| I tested whether the preference for ketone body oxidation (KBO) over fatty acid oxidation (FAO) in elasmobranch extrahepatic tissues is related to their urea-based osmoregulation by measuring plasma non-esterified fatty acids (NEFA) and maximal enzyme activities in tissues from marine (high urea) and freshwater (low urea) elasmobranchs. All species possessed the same metabolic organization (MO): low plasma NEFA levels, low or absent muscle carnitine palmitoyltransferase (a FAO indicator), and high KBO capacity. Salinity acclimation (=increased urea) of a euryhaline stingray caused no metabolic reorganization. My findings suggest urea was unimportant in the evolution of the organization of FAO and KBO in elasmobranchs. I show that, contrary to previous studies, extrahepatic FAQ occurs in elasmobranch kidney and rectal gland. I measured a similar MO in a holocephalan, which diverged from elasmobranchs >400 Mya, suggesting that this MO was also present in the earliest chondrichthyans. The MO of chondrichthyans may partly reflect ancestral fish characteristics. |
