The relative length of the cardiac bulbus arteriosus reflects phylogenetic relationships among elasmobranchs

Previous work on the anatomy of embryonic and adult hearts of the shark Galeus atlanticus showed that the cardiac outflow tract consists not of a single component but of two, the myocardial conus arteriosus and the nonmyocardial bulbus arteriosus. However, it was still uncertain whether a bulbus occurs regularly in elasmobranchs. The present study was designed to gain new insight into this issue. Moreover, we investigated the interspecific variation in the relative length of the bulbus in order to decide whether such variation is related to the lifestyle or to the phylogenetic position of sharks and rays. The material examined consisted of hearts of 42 species belonging to seven families of squalomorphs, six families of galeomorphs and six families of batoids. Our findings confirmed that the bulbus arteriosus is indeed a cardiac outflow tract component commonly present in all elasmobranchs examined. In addition, we propose that the relative length of the bulbus arteriosus is rather related to the phylogenetic position of the species than to its lifestyle. Overall, the bulbus arteriosus is notably longer in galeomorphs than in squalomorphs and batoids, with the exception of the two representatives of Torpedo, which possess a bulbus arteriosus longer than other batoids. Likewise, the anatomical origin of the outflow tract from the ventricle is closely related to the relative length of the bulbus. We suggest that both anatomical traits are associated with the phylogenetic position of the sharks and rays investigated.