1. Abstract
Background In both larval and adult anurans, blood separation and respiratory physiology have remained an enigma. While various blood separation mechanisms have been proposed, the same structure is seen as playing a key role: the conus arteriosus. However, previous findings on its internal structure are contradictory, depending on the specifics of the 2D imaging methods used by different authors. To resolve this problem, we used high-resolution X-ray microtomography of whole Bufo bufo specimens to acquire the first detailed 3D descriptions of this complex structure through metamorphosis.
Results In early tadpoles two small valvular openings develop at the ventricular-conal junction, providing two paths separated by the septum coni and continuing into the aortic arches. Thus, structures to support segregated pulmonary circulation are fully developed well before the lungs appear. The external gills undergo partial resorption and retreat asymmetrically into a gill chamber formed by a hyoidal cover, leaving only a single opening on the left side, the opercular spout.
Conclusions The timing of events in Bufo circulatory development does not track the changing modes of respiration used by the developing tadpole. In particular, a system capable of double circulation carries only oxygen-depleted blood for a significant portion of the tadpole stage.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Data availability statement: The illustrated image data will be archived in the Zenodo repository [links to be added].
Funding statement: This work was not supported by external funding.
Conflict of interest disclosure: The authors declare no conflicts of interest.
Ethics approval statement: All applicable international guidelines were followed. Only museum specimens were used in this research.
Patient consent statement: N/A
Permission to reproduce material from other sources: N/A