A school of silverside fish can look like a disco ball. New research suggests their skin may be doing more than bouncing light back.
Masakazu Iwasaka, an interdisciplinary engineer at Hiroshima University in Japan, says he has found specialized pores in fish skin reacting to light, not only reflecting it.
In a new preprint paper that has not been peer-reviewed or formally published, Iwasaka studied cobalt silverside fish, Hypoatherina tsurugae, found in relatively shallow ocean waters around Japan and Korea.
The fish have iridophore cells embedded in their skin, densely packed with guanine crystals. Those crystals reflect light at different angles to produce color through their structural arrangement.
In previous studies on these fish, Iwasaka recorded iridiphore spots along the animals’ dorsal trunk scales that showed “rapid and repetitive light reflection at frequencies of several hertz, independent of body motion”.
He named three states in those spots: “static bright”, “dynamically twinkling”, and “dark”. He suspected the states might change with environmental light levels and designed the new study to test that idea.
Iwasaka conducted experiments on 22 wild-caught fish kept in a standard lab aquarium. Some were briefly anesthetized, examined with a microscope-lens camera under changing light conditions, and then returned to the main aquarium.
He first aimed a white LED at the fish, the same kind of light used to illuminate the room and aquarium. The iridophores stayed in their “static bright” state until the direct light was switched off, then quickly shifted to “dark”.
In that “dark” state, the guanine crystals stopped reflecting light with the same intensity, as if they had shifted slightly. Before long, they adjusted to the room’s ambient white light and returned to “static bright” mode.
Iwasaka then repeated the experiment with blue, green and red LED lights, along with a blue laser and two kinds of green lasers.
“Spectral analysis revealed that this quenching response was most sensitive to blue light compared with green and red illumination,” Iwasaka describes in his paper.
He said the iridophores returned to a twinkling state within about 10 seconds after the direct light source was cut.
“The speed and reversibility of this response suggest that mechanisms beyond slow intracellular structural rearrangements are involved and raise the possibility of neural modulation in addition to intrinsic photoreceptive processes,” Iwasaka writes.
He suspects opsins, a group of light-sensitive proteins known for their role in retinas, may be involved. But the study did not test the molecular mechanisms behind the skin’s light reactivity.
The study was uploaded to the preprint server bioRxiv and has not gone through peer review or formal publication. It could still contain errors or omissions and has not been endorsed by the scientific community.
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