The impact of evolutionary trade-offs among bioluminescent organs and body shape in the deep sea: A case study on lanternfishes

Functional innovations play major roles in constraining or contributing to the evolutionary diversification of organismal morphology. Among fish key innovations, bioluminescence is critical to the success of many deep-sea fishes. Understanding evolutionary dynamics of deep-sea fish lineages and the role bioluminescent light organs have on their evolution is a continuing effort of many deep-sea biologists. This study focused on assessing how bioluminescent structures involved with sexual communication may be impacting the evolution of body shape in the lanternfishes (Myctophiformes), a lineage of fishes possessing some of the most extensive sexual dimorphism in bioluminescent structures seen in deep-sea organisms. Results from this study using geometric morphometrics integrated with a phylogenetic hypothesis of lanternfish relationships suggest there is significant evolutionary modularity between the caudal peduncle and the rest of the body within lanternfishes. Our results also show that most lanternfish species lacking caudal light organs cluster together in morphospace ( Diaphus , Hintonia , Neoscopelidae), possess enlarged headlight organs ( Diaphus ), and exhibit lower phylogenetic signal than species possessing caudal light organs. These results support the hypothesis that lanternfish body-shape evolution is influenced by selective pressures on their light-producing tail organs and that the importance of caudal light organs in lanternfish communication may result in differential selection on the caudal area in species that possess them. One possible interpretation for why lanternfishes without caudal light organs coalesce in morphospace may be that by removing the caudal light organ, natural selection would select for a lanternfish caudal morphology optimized for the physical landscape of the midwater that they live in, favoring more energy efficient and slow sustained swimming styles. • Tail light organ presence may affect body-shape evolution in lanternfishes. • Lanternfishes exhibit modularity between the caudal peduncle and the rest of the body. • Lanternfishes possessing caudal light organs show higher phylogenetic signal than those lacking caudal light organs. • Most lanternfish species without tail-light organs cluster together in morphospace. • Propose differential selection on the caudal area between species with and without tail-light organs.