Another study supports reef fish see red
The newly published research paper in the Proceedings of the Royal Society B contrasted two non-exclusive hypotheses (meaning they are not opposing hypotheses and both could be true) for the red fluorescence around reef fishes' eyes:
- That UV absorption by fluorescent pigments offers significant photoprotection in shallow water, where UV irradiance is strongest (ie. converting harmful UV light to harmless red light)
- That red fluorescence enhances visual contrast at depths below −10 m, where most light in the ‘red’ 600–700 nm range has been absorbed.
To test these hypotheses, scientists measured the red fluorescence around the eyes of a wide variety of reef fish from the Red Sea, Indo-Pacific, and Mediterranean collected at depths of 5 meters and 20 meters. These fish included gobies, dragonets, pipefish, and hawkfish.
Their analysis showed that the vast majority of fish collected from deeper waters exhibited significantly brighter red fluorescence than their shallow water counterparts. This finding suggests that the red fluorescence is more likely employed primarily as a visual tool (hypothesis #2) rather than as UV protection. As mentioned, these two hypotheses are non-exclusive/non-competing, and the scientists made it a point to emphasize that red fluorescence may also serve a secondary role of UV protection.
We don't yet know what reef fish use red fluorescence for. Perhaps it's to identify other fish underwater where red light would provide high contrast against the deep blue ambient light (imagine: neon paint under black light). Perhaps it's a visual tool to detect cryptic prey. Whatever the case may be, there is a growing body of evidence that shows that reef fish not only can see red but that red is important to their behavior and ultimately survival.