Maurolicus_muelleri-1_SRX3161870_TrinityAssemblyTrinity assembly of Maurolicus muelleri-1 eye transcriptome - 100bp PE Illumina HiSeq2000 runMaurolicus_muelleri-4_SRX3161914_TrinityAssemblyTrinity assembly of Maurolicus muelleri-4 eye transcriptome - 100bp PE Illumina HiSeq2000 runMaurolicus_mucronatus-13_SRX3162896_TrinityAssemblyTrinity assembly of Maurolicus mucronatus-13 eye transcriptome - 100bp PE Illumina HiSeq2000 runMaurolicus_mucronatus-14_SRX3162897_TrinityAssemblyTrinity assembly of Maurolicus mucronatus-14 eye transcriptome - 100bp PE Illumina HiSeq2000 runMaurolicus_spp_OpsinDatasetAlignmentSequence alignment for gene coding region (CDS) inferred vertebrate opsin gene phylogenyMaurolicus_spp_OpsinTree_RAxMLGene coding region (CDS) inferred vertebrate opsin gene phylogeny based on a ML reconstruction in RAxMLMaurolicus_spp_OpsinTree_GTRgammaGene coding region (CDS) inferred vertebrate opsin gene phylogeny based on a Bayesian reconstruction using a GTR & g modelMaurolicus_spp_GalphaTransducinDatasetAlignmentSequence alignment for the vertebrate Gα transducin gene phylogenyMaurolicus_spp_GalphaTransducinTree_RAxMLVertebrate Gα transducin gene phylogeny based on a ML reconstruction using RAxMLMaurolicus_spp_GalphaTransducinTree_GTRgammaVertebrate Gα transducin gene phylogeny based on a Bayesian reconstruction using a GTR & g model of sequence evolutionMaurolicus_spp_ArrestinDatasetAlignmentSequence alignment for vertebrate arrestin gene phylogenyMaurolicus_spp_ArrestinTree_RAxMLVertebrate arrestin gene phylogeny based on a ML reconstruction using RAxMLMaurolicus_spp_ArrestinTree_GTRgammaVertebrate arrestin gene phylogeny based on a Bayesian reconstruction using a GTR & gamma model of sequence evolutionMaurolicus_spp_GArrestinTree_GTRgamma.treMaurolicus_spp_OpsinGeneConversionDatasetAlignmentAlignment for rh2 opsin gene conversion phylogenyMaurolicus_spp_OpsinGeneConversionTree_RAxMLrh2 opsin gene conversion phylogeny based on a ML reconstruction using RAxMLMaurolicus_spp_OpsinGeneConversionTree_GTRgammarh2 opsin gene conversion phylogeny based on a Bayesian reconstruction using a GTR & gamma model of sequence evolution

Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, Maurolicus spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides’ habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.