Imagine you’re in the car on a rainy day looking out the window. What do you see? Then, pull your focus back to window itself and the raindrops beading up. Now what do you see? An entirely new world, right? Smaller, but a world just the same, individual droplets all interacting. A world you might not have even noticed was there, if you hadn’t looked closely.
It was a discovery, much like that classic raindrop riddled one from childhood, that rocked our world in the Department of Invertebrate Zoology (IZ) AquaRoom just a few short weeks ago. Looking in at the Cassiopea jellyfish and octocoral being cultivated inside the AquaRoom’s 55 gallon saltwater tank, it was all too easy to miss what was, quite literally, right under our noses; but, with the help of time lapse photography zoomed in to focus on the tank glass, an entirely new world was revealed. One of flatworms, isopods, and placozoans - each with a presence larger than life once given the spotlight.
It was in early February that a species of distinctly orange flatworm seemed to randomly appear. Curator Allen G. Collins first noticed the tiny organisms twisting and turning on the tank glass, and with IZ interns Lauren Thomas, Allie Straus, and Kela Bakari hard at work studying individuals under a microscope and thinking through important questions on what makes these captivating creatures tick, the AquaRoom team is making some exciting steps towards grasping the complexity of this tiny world. Armed with a Nikon D7200 camera and 60mm lens shooting 1 image every 3 seconds, the interns are capturing time lapse videos like this one that we hope will enable new insight into these organisms’ activities. There’s so much more to that 55-gallon tank than meets the eye!
In this video you can see a sampling of the countless interactions happening every moment at this microscopic level!
“I think our flatworms should have a Youtube channel,” Lauren joked after I spent so long mesmerized by these animals sporting vibrant orange, and moody brown hues that I had worm-shaped spots wriggling through my vision. I was inclined to agree-- though the skittering isopods and translucent placozoans reminiscent of The Blob are something to behold, the fiery eye-catching flatworms steal the show. But what do we know about these invertebrates?
Well for starters, flatworms as a whole glide through their environment by moving hair-like structures known as cilia. They have bilateral symmetry, meaning they’re divided into two halves that are almost mirror images of each other. Surprisingly, the flatworms all used to be united within a single phylum, Platyhelminthes (quite literally platy- “broad or flat” helminthes - “worms”), but now we know that one group, Acoelomorpha, has a separate evolutionary history from the other flatworms. While many flatworm species are parasitic which, unfortunately for their microscopic roommates, means they take from another organism in a way that’s harmful-- such as living in or on the body of the organism they are feeding off of, acoelomorphs are free-living worms.
As for our AquaRoom Flatworms, after some literature searches, we concluded that our orange fellows belong to the acoelomorph species Convolutriloba longifissura. This is a species known to reach a maximum length of 6mm. [3] C. longifissura often eat brine shrimp and copepods, and they consume their prey like a vacuum, scooping it right up! [1] Though we relate to how flatworms down their meals, these organisms unlike us have no gut cavity to speak of-- their mouths open up directly into the main layer of tissue in their body. [3] Speaking of food, their interactions with placozoans and other critters traversing the glass are especially interesting: In the time lapse above (and the cropped excerpt below), a flatworm appears to attempt eating a writhing worm that we initially mistook for a placozoan, but then rejects it just as quickly. While the flatworm recoils, the weird worm then seems to make its escape. One of our questions is whether this is a typical interaction between the two species. And of course we need to put that worm under a microscope to learn more about what it is.
This video depicts the mystery worm's harrowing encounter with the Convolutriloba longifissura.
So flatworms live on other organisms, feed off of them, or try to, but do they ever team up? Apparently, they do. Flatworms of Convolutriloba have symbiotic relationships with green algae that live within their tissue, and it’s these same green algae cells that enable the flatworms to naturally fluoresce. [3]
Though it remains a mystery how exactly these flatworms appeared in the tank, it soon became apparent how their numbers flourished so quickly. Flatworms reproduce asexually through division. What sets C. longifissura apart from other Convolutriloba species however, is the stages it undergoes during division. Through a transverse fission the flatworms posterior end splits off, and then within a 24-hour period, the posterior end splits in half via a longitudinal fission. Within 2-3 days, during which the two budding flatworms develop eyes and a mouth, what began as one individual flatworm has now become three. [2]
Of the genus Convolutriloba’s four known species, three of them have been named from specimens found on aquarium walls-- and placozoans were described from aquarium walls, as well. It seems as though we aren’t the only ones exercising our curiosity by staring at the glass walls of aquaria! These organisms may be minute and mysterious, but just like their Cassiopea tank mates, they are ambassadors of a remarkable natural world worthy of the up-close attention.
By-- Alia Payne and Lauren Thomas
FURTHER READING:
1. Åkesson, B., Gschwentner, R., Hendelberg, J., Ladurner, P., Müller, J., & Rieger, R. (2002, January 06). Fission in Convolutriloba longifissura: Asexual reproduction in acoelous turbellarians revisited. Retrieved from https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1463-6395.2001.00084.x
2. Identification - macropyga. (n.d.). Retrieved from https://sites.google.com/site/macropyga/identification
3. Euichi Hirose and Mamiko Hirose "Body Colors and Algal Distribution in the Acoel Flatworm Convolutriloba longifissura: Histology and Ultrastructure," Zoological Science 24(12), (1 December 2007). https://doi.org/10.2108/zsj.24.1241
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