Morphological And Molecular Analyses of an Anatomical Novelty: The Pelvic Fin Filaments of the South American Lungfish

Abstract

The pelvic fins of male South American lungfish, Lepidosiren paradoxa, are adorned with a distinctive array of filaments, which grow and become highly vascularized during the breeding season. The resemblance between these pelvic fin filaments (PFFs) and external gills of other vertebrates suggested that this gill-like structure was used for physiological gas exchange. It has been proposed that the unique pelvic fin of male L. paradoxa is used for release of oxygen from its blood into the environment in order to aerate its nesting brood, or, conversely, as an auxiliary respiratory organ by absorbing oxygen from the environment into its bloodstream. Here, we employed histology, scanning electron microscopy (SEM) and quantitative PCR (qPCR) to assess whether the morphology and molecular profile of PFFs are compatible with a role in gas exchange. First, we closely examined its external morphology and showed that PFFs develop from short papillae during the rainy season, but remain covered by a thick nonvascularized epithelium. Histological examination confirmed that capillaries within the filaments are separated from the exterior by a basement membrane and a stratified epithelium composed of four to five cell layers. In addition, SEM analysis revealed significant differences between the fin filament epithelium and typical gill epithelium. Finally, our qPCR results showed that five genes commonly expressed in gills were downregulated in PFFs relative to their expression in regular pectoral fin epidermis. Collectively, our results do not directly support a role for PFFs, commonly referred to as "limb gills", in oxygen release or uptake.

Globin E is a myoglobin-related, respiratory protein highly expressed in lungfish oocytes

“…However, given the overall similarity between pelvic fin filaments and external gills, it was also proposed that pelvic fin filaments were used to aerate the eggs by releasing oxygen from the male’s blood into the water 38 . Recently, however, it was shown that Lepidosiren pelvic fin filaments neither have the morphology compatible for oxygen diffusion nor the gene expression profile typical of gill filaments 39 . Therefore, it is conceivable that GbE may help to extract O 2 from the water to support the development of the embryo, which may be considered analogous to the function of the embryonic Hb in tetrapods 3 .…”

A Morphological and Histological Investigation of Imperfect Lungfish Fin Regeneration

“…The function of these fin attachments has not yet been fully clarified. Suggestions that they serve as "limb gills" for the release or uptake of oxygen could not be directly confirmed (Lima et al, 2017).…”