NISHIGUCHI, M.K.*; BOLETZKY, S.v.: Enlightenment of old ideas from new investigations: The evolution of bacteriogenic light organs

Bioluminescence is widespread among many different types of marine organisms. Metazoans contain two types of luminescence production: bacteriogenic (symbiotic with bacteria), or autogenic, via the production of a luminous secretion or the intrinsic properties of luminous cells. Several species in two families of squids, the Loliginidae and the Sepiolidae (Mollusca: Cephalopoda) harbor bacteriogenic light organs that are found central in the mantle cavity. These light organs are unique in function, that is, the morphology and the complexity suggests that the organ has evolved to enhance and direct light emission from bacteria that are harbored inside. Although light organs are widespread among taxa within the Sepiolidae, the origin and development of this important feature is not well studied. We have compared light organ morphology from several closely related taxa within the Sepiolidae and the Loliginidae, as well as combining phylogenetic data to determine whether this character was an ancestral trait that was repeatedly lost among both families, or whether it has evolved independently as an adaptation to the pelagic and benthic lifestyles. By comparing other closely related extant taxa that do not contain symbiotic light organs, we have hypothesized that the ancestral state of sepiolid and loliginid light organs most likely evolved from part of a separate accessory gland open to the environment that allowed colonization of bacteria to occur and influence the eventual development of the modern light organ.