the green water phenomenon. Do you remember from Biology class the beautiful green Buckminster Fuller-like geodesic sphere of Volvox, containing its daughter colonies? But mostly green water is considered a temporary minor irritation.

It's interesting that "green water" can present a problem in a newly-set up tank, but rarely in an established tank that has a flourishing microfauna. Why should this be? And why, after medication has been used to eliminate a plague of Ich or Velvet, should green water sometimes make an unexpected reappearance, even in an established tank? Well, though single-celled algae are partly responsible for "green water," most of the dense green tint comes from Euglaena and its close relatives. The medications countering Ich may have some slight effect on algae, but they have much more effect on the euglenoids. And they are devastating to the free-swimming plankton, the population of all-but-microscopic ciliates, rotifers and crustacea, especially copepods, that graze on Euglæna and keep the population in balance. With their grazers gone, the decimated population of euglenoids rebounds like a fallow pasture, for these protists multiply by cellular division rather than any form of sex. Until the grazing population also recovers, the water turns thick and green. Then, when the rotifers and copepods have returned in strength, the euglenoid population collapses under grazing pressures, and the "green water" mysteriously "goes away."

Because green water results from unscavenged nutrients combined with plentiful light, the darkness cure can work in a pinch. Turn off all lights and swaddle the tank in towels to exclude even ambient light from the room. Give it five days in total darkness. Don't even switch the light on for feeding. The fish will survive. The plants might be stressed. The reasoning behind this mumbo-jumbo is that algae and euglenoids don't have the storage facilities "higher" plants have.

There's a ghost from this darkness cure in the misunderstanding I hear sometimes, that ambient light spilled from the room can contribute to green water under normal circumstances. I doubt it. Light of the right wavelengths has to be of a certain minimum intensity before photosynthesis is triggered. Brilliant daylight from a nearby window could be a problem. But light from the room isn't intense enough. It's worth mentioning again, if you're unwisely considering reducing the intensity of light to control algae over the longer run, that algae are able to photosynthesize at levels of light too low for vascular plants to use.