Fig. 7: Proposed expansion of Marshall and Johnsen’s criteria for ecological significance of biofluorescence.

We propose the presented dichotomous key to incorporate the complexity of the previously defined criteria. Marshall and Johnsen’s previous checklist for ecological significance of fluorescence is represented by the left most path of the key. Note the addition of different receiver contexts (conspecific vs. heterospecific; left vs. right panels) and the opportunity for not meeting certain criteria to be signals of cryptic coloration (via “NO” paths). Our newly proposed key now incorporates both cryptic and conspicuous signals as ecologically significant explanations for the evolution of biofluorescence. We also propose an expansion of the term “background environment” in Criterion 2 to include both the habitat environment and the individual’s skin environment as, depending on context and size/extent of biofluorescent patch, either could be the relevant background against which the signal is being viewed. Created in BioRender. Whitcher, C. (2024) BioRender.com/x53d269.