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In a study published in Current Biology, scientists show that this organism reorganizes in seconds the orientation of the structures controlling the beating of its cilia. This mechanism, triggered by the entry of calcium into cells, could be one of the oldest animal locomotion strategies.
Image Wikimedia
An extremely simple animal... but very mobile
Among all known animals, Trichoplax stands out as an exception. This tiny marine organism has an extremely simple organization: it has neither muscles, organs, nor even a nervous system. Its flat and changeable shape resembles a kind of living "pancake" capable of constantly deforming.
Despite this apparent simplicity, Trichoplax actively explores its environment. It moves quickly along the seafloor, changes direction, and reacts in a coordinated manner when touched or attacked. Understanding how such a rudimentary organism can produce such behaviors was until now a puzzle.
Thousands of cilia to move forward
To move, Trichoplax uses thousands of microscopic cilia on its underside. By beating in a coordinated fashion, these vibrating structures propel the animal along the seafloor, much like a multitude of tiny legs.
© Marvin Leria et al.
In most animals that use this mode of locomotion, the direction of ciliary beating is determined very early during embryonic development. It depends on small structures at the base of each cilium called basal bodies, whose orientation then remains stable throughout life.
In a study published in Current Biology, scientists show that it is quite different in Trichoplax.
An ultra-fast reorganization at the scale of the whole organism
The work reveals that the orientation of the basal bodies in Trichoplax is, on the contrary, extremely dynamic. At any given moment, it directly reflects the direction in which the animal is moving.
When a mechanical stimulus occurs, for example when the animal is touched or injured, all the basal bodies realign in just a few seconds. This reorganization immediately changes the direction of ciliary beating, allowing the organism to flee in the opposite direction to the stimulus.
The scientists also show that local variations in the orientation of the basal bodies contribute to the animal's rapid shape changes. This phenomenon involves remarkable coordination among tens of thousands of cells, each precisely adjusting the orientation of its cilia, and all without the help of any central nervous system.
Calcium as the conductor
At the heart of this mechanism is a well-known player in cellular function: calcium.
Mechanical stimuli cause calcium to enter cells via specialized channels in their membrane. This signal acts as a trigger that initiates the reorientation of the basal bodies and thus the change in direction of ciliary beating.
From cell to cell, this reorganization quickly propagates through the entire tissue, allowing the whole organism to change its trajectory in seconds.
When scientists prevent calcium from entering cells or make this element unavailable, Trichoplax loses this rapid reorientation ability. The animal then can no longer effectively adjust its movements.
A novel mechanism in the animal kingdom
Such a mechanism of rapid reorganization of cellular structures had never before been observed in animals. Comparable phenomena were known, but they generally occurred over much longer periods, from several hours to several days.
This study thus shows that an extremely simple organism can produce sophisticated coordinated behaviors without a brain or nervous system. It also sheds new light on the fundamental mechanisms of locomotion and on the strategies that the earliest animals may have used to interact with their environment.
More broadly, this work indicates that simple local rules, combined with diffusible physicochemical signals like calcium, can suffice to generate complex collective behaviors at the scale of an entire organism.