A small desk aquarium, besides being home to egotistical and self-important critters, is an endless source of wonder and learning. Since we cannot breathe water, we tend to see ponds and lakes from the top down – in other words, not very deeply. Even a small aquarium like mine gives amazing insights into the vertical life of a pond, from the insects on the surface to graceful worms on the bottom.
The Planarian worm is one of the most-studied and best-known organisms in the history of science, and is well-known to both researchers and students of high school biology. What is not often appreciated, however, is the grace and rapidity with which these creature move, as shown in this video shot through the 4X cell phone magnifier:
The planarian worm’s stately and measured glide is made possible by cilia on its underside, which propel it smoothly, without the wriggling motion of its more evolved, segmented annelid relatives. This ciliary locomotory mechanism works as well at the air-water interface as on glass or a plant stem, allowing the ribbon-flat planarian to glide upside down along the surface of the water:
The worm shown here is a large adult, almost 2cm in length when fully extended.
One of the most dramatic of the protozoa is the Stentor, or “Trumpet Animalcule.” Reaching lengths of up to 2mm, Stentor is one of the largest known unicellular organisms. They are common in freshwater environments, and are readily found either attached to roots and stems as a large, trumpet-shaped ciliate, or in a more ovoid form as a free-swimming organism looking for a place to call home. This video shows two Stentors in the aquarium, attached to a small duckweed root:
The Ciliate Resouce Archive. “Stentor.” http://www.uoguelph.ca/~ciliates/repgenera/stentor.html
Walker, D. “Protozoa – The Stentor.” https://www.youtube.com/watch?v=SrNkazUGHfw
Visible Vorticella Colonies:
One thinks of Vorticella as tiny, invisible organisms. However, when they aggregate in large numbers, they can form colonies visible with the naked eye:
Seen squashed under a coverslip, these Vorticella are almost unable to move and, mashed together, are very unhappy, do not open their cilia, and appear roundish. Note the brownish particulate material within the cytoplasm. :The restricted movement of the individual colony members can be seen on this video. Note that the restriction of the colony members’ movement allows the tangle of stalks to be seen. These also seem to be pigmented brown, where they are normally translucent and hard to see without darkfield or phase illumination:
(Suggestion from Guy Moody, Amateur Microscopy member from UK: “Do you use a ‘live’ cell? They are very easy to make. You can glue with Glassbond two cover slips on a slide with the gap between them just shorter than a cover slip. You put a drop of water in the space and put a cover slip on top. Once the cover slip is on you can add more water from the side with a pipette and keep the culture going for as long as you like.“)
These colonies are mobile and can form and dissociate reasonably quickly. After keeping the aquarium on restricted rations during the summer heat, added ~800 ml of fresh hay infusion; within 24 hours, the water became mildly cloudy, as the bacterial count rose to feed the copepods, which had become scant over the summer.
However, at the same time, the largest Vorticella colony began to look ragged, and by 48 hours was gone, leaving behind only a dirty, mucilaginous blob, the Vort equivalent of a pile of beer cans and old tires:
So where had this colony gone so quickly? Examining a duckweed root taken from the surface, a few inches away from the abandoned colony, revealed many happy Vorticella:
Enriching the water seemed to disperse the colony – so is colony formation related to food supply? This is one of the questions that make protozoology and fresh water biology so fascinating.