Evolution of the light organ system in ponyfishes (Teleostei: Leiognathidae)

Abstract

Members of the leiognathid subfamily Gazzinae, comprising approximately two-thirds of ponyfish species, are sexually dimorphic with regard to features of the light organ system (LOS). In Gazzinae, the circumesophageal light organ (LO) of males is enlarged and varies in shape compared with similarly sized conspecific females. In association with male species-specific translucent external patches on the head and flank, these sexually dimorphic LO features are hypothesized to be correlated with species-specific luminescence displays. Anatomical differences in LO shape, volume, and orientation, and its association with the gas bladder and other internal structures that function in light emission, are compared to observations of luminescence displays for every major lineage within Leiognathidae. We reconstruct the character evolution of both internal and external morphological features of the LOS to investigate the evolution of LO sexual dimorphism and morphology. Both internal and external sexual dimorphism in the ponyfish LOs were recovered as most likely to have evolved in the common ancestor of Leiognathidae, and likelihood-based correlation analyses indicate that the evolution of internal and external dimorphism in males is statistically correlated. Magnetic resonance imaging technology was applied to examine the unique internal LOs of ponyfishes in situ, which provides a new metric (LO index) for comparison of LO structure across lineages.

The Digital Fish Library: Using MRI to Digitize, Database, and Document the Morphological Diversity of Fish

“…CT and MRI are unhampered by limitations relating to the depth of signal penetration and therefore do not necessarily require any special preparation of specimens prior to imaging. This has great advantages where the dissection of specimens is not desired, as is frequently the case in museum collections [14], [25], [52], or even possible where rare or endangered species are involved [57]. These technologies also offer an efficient method for permanent storage of large volumes of quantitative digital data that requires very little physical space, promotes the use of sophisticated data analysis and visualization methods, and allows the dissemination of this data online in standard formats [5], [50].…”

“…While non-invasive MR imaging and tissue segmentation methods have already been established as a tool for in situ characterization and quantification of fish soft tissues [22]–[33], [37], [61], the DFL is the first to demonstrate the feasibility of these methods for imaging a large and diverse array of preserved specimens for comparative morphology [8]–[14]. We have shown that large stores of fixed material from numerous public or private biological collections are now candidates for soft-tissue analysis with high-field MRI, thereby increasing their value as scientific resources.…”

“…High resolution MRI is slowly but increasingly contributing volumetric image data for comparative morphological studies of soft tissue systems in other taxa [7], [51], [57], and has enormous potential to contribute similar data in a broad range of fishes. For example, working in collaboration with the DFL, Chakrabarty et al [14] has successfully applied high resolution MRI and segmentation methods to analyze the 3D morphology of the unique light organ systems (LOS) of ponyfishes, including a specimen holotype, acquired from a museum. This MRI dataset was used to reconstruct the character evolution of both internal and external features of LOS to investigate the evolution of sexual dimorphism in this soft tissue system.…”

“…[8], [9], [11], [32]), or that were exposed to alcohol in museum collections (e.g. [14], [25]–[27], [31]), processes that are known to significantly degrade tissue MRI responses, including reducing both the SNR and the contrast-to-noise ratio (CNR) [28], [33], [34]. Furthermore, with the exception of Chakrabarty et al [14], authors did not take advantage of chemical contrast agents commonly used in clinical and biomedical MRI [35].…”

“…[14], [25]–[27], [31]), processes that are known to significantly degrade tissue MRI responses, including reducing both the SNR and the contrast-to-noise ratio (CNR) [28], [33], [34]. Furthermore, with the exception of Chakrabarty et al [14], authors did not take advantage of chemical contrast agents commonly used in clinical and biomedical MRI [35]. These agents can enhance the MR responses of tissues and improve their anatomical contrast [36], [37], and may therefore be useful for mitigating some of the adverse effects of preserving and storing fishes in museums.…”