Feeding in billfishes: inferring the role of the rostrum from a biomechanical standpoint

Abstract

Perhaps the most striking feature of billfishes is the extreme elongation of the premaxillary bones forming their rostra. Surprisingly, the exact role of this structure in feeding is still controversial. The goal of this study is to investigate the use of the rostrum from a functional, biomechanical and morphological standpoint to ultimately infer its possible role during feeding. Using beam theory, experimental and theoretical loading tests were performed on the rostra from two morphologically different billfish, the blue marlin (Makaira nigricans) and the swordfish (Xiphias gladius). Two loading regimes were applied (dorsoventral and lateral) to simulate possible striking behaviors. Histological samples and material properties of the rostra were obtained along their lengths to further characterize structure and mechanical performance. Intraspecific results show similar stress distributions for most regions of the rostra, suggesting that this structure may be designed to withstand continuous loadings with no particular region of stress concentration. Although material stiffness increased distally, flexural stiffness increased proximally owing to higher second moment of area. The blue marlin rostrum was stiffer and resisted considerably higher loads for both loading planes compared with that of the swordfish. However, when a continuous load along the rostrum was considered, simulating the rostrum swinging through the water, swordfish exhibited lower stress and drag during lateral loading. Our combined results suggest that the swordfish rostrum is suited for lateral swiping to incapacitate their prey, whereas the blue marlin rostrum is better suited to strike prey from a wider variety of directions.

Lubricating the swordfish head

“…The roughness has been interpreted as a micro turbulence generator suppressing flow separation and reducing drag (Videler, 1995). A recent study using computed tomography scans and biomechanical analysis confirmed the suitability of the sword as a tool to slash prey, but also revealed a weak, poorly mineralised area near the base of the rostrum just in front of the eyes (Habegger et al, 2015). The results of this study made us aware of the significance of anatomical features in the same area we found between 1996 and 2007.…”

Revisiting the links between bone remodelling and osteocytes: insights from across phyla

“…D) (Atkins et al , ). It is unknown whether billfish rostra suffer high strains or stresses, but this is likely, given the massive accelerations and cantilever bending they apply to their bills in rapid, slashing pursuit of small fishes (Domenici et al , ; Habegger et al , ). To our knowledge, there have been no studies of damage and bone material failure modes in fish bone, so it is possible that this remodelling morphology is not purely damage‐driven (e.g.…”

Aggressive interactions between juvenile swordfishes and blue sharks in the Western Mediterranean: a widespread phenomenon?

“…This is difficult to ascertain. On one hand, several studies suggest that the swordfish' bill has important hydrodynamic and feeding-related functions (Habegger et al 2015, Videler et al 2016) thus bill breakage could be seriously disadvantageous for swordfish. On the other hand, at least in billfishes, there are several reports of apparently healthy individuals with damaged, malformed or even missing rostra (Frazier et al 1994).…”

“…Assuming an isometric relationship in the growth of the snout, total snout length (TSL) was calculated for each case in relation to the distance from the tip to the breaking point. To add further correction, we observed the arrangement of the paired nutrient canals at the breaking point, as shown in cross sections, to compare it with a swordfish analysed by Habegger et al (2015). The allometric regression between TSL and body length (BL), from post operculum to tail fork, was obtained following McGowan (1988).…”