Interspecific and intraspecific variability in placoid scale morphology in relation to body form variability in squaliformes

An ontogenetic series of nine species; Centrophorus granulosus, Centroscymnus coelolepis, Dalatias licha, Deania calcea, Echinorhinus cookei, Isistius brasiliensis, Oxynotus centrina and Squalus acanthias were studied to determine swimming capabilities, boundary-layer flow conditions and placoid scale functional morphologies. Body morphometric variables included the girth and the distance from snout to the orbitals, origin of the median and paired fins, and the caudal peduncle, body and fin surface area, fin aspect ratios and caudal fin angles. Placoid scales were sampled from sixteen regions across the body. Morphometric variables included the number of scales per area of integument, scale crown width, length and area, and scale weight. Body shape morphometry coupled with published natural histories suggest a majority of the species examined are probably capable of moderate to fast swimming speeds. There was a correlation between low fin aspect ratios and near neutral buoyancy among the species examined suggesting functions other than dynamic lift for the paired fins. Likewise, there was a trend for these fins to be reduced in size suggesting decreased drag. Freed from functioning in generating lift, the pectoral fins probably provide a high degree of vertical directional control. Larger fins with relatively high aspect ratios were correlated with negatively buoyant species suggesting a role in providing dynamic lift. These sharks were more typical in body form of coastal pelagic or benthic sharks of the order Carcharinidae. The squaloid sharks have been recognized to possess the most morphologically diverse placoid scales. Among the nine species examined, three general crown morphologies were distinguished; traditional plate-like crowns, spike or thorn-like crowns and concave trapezoidal crowns. Ontogenetic, sexual dimorphic, and spatial variability within a specimen was observed in all species and interspecific variability was observed in the trends of placoid scale variability. Placoid scale functional morphologies are hypothesised from intraspecific and interspecific variability, body shape morphologies and natural histories.