The presence of lateral photophores correlates with increased speciation in deep-sea bioluminescent sharks

Abstract

The vast majority of species within the lanternshark genus Etmopterus harbour complex luminescent markings on their flanks, whose functional significance has long remained obscure. Recent studies, however, suggest these enigmatic photophore aggregations to play a role in intraspecific communication. Using visual modelling based on in vivo luminescence measurements from a common lanternshark species, we show that etmopterid flank markings can potentially work as a medium range signal for intraspecific detection/recognition. In addition, using molecular phylogenetic analyses, we demonstrate that the Etmopterus clade exhibits a greater than expected species richness given its age. This is not the case for other bioluminescent shark clades with no (or only few) species with flank markings. Our results therefore suggest that etmopterid flank markings may provide a way for reproductive isolation and hence may have facilitated speciation in the deep-sea.

Keywords: bioluminescence; diversification; evolution; shark; visual detection.

Figure 1.

In vivo recordings and visual modelling of Etmopterus spinax luminescence. (a) Correlation between lateral and ventral (counterilluminating) photophore luminescence intensity. (b) Target (t) and background (b) pixels used in the theoretical visual modelling: t₁=bioluminescent detection pixel, t₂=gross discrimination pixel (allowing more than 50% of the lateral pattern to be seen), t₃=fine discrimination pixel (allowing more than 95% of the lateral pattern to be seen), t₄=non-bioluminescent (black) detection pixel. Scale bar, 5 cm. (c) Detection distance of target pixels at counterillumination depth according to the observer's pupil diameter. The shaded area represents the pupil diameter range observed from a complete E. spinax ontogenetic series.

Figure 2.

Expected species richness curve of bioluminescent sharks indicating the 95% CI for the expected number of species given clade age. Values in parentheses indicate, for each clade, the ratio between the number of species with flank markings (bold) versus the total species number. Only the Etmopterus clade, which contains the highest proportion of species with flank markings (97%), shows a significantly (p<0.01) higher species richness than expected. Flank markings are represented in red on shark drawings and on insets above them (enlarged view). To illustrate the morphological diversity of flank markings within Etmopterus genus, the flank marking shapes of four species were represented.