The full width of the frame is 2mm across. (measured by pointing it at a ruler using the same magnification). That makes the larger objects around 40-50um in diameter, and the little rod-shaped guys about 20um long and 5um wide.
These sizes put both the larger and smaller guys within the size range of dinoflagellates. It also puts the larger guys in the size range of coral or other invert larvae.
If this is in a saucer, they are probably moving on their own power, although some of the smaller movements could just be due to brownian motion. I don't see any threads in the video, but I understand why you say that as it does look like most of the larger guys are kind of twisting around the same spot. It is certainly possible that they are connecting themselves to the substrate somehow, but I really can't tell from the video.
If I were to get a regular, compound microscope, can it be used for top-illuminated objects like this or do slides have to be prepared?
One can illuminate sample from the top while using a compound microscope. However, you will need to use slides anyway if you want to get higher magnification images because the higher the magnification, the closer the lens must be to your sample. By the time you get up to 200x magnification (20X objective + 10X eyepieces), your objective would be in the water in your saucer and I'm sure the saltwater would ruin your objective! But slides and cover slips are dirt cheap and can even be reused for your purposes. I could just give you a few if getting those is a problem.
Some advice for those shopping for a compound microscope to explore the tiny world around us:
1) newer != better
2) off brand chinese microscopes are often lower quality
3) you get what you pay for (to an extent)
4) the best microscopes will have 4 different contrast settings: Bright Field (BF), Dark Field (DF), Phase Contrast (PC, often referred to as just "phase"), and Differential Interference Contrast (DIC). All microscopes offer BF, because this is simply shining light through the sample towards your objectives. DF is just like looking at a negative version of BF. PC allows you to see differences in the refractive index of different parts of your sample. This means that you can often see little organelles and flagella and things using PC that you could not see with BF. Both BF and PC only give you a 2D image, but DIC gives you a sort of 3D image based upon the difference in refractive indicies of different parts of your sample. It isn't a true 3D rendering of the sample, but it does give you a different view.
Here is an example of the same human cheek cells in BF (a), PC (b), and a technique similar to DIC (c):
(Source:
https://micro.magnet.fsu.edu/primer/techniques/contrast.html)
My parents purchased a compound light microscope for my 16th birthday. In retrospect, although it looks cool, it isn't a great instrument. It was a relatively cheap ($300) chinese knock-off that only does BF. I would not purchase a compound microscope that doesn't at least have PC. DIC is a nice touch, but comes at a higher price. Without PC, you will have trouble visualizing the details of samples without killing and staining them, which is probably more trouble than beginner microscopists want to get into. Plus then you can't watch them swim around
If I were in the market now, I would purchase a used on brand micrsocope, just like this one on
ebay. A decent used microscope like this will probably put you out at least $500, but I don't really think it's worth it to buy a crappy one. Feel free to ask me about any specific microscope you're thinking about purchasing. Just a final disclaimer: I am not a microscope expert. I know the basics of how to use them, and that's about it!
