Functional Morphology and Feeding Mechanics of Billfishes

Billfishes (marlins, spearfishes, sailfishes and swordfish) are one of the fastest and largest marine apex predators, and perhaps their most recognizable attribute is their bill or rostrum. The proposed function for this novel structure has ranged from hydrodynamic enhancement to defensive weaponry. However, the most supported hypothesis for its function has been linked to feeding. Billfishes have been observed to subdue their prey with their rostrum, either stunning or cutting them into pieces before ingestion. Due to their large body sizes and pelagic lifestyles a thorough investigation of the function of this structure has been logistically challenging. The goal of my dissertation is to investigate the role of the rostrum during feeding from a functional, mechanical and morphological standpoint. By the use of interdisciplinary approaches that blend engineering with biology, the function of the rostrum and billfish putative feeding behavior was investigated. By the use of different approaches that involve morphological characterizations, histology, estimation of performance measurements such as bite force and the investigation architectural tradeoffs from geometric morphometrics analysis, my dissertation aims to characterized the role of the rostrum in billfishes as a possible adaptation for feeding. Results showed that the rostrum in billfishes is mechanically capable of acting as a feeding weapon; continuous stress distribution along its length suggest no particular point that could lead to breakage during feeding. Finite element analysis, as well as bending experiments suggest feeding behavior may be species specific and strictly associated with rostrum morphology. While istiophorids may be morphologically suited to strike their prey with a wide range of motions, swordfish appear to be specialized from a mechanical and hydrodynamic standpoint to hit their prey with lateral strikes. Biting performance is relatively low in these top predators compared to other non-billfish species suggesting the rostrum may facilitate prey processing reducing the need for powerful biting. However contrary to my expectations rostrum length was not a predictor of bite force. Skull variation was evident among billfish species. Swordfish, the species with the longest rostrum, had the smallest head and the lowest relative bite force whereas blue marlin, the species with the stiffer, most compact rostrum, had the largest head and one of the greatest relative bite forces. The shortbill spearfish showed a relatively low bite force indicating predatory success in this species may be linked to an extended lower jaw that may facilitate a speed efficient jaw during prey capture. Whether the rostrum in billfishes has evolved as an adaptation for feeding, remains uncertain. However results from this study demonstrate that rostrum material properties, morphology and head architecture, in addition to relatively low biting performance in billfishes, favor a role of prey capture for the rostrum