Meeting Abstract
Corydoras julii is a small, Amazonian catfish armed with 44-48 bony scales called scutes on each side. The scutes are thin (~1 mm thickness), highly mineralized (55-60% mineralization), and do not imbricate (overlap) much with one another (~30% overlap, compared to >50% in other armored fishes). The scutes are mechanically reinforced with a superficial layer of a highly mineralized tissue called hyaloine. With such thin and rigid scutes, its armor may not be tough enough to prevent puncture. The purpose of this study was to study the mechanical properties and material composition of the C. julii armor. This was achieved by performing puncture tests on the scutes using an Instron 5942 equipped with an 18-gauge needle, approximately the same diameter as a red-bellied piranha (Pygocentrus nattereri) tooth. Puncture tests showed that posterior scutes are weaker (Young’s Modulus=116 MPa and Ultimate Strength=1.06 N) than scutes located on the anterior (162 MPa, 1.35 N) or middle (144 MPa, 1.29 N) regions. Only 2% of punctures went through two scutes, showing the synergistic effects of fish armor’s mechanical properties and that scale thickness affects puncture resistance. Feeding trials between P. nattereri and C. julii were performed to test whether C. julii armor could withstand in vivo bites from a predator. In feeding trials, P. nattereri targeted the weakest area of the armor, at the tail, and punctured through the armor after an average of 9.1 (+/- 3.5) bites. Additionally, they slide their armor into place so that there is additional reinforcement from scute imbrication and splay their pectoral spines in anticipation of an attack. The C. julii armor could not withstand repeated attacks from P. nattereri; however, it may be strong enough to withstand an initial predation event, allowing for them to escape while the predator reorients itself for another strike.
