Comparative investigation into the morphology of oral papillae and denticles of four species of lamnid and sphyrnid sharks

Abstract

The microstructures of the oral cavity in sharks have received relatively little study, despite their potential functional importance for gustation, feeding, and ventilation. Accordingly, here we conducted a preliminary comparative investigation into the structure and organization of oral papillae and denticles found on the ventral surface of the oral cavity in four species of shark (bigeye thresher, shortfin mako, scalloped hammerhead, and smooth hammerhead). Despite a limited sample size, differences in complexity and ornamentation of oral papillae and denticles were found across the four species. The scalloped hammerhead shark exhibited the largest oral papillae compared to the bigeye thresher and the shortfin mako. The most complex oral denticles, in terms of number of ridges and microstructures, were found in the scalloped hammerhead, followed by the bigeye thresher, smooth hammerhead and shortfin mako. For smooth hammerheads, in which samples were available from both juveniles and adults, differences in denticle microstructures were found suggesting possible ontogenetic variations. These results suggest that shape, size and arrangement of oral papillae and denticles may be related to ecology and phylogeny of the species studied. Based on these emerging patterns we discuss several plausible hypotheses relating to the function of these structures for consideration in future studies.