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    The Trippy Social Life of the Worm That Is Also a Plant

    Written by

    Michael Byrne


    As far as nature just hacking together some beneficial traits, it's hard to beat the so-called "mint-sauce worm." Said worm, which is known properly as Symsagittifera roscoffensis (formerly Convoluta roscoffensis, which will make a lot of sense in a moment), is often thought of as a plant-animal hybrid (a convolution of plant and animal characteristics, that is) for its peculiar ability for deriving energy from the photosynthetic energy-harvesting processes of algae living within its worm gut.

    In other words, the S. roscoffensis doesn't just go out and eat some food, it goes out and eats some food that goes on to produce energy for its host via photosynthesis while living within the protective and sufficiently transparent confines of said host's body. The benefits of the relationship, known as photosymbiosis, go both ways as the worm offers its algal cargo protection and all-around optimal conditions for being a sea-plant.

    Biologists are obviously pretty interested in the lifestyles of the mint-sauce worm, which is so nicknamed because of the bright green appearance of large, dense collections of the acoelomorphs. Research out this week from scientists at the University of Bristol describes the "social sunbathing" characteristics of the worms, or how the worms are able to work together to form beneficial organizations of "super-organismic seaweed." Interestingly, the worms are able to form these faux-seaweed mats in areas where normal seaweed is unable to anchor and collect.

    If you were to be hanging around certain worm-optimal beaches, especially well-protected sand beaches along the coasts of Wales and the UK's Channel Islands, mint-sauce worm-mats might not look all that different from regular old algae. The worms are transparent, leaving only the algae within to provide color.

    Image: Franks et al.

    Watch these apparent mats of algae over time, and things start to get a bit weird. When the tide is out, the S. roscoffensis can be found chilling in mat form along the wet sand, but when the water returns, they respond by burying themselves in said sand. This is the more or less the day-night-cycle of the mint-sauce worm—the tide comes in and it's time to snooze, and then the tide goes out and it's time for another worm beach party. It sounds better than living in some dog's intestines, at least.

    In any case, the point of the research was to come up with a good computer simulation of how this socialization happens. It turns out that the worms go through a couple of phases on their way to full-on beach occupation, starting with small mint-sauce "flotillas" that in turn organize into the trippy green whorls seen above. These whorls then function kind of like mills, wherein surrounding worm clumps are gathered and incorporated into the growing worm-mat. These dense biofilms in turn offer the worms safety and numbers and a collective structure that winds up looking a lot like what algae form on their own, sans worm hosts.

    "These marine aceol flat worms rely for all of their nutrition on the algae within their bodies: hence their common name," the paper explains. "We show that individual worms interact with one another to coordinate their movements so that even at low densities they begin to swim in small polarized groups and at increasing densities such flotillas turn into circular mills."

    As the study authors note, S. roscoffensis is used currently by scientists as a model organism, offering a view into the processes of stem cell regeneration, in particular. In addition to increasing our understanding of a profoundly odd and ingenious adaptation, understanding the socialization of mint-sauce worms should increase their power as a tool for understanding everything from stem cell biology to the impacts of climate change on marine life.