John Sparks' Leiognathidae Research

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This project is designed to examine the evolution, coevolutionary interactions, and biogeography of the symbiosis between fishes of the family Leiognathidae (ponyfishes) and the luminous marine prokaryote Photobacterium leiognathi. The fish host is abundant, economically important, and widely distributed throughout the Indo-Pacific. The fish harbors a dense population of the bacterial symbiont in an internal circumesophageal structure, called a light organ, providing the bacteria with oxygen and nutrients, and uses bacterial luminescence in various displays associated with predation, anti-predation and sex-specific signaling. Males of most ponyfish species exhibit greatly enlarged, structurally complex and species-specific light organs, whereas those of females are comparatively poorly developed. It is hypothesized that sexual selection for male luminescence signaling is correlated with morphological diversification of the light organ system (LOS) and speciation in ponyfishes. Although the evolution and functions of a sexually-dimorphic bioluminescent system based on species-specific male signaling are well documented in fireflies, in vertebrates these systems are poorly understood. Ponyfishes provide an excellent system in which to address these questions. The evolutionary relationships of ponyfishes will be reconstructed using DNA sequence data and phenotypic features of the fish's LOS, and examined in light of symbiont patterns of relationship. This project will provide insight into the evolution of luminescence-signaling systems in fishes, the role of sexual selection in ponyfish diversification, coevolutionary interactions in a symbiosis in which the bacterium is acquired with each new generation, and the role of the symbiosis in the biogeographic distribution of the fish and bacterium. Below is the Sparks et al 2005 Cladistics cladogram, highlighting the placement of leiognathids and the interrelationships of the majors clades and genera within the Leiognathidae.

Given the taxonomic and phylogenetic confusion in the group, we have started to travel the Indo-Pacifc to collect ponfishes. These have included seven expeditions to date by either John Sparks and Leo Smith (Madagascar 2003), John Sparks and Prosanta Chakrabarty (Taiwan, 2006, Singapore, 2007, Thailand 2007, and Malaysia 2007) or Prosanta Chakrabarty alone (Taiwan 2007 and Sri Lanka 2007).


ponytree