John Sparks's Ponyfish (Leiognathidae) Bioluminescence Research
This NSF-funded project examines 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, predator avoidance, 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 have be reconstructed using DNA sequence data from both the mitochondrial and nuclear genomes and phenotypic features of the fish's LOS, and examined in light of symbiont patterns of relationship. This project has provided 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 Leiognathidae.
Given a history of taxonomic and phylogenetic confusion in the group, we have collected extensively throughout the Indo-Pacifc range of ponyfishes. To date, 12 ponyfish collecting expeditions have been undertaken by either John Sparks, Leo Smith, and Kevin Tang (Madagascar 2003), John Sparks and Prosanta Chakrabarty (Taiwan, 2006, Singapore, 2007, Thailand 2007, Malaysia 2007, Madagascar 2008), Prosanta Chakrabarty and Hans Ho (Taiwan 2007), Prosanta Chakrabarty (Sri Lanka 2007), and John Sparks, Zachary Baldwin, and Robert Schelly (Vietnam 2010, Qatar 2008, 2010, and Madagascar 2011). These collections include several species of ponyfishes new to science.