Sexually dimorphic eye-size in Dragonfishes, a response to a bioluminescent signaling gap

Deep-sea fishes must overcome extremely large nearest-neighbor distances and darkness to find mates. Sexual dimorphism in the size of luminescent structures in many deep-sea taxa, including dragonfishes (family Stomiidae), indicates reproductive behaviors may be mediated by visual signaling. This presents a paradox: if male photophores are larger, females may find males at shorter distances than males find females. Solutions to this gap may include females closing this gap or by males gathering more photons with a larger eye. We examine the eye size of two species of dragonfishes (Malacosteus niger and Phostomias guernei) for sexual dimorphism and employ a model of detection distance to evaluate the potential for such dimorphism to bridge the detection gap. This model incorporates the flux of sexually dimorphic postorbital photophores and eye lens size to predict detection distances. In both species, we found a significant visual detection gap in which females find males before males find females and that male lens size is larger, marking the second known case of size dimorphism in the actinopterygian visual system. Our results indicate the larger eye affords males a significant improvement in detection distance. We conclude that this dimorphic phenotype may have evolved to close the detection gap.

Methods
Specimen Data

Postorbital photophore area, lens size, and standard length (SL) data were taken from 42 spec- imens of M. niger (7.6–16.4 cm SL; 25 male, 16 female; Table S1) and 63 specimens of P. guernei (4.4–14.0 cm SL; 30 male, 32 female) obtained from Harvard University’s Museum of Comparative Zoology. We took images of the head of each specimen with a Nikon D6 digital camera and measured the area of the postorbital photophore in cm2 using ImageJ (Schneider et al., 2012). These data are contained in the file `eye_PO_data.csv`.

Flux Data

The flux (E) of the postorbital photophore of both species is unknown, but we presume it varies with photophore size as in other deep-sea teleosts. Therefore, we predicted the flux of each individual based on data for the postorbital photophore of Aristostomias scintillans, a close relative of both species, from Mensinger and Case (1990, J. of Exp. Mar. Biol. & Ecol. 144:1–15). The data for this study are in `mes.cas.flux.dat.csv`.