Morpho-functional diversity in Diaphus spp. (Pisces: Myctophidae) from the central Atlantic Ocean: Ecological and evolutionary implications

Abstract Insight into the evolutionary radiation of organisms is partly underpinned by theories on the adaptive divergence of sensory systems, and the ecological, physiological, and behavioural adaptations to both environmental conditions and to competition for limited resources. The aim of this study is to determine the degree of phenotypic variability and functional niche overlap between Diaphus (Myctophidae) species that could account for the high level of speciation within this genus. The Diaphus specimens which were used for our study, were obtained from a survey transect across the central Atlantic Ocean. We analysed the morpho-functional features of fish body shape and sagittae otolith shape of these specimens. Although our study included only 9 species, our findings revealed a high degree of morpho-functional variability in the fish body and otolith shapes, both of which are coupled with the variations of the bioluminescent organs of the head, especially the suborbital organ situated under eye and called So -photophore. Two morphotypes were identified: ‘ Diaphus -deep’ (morphotype-1) with an So -photophore, and defined by a larger otolith and a deep body, larger head, eye and mouth; and ‘ Diaphus -slender’ (morphotype-2) without an So -photophore, and characterized by an increased development of anterior-dorsal margin of otolith and an enlarged body with small head, eye and mouth. Our analyses supported both the sensory drive and niche hypotheses as forces promoting the radiation within the genus Diaphus . Future studies involving a greater number of species are necessary to advance the knowledge of speciation mechanisms in myctophids.