A shimmering school of silver fish might evoke the image of a disco ball—a dazzling spectacle composed of countless tiny mirrors. However, recent research suggests that the skin of these fish is capable of much more than merely reflecting light.
Masakazu Iwasaka, an interdisciplinary engineer at Hiroshima University in Japan, has spent decades investigating how light interacts with fish bodies. In a new preprint paper posted on the bioRxiv server, which has yet to undergo peer review or be officially published, Iwasaka claims to have discovered specialized pores in fish skin that react to light rather than just reflecting it.
He observed this remarkable phenomenon in the silver fish species Hypoatherina tsurugae, which inhabits the relatively shallow ocean waters around Japan and Korea. Their skin contains special cells known as iridophore cells, densely packed with guanine crystals. These crystals reflect light at various angles, creating color effects not through pigment, as our skin’s melanin does, but through their structural arrangement, which refracts and reflects light at different wavelengths.
In previous studies of these fish, Iwasaka documented iridophore patches along the scales on the dorsal part of the animals’ bodies, which exhibited “rapid and repetitive light reflection at frequencies of several hertz, independent of body movement.” These patches switch between three states, which Iwasaka rather poetically termed “static bright,” “dynamic flickering,” and “dark.” He hypothesized that these states might change depending on the ambient light levels and designed this new study to test that theory.
Iwasaka conducted his experiments on 22 wild-caught fish housed in a standard laboratory aquarium. To examine the fish’s skin under a microscope with varying light conditions, some were briefly anesthetized, studied with a microscope equipped with a camera, and then returned to the main aquarium. Initially, he shone a white LED at the fish, which illuminated the room and aquarium as a whole.
The iridophores remained in their “static bright” state until the direct light was switched off, at which point they quickly shifted to “dark” mode, much like how our pupils adjust when someone turns off the light.
In this “dark” state, the guanine crystals ceased to reflect light with the same intensity, as if they had been slightly displaced. But they soon adapted again to the room’s ambient white light, reverting to the “static bright” mode.
Iwasaka then repeated this experiment, testing the iridophores’ response to blue, green, and red LED light, as well as a blue laser and two types of green lasers.
“Spectral analysis revealed that this quenching response is most sensitive to blue light compared to green and red illumination,” Iwasaka describes.
According to him, in these experiments, the iridophores returned to a flickering state approximately 10 seconds after the direct exposure to the light source ceased.
“The speed and reversibility of this response suggest that mechanisms beyond slow intracellular structural rearrangements are involved, pointing to the possibility of neural modulation in addition to intrinsic photoreceptor processes,” Iwasaka writes.
He speculates that opsins—a group of light-sensitive proteins best known for their role in the retina—may be involved. However, this theory goes beyond the scope of the present study, which did not examine the molecular mechanisms underlying the light reactivity of the fish’s skin.
Please note that this research has not yet undergone the rigorous process of scientific peer review or official publication. This means it may contain errors, omissions, and has not yet received approval from the scientific community.
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