Gastrotrichs – the ephemeral little mermaids of nature

I’ve been checking on my hay infusion for quite some time now. On my last post, I described my observation that the bdelloid rotifer population has boomed so much that they became visible to the naked eye. Alongside this development, I also noticed that the population and diversity of protozoans have significantly decreased. Now, it’s been a month since I started this “little experiment”. During my recent observations using the foldscope, I still noticed that bdelloid rotifers were the predominant inhabitants of my infusion, though their population has slightly decreased and they tended to concentrate at the periphery and at the bottom of the jar. On the other hand, the protozoan population was at an all-time low.

But while I was in the middle of noting these observations, something else caught my attention. I accidentally caught a glimpse of a peculiar creature I’ve never seen before:

I mean, just look at it! I got so mesmerized by how elegantly it swam across my field of view. At first, I thought it was just another type of protozoan, as it was about the same size as other ciliates I already know. But upon looking at it more closely, I realized that not only did it move differently compared to your average protozoan, it also displayed what looks like a tail fin!

I became so intrigued by it that I quickly searched for what it is on the internet. At first, since I thought it swam like a dolphin, I used the keywords “dolphin-like ciliate” and “dolphin-like protozoan“, but they didn’t yield any images that resemble what I was looking for. I then used more specific descriptions, like “protozoa that moves like a dolphin“, but my attempts weren’t successful either. I then figured that perhaps it was not a protozoa nor even a single-celled microorganism to begin with, so I changed my keywords to more general terms like “dolphin-like microbe” and “microorganism that moves like a dolphin“. At some point I even thought that it might be a smaller type of rotifer, since bdelloids also sport a pair of fin-like toes or spurs, so I changed my search term to “dolphin-like rotifer“, but still to no avail. I then decided that a “dolphin” might be a little too far-fetched a comparison, so I thought of an even simpler lookalike, such as a fish, and eventually I changed all my search terms to begin with “fish-like-“. After several days of still not finding any similar images on the internet, a creature that “looks-like-a-fish-but-moves-like-a-dolphin” finally came to mind – a mermaid!

While still wondering why I never thought of that all along, I immediately began my online search all over again, but this time starting with the keywords “mermaid-like-“… until finally, I found a similar-looking image! Thanks to the winning search term “mermaid-like rotifer“, I learned at last what that bizarre creature is called – a GASTROTRICH. For some of you, the term might already be familiar from biology class, but it didn’t ring a bell for me. I can’t recall being taught about such creatures, at least not when I learned about rotifers. Yet, I realized two important things from this search: 1) that the internet agrees with me that they do look like mermaids; and 2) that while indeed they are NOT rotifers, they are also microscopic, multicellular animals just like them!

I tried to look for another gastrotrich in my hay infusion, and fortunately I was able to find several more. I took pictures and videos of them, but strangely enough, I wasn’t able to spot another one that swam so “mermaid-like” as the first one I found. They seemed to be shy around bright light and tended to swim away from view, so I had a little difficulty taking snapshots of them! They were also tricky to spot since they were so small (less than 0.1 mm so they weren’t visible to the naked eye) and almost transparent, like ghostly swimmers that suddenly appear out of nowhere and then disappear without a trace.

Gastrotrichs may be one of the smallest animals and multicellular creatures in the world, as they are even smaller than unicellular microbes like Paramecium. Their posterior end which looks like a tail fin is actually a pair of tubes enclosing adhesive or cement glands; one gland secretes the ‘glue’ that helps them adhere to a substrate, while the other gland secretes a substance that removes the adhesion. Most species, especially freshwater dwelling ones or the Chaetonotid types, are able to swim smoothly through water partly due to their adhesive glands only being present at the back of their body, but mostly due to the rhythmic action of multiple cilia present in the ventral or belly side of their body. This is the very reason that their name was derived from the words “gaster” (belly) and “thrix” (hair), and also why they are called hairybacks, though their hairiness is not always apparent.

Like rotifers, they usually feed on detritus, small dead or living organic material, bacteria, or smaller protozoa. This could be the reason why they suddenly made their appearance in my infusion at around the same time the rotifers increased in population, as they share the same diet. Since most of the decaying matter which they feed on either float on the surface of the water or settle at the bottom near sediments, predictably they can be found swimming at these areas as well. Just as expected in my infusion, I mostly caught them on the surface or at the bottom of the jar, but also at the sides where rotifers resided too.

Unfortunately, I learned that they are also among the shortest living animals in the world. On average, their lifespan only lasts for 3 days! Within this very short period of time, they will have laid an egg that will hatch into miniature versions of the adult, already having a fixed number of cells that will no longer divide but only enlarge as they grow and reach sexual maturity by the third day. They will be outlived by most micro-animals like rotifers, who themselves don’t have too much time to live either, only lasting for several days to weeks under normal conditions. This could probably explain why as the days go by, I see fewer and fewer gastrotrichs in my infusion until I can hardly find a single one, while the bdelloid rotifer population didn’t seem to care. But of course, it might just be that there are significantly fewer gastrotrichs in my infusion than rotifers to begin with, and that they were outcompeted by the sheer number of rotifers in their common food source. Be that as it may, I still consider myself lucky that even in a very brief moment, I was able to witness these majestic little creatures and wish that I could swim even half as good as gastrotrichs do.

Link to my previous post:

4 Comments Add yours

  1. Manu Prakash says:

    Just absolutely beautiful post!! Loved your “dolphin” saga to find the species. It’s incredible how this look alike search is very good – I have searched ciliates before and called them – “turtle” look alike ciliate”.


    1. Cedrick says:

      Thanks, Manu! Persistence is the key (with some bit of imagination) 😊

  2. Manu Prakash says:


    I wanted to share a new taxonomic key for genera and families for species identification.

    Beautiful images to identify!


    1. Cedrick says:

      Thank you so much! Will try to identify the gastrotrichs I found and will also try to look for more. I just realized not all of them look like mermaids, and I might even confuse others to be worms. But at least spotting a mermaid-looking species introduced me to a whole new group of animals I never knew before.

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