Release Subtitle:
Zoologists explore the mechanism and development of gravity-sensing ability in marine acoel flatworms
Release Summary Text:
All life forms are endowed with the ability to sense gravity. However,
the mechanism is not well-understood in acoels, a group of marine
flatworms that represent a primitive invertebrate (without backbone)
lifeform. In a new study, zoologists from Okayama University suggest
necessary conditions for this ability to develop in hatchlings of an
acoel species native to Okayama sea coasts, opening doors to
understanding evolution better and pathology treatment applications in
humans.
Full text of release:
All living organisms are equipped with sensory organs to detect changes
in their surrounding environment. It may not immediately strike us as
obvious but, similar to how we can sense heat, cold, light, and
darkness, we are also extremely adept at sensing gravity. In our case,
it is our inner ear that does this job, helping us maintain balance,
posture, and orientation in space. But, what about other organisms, for
instance invertebrates that lack a backbone?
The gravity sensing organ in some aquatic invertebrates, known as a
“statocyst,” is, in fact, rather fascinating. The statocyst is
essentially a fluid-filled sac with sensory cells lining its inner wall
and a small, mineralized mass called “statolith” contained inside.
During any body movement, the statolith moves and consequently comes in
contact with sensory cells in the inner wall, deflecting them. The
deflections, in turn, activate the neurons (nerve cells), which then
relay signals to the brain about changes in body orientation.
However, exactly how the sensory cells stimulate the neurons is not
particularly clear for acoel flatworms—soft-bodied, marine animals with a
simple anatomy, which represent one of the earliest extant life forms
with bilateral (left-right) symmetry. What zoologists know so far, based
on the finding that juvenile acoel flatworms occasionally fail to sense
gravity, is that the ability is acquired sometime after hatching from
the eggs.
In a new study published in Zoomorphology,
scientists from Okayama University, Japan led by Prof. Motonori Ando
have now taken a stab at understanding these curious creatures better.
But what exactly is so attractive about acoel flatworms? Prof. Ando
explains, “Understanding the stimulus
response mechanism of Acoela can uncover a fundamental biological
control mechanism that dates back to the origin of bilaterian animals,
including humans. These organisms, therefore, are key to unravelling the
process of evolution.”
For their study, the scientists used an acoel species called Praesagittifera naikaiensis or P. naikaiensis that is endemic to the Seto Island Sea coasts at Okayama. “The mysterious body plan of P. naikaiensis could be key to connecting Okayama and the world’s natural environment,” says Prof. Ando.
To examine the relationship between the statocyst and nervous system of P. naikaiensis,
the scientists had to make them both visible, a task usually
accomplished by a “marker” or a “label.” However, due to a lack of any
suitable label for the statocyst, they adopted a different strategy in
which they labeled instead the basal lamina,
the layer on which the sensory cells sit. As for the nervous system,
they labeled the nerve terminals using a well-known marker. Finally,
they studied the specimen using confocal microscopy, a technique in
which light is focused on to a defined spot at a specific depth to
stimulate only local markers.
The results were illuminating. The scientists found that the acoel
flatworm developed a gravity-sensing ability within 0 to 7 days after
hatching, with the statolith forming after hatching. The statocyst
comprised longitudinal and transverse nerve cords, forming what is
called a “commissural brain” and a “statocyst-associated-commissure” (stc)
characterized by transverse fibers. They hypothesized that a
gravity-sensing ability developed when: 1) the statolith acquired a
sufficient concentration of calcium salts, 2) stc functioned as the signal-relaying neurons, and 3) the sensory cells were present outside the sac and stimulated indirectly by the statolith through the basal lamina and stc.
Inspired by these findings, Prof. Ando has envisioned future research
directions and even practical applications of their study. “It
has been reported that closely related species of this organism inhabit
the North Sea coast, the Mediterranean coast, and the east coast of
North America. Since there is great interest about the commonality of
their habitats, we can extend our research to a more global level, using
these animals as a novel bioassay system for the environment they live
in, especially in the face of the accelerated pace of climate change and
anthropogenic habitat degradation. Furthermore, acoel flatworms could
be an excellent biological model for studying diseases caused in humans
due to abnormalities of sensory hair cells,” says an excited Prof. Ando.
It seems modern science is just warming up to the myriad mysteries of this minute worm!
Release URL:
https://www.eurekalert.org/pub_releases/2021-05/ou-tuf052021.php
Reference:
Title of original paper: Structural analysis of the statocyst and nervous system of Praesagittifera naikaiensis, an acoel flatworm, during development after hatching
Journal: Zoomorphology
DOI: http://dx.doi.org/10.1007/s00435-021-00521-9
Contact Person: ANDO Motonori
ANDO Motonori is a Professor at the Graduate School of Education and the Graduate School of Environmental and Life Science at Okayama University, Japan since 2012. He received his Ph.D. in medicine and was an assistant professor at Kochi University School of Medicine from 1990 to 2005. He joined Okayama University as associate professor in 2005. His research interests include cell physiology, cell movement, membrane transport, environmental response, and natural science education. He has published 68 papers and 2 books and has 1 patent and has more than 1200 citations to his credit.
https://www.okayama-u.ac.jp/eng/research_highlights/index_id136.html
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