Glowstick Sharks

Derivation from a Common Ancestor: Homologous Evolution of Biofluorescence in Sharks

[Fun fact: I actually made a Canva for this, but my links didn't transfer over :( and I wasn't about to re-copy+paste everything. So if you'd rather view the graphic (without links), you can find it >here< .]

Taking a dive into the depths of the ocean, the water becomes continuously bluer and darker as red wavelengths from the sun are filtered out. In order to combat the increasing darkness, biofluorescence among marine life is widely expressed across taxa - more than 180 species of fish! (Sparks et al., 2014). Not to be equated with bioluminescence where light is self-generated by a series of chemical reactions, biofluorescence occurs when green fluorescent proteins (GFPs) along with brominated tryptophan-kynurenine metabolites absorb filtered blue UV light from the sun and re-emit lower energy green light (Park et al., 2019). A more detailed description of the mechanism of biofluorescence can be found here. GFPs and similar biofluorescent-producing metabolites have independently evolved in many metazoic phyla including Cnidaria, Arthropoda, and Chordata (Macel et al., 2020). Consequently, this homology in marine evolution implies the presence of GFPs in a common ancestor of each of these species.

Imaging of fluorescent sharks and scales (Park et al., 2019)

On a more finite scale, scientists recently discovered and studied the role of brominated tryptophan-kynurenine metabolites in two species of sharks from different regions: the eastern Pacific swell shark (Cephaloscyllium ventriosum) and the western Atlantic chain catshark (Scyliorhinus retifer) (Park et al., 2019). The finding of both GFPs and these specific metabolites in sharks living in completely separate regions suggests that each species required the function for similar reasons. Although not visible to the naked human eye unless viewed under UV light, the sharks are capable of seeing the fluorescent patterns on other marine animals. According to Park et al., the biofluorescence in these shark species has possible functions in intraspecific communication, dimorphic patterns that specify sex, interspecific identification, and even immune responses against microbial attacks (Gruber et al., 2016). To again reiterate, homologous evolution occurs when a similarly structural or physiological trait evolves in different and independent species whose utilization of the trait's function may vary. Therefore, the similarity in the expression of GFPs in these two sharks, as well as the other marine creatures mentioned, gives rise to evidence pointing towards a common ancestor of these species.

Phylogenetic tree for GFP expression (Macel et al. 2020)

Personally, I have a deep-rooted fascination with both biofluorescence and bioluminescence, always coming back to the topic for various projects throughout my education, and I learn something new about the mechanism everytime. I mean how can you not find glowing sharks cool?! Nevertheless, there's still so much we don't know about the behavioral and ecological functions of bioluminescence and biofluorescence, so there are still numerous secrets to unlock about these ancient creatures.

Works Cited

Gruber, D. F. et al. Biofluorescence in Catsharks (Scyliorhinidae): Fundamental Description and Relevance for Elasmobranch Visual Ecology. Sci. Rep. 6, 24751; doi: 10.1038/srep24751 (2016).

Macel, ML., Ristoratore, F., Locascio, A. et al. Sea as a color palette: the ecology and evolution of fluorescence. Zoological Lett 6, 9 (2020). https://doi.org/10.1186/s40851-020-00161-9

Park, H. B. et al. “Bright Green Biofluorescence in Sharks Derives from Bromo-Kynurenine Metabolism.” IScience, vol. 19, 8 Aug. 2019, pp. 1291–1336., doi:10.1016/j.isci.2019.07.019. Accessed 3 Sept. 2021. 

Sparks, John S., et al. “The Covert World of Fish Biofluorescence: A Phylogenetically Widespread and Phenotypically Variable Phenomenon.” PLoS ONE, vol. 9, no. 1, 8 Jan. 2014, doi:10.1371/journal.pone.0083259.