Effects of ceratotrichia diameter and packing density on chimaera pectoral fin kinematics

Chimaeras - an ancient group of cartilaginous fishes - swim by flapping their pectoral fins in a distinctive locomotory mode, termed flapping flight, that induces an undulatory wave traveling from the leading edge to the trailing edge of the fin. Recent work on bony fishes has shown that fins with internal structure (fin rays) may behave differently than models made of a single material. Our goal was to understand the potential significance of internal structure for the swimming kinematics of chimaeras. We designed an actuation system based on the kinematic patterns of real chimaeras and examined artificial fins of varying fin ray diameter and packing density to investigate how internal anatomy influences pectoral fin tip amplitude, leading-edge curvature, and the induction of undulatory waves. Both diameter and packing density influenced fin kinematics, but diameter had a much larger effect on fin tip amplitude over the tested ranges. Additionally, established rules for undulatory waves outlined in the flexible foil literature did not hold for our fin models. Our work provides new insights into the anatomical parameters that could influence the evolution of chimaera flapping flight, and pectoral fin locomotion more broadly, and provides direction for possible biomimetic applications.