Finding fish fluorescently

Very early in my teaching career at Carleton College in Minnesota, I was thrust into the position of teaching students about things that I knew very little about.  I quickly learned that things went well, so long as I confessed my ignorance – the very bright students at that college were always happy to help me with my education. My ignorance of things biological stemmed from my undergraduate training in psychology, which had only a smattering of biology and chemistry in the coursework.  So when we extracted chlorophyll from a plant, shone a bright high-energy (probably UV) light on it, and it glowed a beautiful red, my reaction was “wooo…, that’s cool.”  My colleague, who was much more broadly trained, explained that this process, biofluorescence, occurred because the chlorophyll’s electrons were excited by the high energy light, and that they emitted the red light when they returned to a lower-energy state.

Marteenfig1Solenostomus cyanopterus

Robust ghost pipefish, Solenostomus cyanopterus, is cryptic in ambient daylight (left), but biofluoresces red when lit at night by a high-intensity LED torch (right).


Many threatened or endangered marine species are cryptic, providing challenges to conservation biologists who must assess the abundance of these species.  Usually, marine biologists use underwater visual censuses to measure abundance and distribution of marine species, but small or cryptic species are often missed or undercounted.  Maarten de Brauwer reasoned that conservation biologists could use biofluorescence as a tool to find small or cryptic marine organisms.  He knew from a paper that recently came out in the literature, and from his own experience as a diver, that a number of cryptic species do fluoresce. But how large is that number?


A diver searches for biofluorescent species. Credit: J. A. Hobbs

DeBrauer, working with five other researchers, surveyed reef fish at four locations in Indonesia, as well as two locations outside Indonesia (Christmas Island and the Cocos Islands).  Indonesia was a conservation priority as it contains the world’s greatest abundance of marine fish species. Using high-energy LED torches, the researchers surveyed 31 sites at the six locations, assessing each fish they detected for whether it was cryptic or non-cryptic, and whether it fluoresced. Of 95 cryptic species, 83 fluoresced.  In contrast, only 12 of 135 non-cryptic species fluoresced.


Number of cryptic and non-cryptic species showing biofluorescence in the survey.

Why are cryptic species more likely to biofluoresce?  As it turns out, we don’t know the answer to this question.  De Brauwer suggests that some small species, like gobies and triplefins, may use flourescence, which is particularly well-defined around the head region, as a way of communicating without predator detection.  These species fluoresce in red, a very-short-range light, so predators won’t see them unless they are very close. Some species of scorpionfish that live in algae and seagrass also fluoresce red, which allows them to blend in well with the red fluorescence emitted by the algal and seagrass chlorophyll.

Having shown that cryptic species tend to bioflouresce, the next challenge was to see whether bioflourescence surveys worked better than standard underwater visual censuses. First, the researchers focused their efforts on two species of pygmy seahorses (Hippocanpus bargibanti and H. denise) that live on seafans, searching for two minutes, either with or without a flourescence torch.  They followed with a similar study on two species of reef fish, the largemouth triplefin (Ucla xenogrammus) and the highfin triplefin (Enneapterygius tutuilae); but this time surveying 20m x 2m transects, either with or without a fluorescence torch.


A diver searches a seafan for pygmy seahorses. Credit: J. A. Hobbs.

Unfortunately, the pygmy seahorses are tiny (as you might suspect from their name) and probably rare, so only 32 H. bargibanti and 7 H. denise were detected. These seahorses fluoresce red primarily in their tail region and green from their eyes.


Two cryptic pygmy seahorses “seen” under ambient light (left, circled in red) and in the underwater biofluorescence census (right).

The numbers of H. denise were too small to include in the analysis. But for the other three species, the bioflourescence surveys detected more individuals than did the underwater visual surveys.


Mean number of individual H. bargibanti (left), U. xenogrammus (center) and E. tutuilae (right) detected with underwater visual surveys (UVC) vs. underwater biofluorescence surveys (UBC).

The researchers discovered that bioflourescence is very common in these cryptic and rare species, which means this technique can be used to assess abundance in species most likely to be overlooked using standard underwater visual surveys. The International Union for the Conservation of Nature, which (among other tasks) is responsible for assessing the extinction risk of species worldwide, has only been able to do so for less than 44% of fish belonging to three large cryptic families of reef fish.  Of 2000 species in these three families, 21% are listed as data-deficient because they have been so difficult to survey.  This novel approach should help inform conservation biologists about species that are in dire straits, so they can focus conservation efforts in a productive and useful direction.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Brauwer, M., Hobbs, J. A., Ambo‐Rappe, R., Jompa, J., Harvey, E. S. and McIlwain, J. L. (2018), Biofluorescence as a survey tool for cryptic marine species. Conservation Biology, 32: 706-715. doi:10.1111/cobi.13033. You should also check out Dr. De Brauwer’s blog at Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2018 by the Society for Conservation Biology. All rights reserved.