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Glass fish lack skull bones

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The tiny fish Danionella dracula has a special heterochrony and an unusual anatomy, which make the animal interesting for neurophysiological research and developmental biology.

Dresden (Germany). Many fish, but also other animals, only develop in the larval stage before they reach the adult form. The axolotl, the grotto olm and a few other exceptions are exceptions to this, which are mature as larvae and retain their larval characteristics throughout life. This is particularly advantageous in caves and other environments with few resources.

Scientists from the Senckenberg Research Institute and nature museums have now discovered a particular form of this time-shifted development (heterochrony) in the fish Danionella dracula, which was first discovered in East Asia in 2009. For their study, the researchers bred the eleven to 17 millimeter fish, which is one of the smallest representatives of vertebrates, in the laboratory. This made it possible to observe the development of the larvae, their growth and maturation in detail.

Minifish bones are missing

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According to their publication in the journal Developmental Dynamics , the team was able to observe that Danionella dracula not only retains many larval features in adulthood, but also that the fish is missing some parts of its body. “Progenesis affected the entire skeleton of Danionella so that 60 skeletal elements are missing, including some bones of the skull,” explains Kevin Conway of Texas A&M University.

Well developed hearing aid

Although whole parts are missing from the skull of the fish, other parts of the body are more pronounced than average. “The ‘Weber Apparatus’, which is responsible for the intra-species communication of the animals, deserves special mention here,” says Ralf Britz from the Senckenberg Natural History Collections in Dresden. It is a hearing aid made up of three tiny bones that are similar to the middle ear ossicles in humans and transmit sound waves from the swim bladder to the inner ear.

“It is precisely this part of the Danionella skeleton that has developed not only normally, but even accelerated. This enormous difference in the speed of development of different organ systems within one and the same organism is very unusual for vertebrates, ”explains Britz. Danionella dracula therefore has a fully developed hearing aid, although the skeleton and many organs remain in the larval state and do not develop fully.

Model organism for neurophysiological research

The special properties make Danionella dracula almost predestined for use as a model organism in neurophysiological research , because a view of the brain is possible when it is alive. At the same time, the tiny fish is also of great interest for developmental biology due to its particular heterochrony.

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