Evolutionary biology of loosejaw dragonfishes (Teleostei: Stomiidae): Biodiversity, phylogenetic relationships, ecology, feeding biomechanics, and far-red visual systems

The loosejaw dragonfishes of the genera Aristostomias, Malacosteus, Pachystomias, and Photostomias possess some of the most intriguing morphologies in the deep sea. Species of the genera Aristostomias, Malacosteus , and Pachystomias possesses far-red emitting photophores and visual pigments sensitive to these long-wave emissions. Perhaps most striking is a lack of a floor to the mouth in the genera Aristostomias, Malacosteus, and Photostomias, the character for which this group is named. It is hypothesized that species with both far-red capabilities and the loosejaw condition stealthily catch prey by sweeping extremely long jaws through a field of light invisible to nearly all other deep-sea organisms. Despite these provocative characters, this group has received little scientific scrutiny in the last 50 years. The purpose of this study was to thoroughly explore the evolutionary biology of loosejaw dragonfishes, including their biodiversity, phylogenetic interrelationships, diel vertical migration (DVM), feeding biomechanics, and evolution of far-red visual systems. A revision of the genera Photostomias and Malacosteus revealed five species new to science. A study of the DVM of Aristostomias, Malacosteus, and Photostomias revealed that Aristostomias and Malacosteus deviate from the typical stomiid DVM pattern and that Malacosteus remains at depth as a strategy to obtain chlorophyll, the raw material for the perception of its most far-shifted bioluminescence. A computer model of the lower jaw system predicted that the loose jaw is not only an adaptation for increased closing speed, but that this innovation allowed for the evolution of extremely long jaws, a morphology integral to the enigmatic loosejaw feeding ecology. Two phylogenetic analyses, one based on 202 morphological characters, the other on four unlinked molecular loci, resulted in a consensus hypothesis with Pachystomias sister to the other three loosejaw genera, and Aristostomias sister to a clade containing Malacosteus and Photostomias. Based on this phylogenetic hypothesis and cranial-nerve innervation, homology of the far-red photophores was rejected and, thus, parallel evolution of long-wave bioluminescence is hypothesized. Reconstruction of ancestral states for tuning site in the gene encoding rhodopsin revealed that a far-red visual system is ancestral for the loosejaws and that Photostomias has reverted to a primitive short-wave stomiiform visual system.