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.