Wherefore Art Thou Gromia?
Last summer I decided to explore a small marine aquarium that
I had been growing algae in for about two years. It sits in an
east window, is plastic and holds just two gallons. I have a
small pipet with five balls in it designed for measuring specific
gravity. Once a week I check specific gravity and add some
filtered tap water if necessary. Living 1100 miles from the
nearest coast, I rely on our local pet shop to provides boxes of
I have three such aquaria sitting in that window and I decided early on to let the algae overrun this one. When I took a sample to examine under the stereo microscope, I found the expected cyclopoid crustacea, intriguing ciliate protozoa, filamentous algae, and unexpectedly, polychaete worms and a number of strange orangeish-red ovate spheroids hidden among the filaments of the mossy algae. At first I thought they were some sort of eggs. Even a very small marine environment can produce strange and fascinating creatures. When I first set up this tank, the polychaetes were few and only about 1/2 inch long. Now, they are abundant and several are 6 inches long!
The aquarium also contains a bright green filamentous alga which reproduces at a tremendous rate and has almost completely covered the sides. Sometimes I take samples of the alga, put it in a culture dish and, using dissecting needles, disassociate the filaments to see what might be hidden in it. This form is one of those amazing "oscillating algae"; in other words, the filaments move! Recently, I let a dish of disassociated filaments sit overnight and the next day, most of the filaments had migrated together forming a dense net. An intriguing phenomenon about which I know almost nothing and about which I am anxious to learn more.
But back to the "eggs". I first found them on just such an occasion while dissecting an algae mat. What interested me about them was they appeared hard rather than gelatinous as one might except of small aquatic eggs. They varied in size from about 0.5 mm. to 2.0 mm and had not been flatted or smashed when I made a slide preparation with a coverglass, so I took a dissecting needle and pressed on the coverglass. Instead of a "squish", there was a satisfying "crunch" and the resultant mess looked rather crystalline in nature. That got me to thinking that perhaps it was some kind of shell or test of some micro-critter.
This first encounter was before I had an inverted plankton microscope and so after producing several squashed and unusable preparations, I finally got one that had just the right amount of fluid and a very thin coverglass. I began to scan the slide at 40x and after a couple of minutes found an orange-red ovoid blur. Remember, this thing is thick by micro-standards. On focusing up to the surface, I was unable to find any real markings or differentiations which might provide a clue. I focused back down and examined the entire periphery of the objectnothing distinctive. I was about to dismiss it as an odd bit of debris, but then decided to scan slowly around the periphery once more. As I came around to the smallest part of the oval, I came across a small network of thread-like (filose) pseudopodia, projecting and criss-crossing, and suddenly fusing (anastomosing). It was one of the most bizarre, alien phenomena I have ever observed. I had read about certain amoeboid organisms behaving in this manner, but I had never before observed it and no description can quite prepare one for the experience. And it doesn't seem to be just my weird sensibilities.
Shortly after my first Gromia "sighting", I
asked a micropaleontologist friend of mine if he had seen Gromia.
His reply: "Oh, yes. It's a classic. It's so unusual that
everyone should see one." The following week, I asked a cell
biologist friend of mine if he had seen Gromia. His
reply: "One of the weirdest critters around. Absolutely
fascinating!" Three weeks after that, my best friend, also
an amateur microscopist came over to visit. I showed him Gromia.
Just prior to his visit, I had acquired a Wild inverted plankton
microscope with phase. I had prepared a couple of micro-chambers
in advance. [For information on making such chambers, see my 1998
article on MICSCAPE titled: "Inverted Microscopes: Some Notes
on David Goldstein's Article"]
When I had it properly set up with phase contrast at 600x, on the net of pseudopodia, I let my friend have a glimpse. After several minutes, he turned to me and said: "That's the strangest organism you've shown me yet. I really could almost believe that it came from another planet!"
We spent the afternoon looking at several samples and then we came upon a magnificent specimen with a pseudopodial net that extended out of the field of view even at 60x. As we increased magnification, we noticed a bulge of protoplasm directly outside of the oral aperture which appeared to be pulsing. As we watched more closely, it seemed like this structure was acting as a pump pushing out some filaments of pseudopodia and retracting others.
At 1500x with phase, we watched particles moving simultaneously in both directions along a single filopodium. Sometimes it was possible to see a filopodium being extended or retracted. In the process of retraction, minute particles, small flagellates, small ciliates, diatoms, and even bits of algal filaments were dragged toward the oral aperture and then into the shell for digestion. According to the literature, most of the waste is compacted into two types of minute structures which are very hard and apparently retained with the shell. Perhaps some of these are used to strengthen the wall of the shell. In any case, one researcher reported chipping a glass knife on an ultramicrotome while trying to section a Gromia shell!
Now, a year later, I still return to that aquarium to hunt for Gromia and I still find them, but in fewer numbers. Even now, when I examine a specimen, I thinkwhat a strange world we live in and how little of it we really know and understand. But, perhaps even more important to me is the little thrill of excitement I experience when I delve into the micro-world. For a while I am a boy again, caught up in wonder and amazement, and not even hearing the call to come to dinner.
Comments to the author Richard Howey welcomed.
The author's other articles on-line can be found by typing in 'Howey' in the search engine of the Article Library, link below.
Images of Gromia can be found on the Protist Information Server.
An 1874 description of Gromia terricola (a freshwater species) and original illustrations by the famous biologist Joseph Leidy on Dr. Sam Bowser's home pages.
Published in the October 1999 edition of Micscape Magazine.
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