Abstract
Development of the bonefish Albula vulpes (A. vulpes) is characterized by an unusual sequence whereby an initial stage of rapid larval growth from hatch to ~65 mm is followed by a significant reduction in length to ~35 mm that precedes transition to the juvenile stage. During the rapid lengthening phase, the larvae are leptocephalus: their body plans display a thin, ribbon-like appearance that resembles that of the common eel. This stage is morphologically and physiologically distinct from later stages. Advantages to this unusual development sequence are currently not well understood. To investigate the potential benefit of the rapid growth stage on energetics, 2D and 3D computational modeling of swimming leptocephalus A. vulpes was performed. The NASA CFD solver FUN3D was used to solve unsteady incompressible Navier-Stokes equations. The idealized 2D fish geometry was represented with a NACA 0003 symmetric airfoil. Geometry for 3D calculations was constructed using digitized body profiles and fineness measurements from field samples. Body conforming meshes were generated around the 2D and 3D geometries and deformed to follow prescribed anguilliform swimming kinematics through a tailbeat cycle. The optimal spatial and temporal discretization’s for the simulations were assessed using sensitivity studies. The simulation test suite was comprised of swimming simulations for a Reynolds range from 36 to 8500 which corresponds to a larval length from 6 mm to 65 mm and specific swim speeds of 1 BL/s and 2 BL/s. The tailbeat frequency (Strouhal) required for steady swimming at a given Reynolds and swim speed was determined using an iterative approach. It was found that over the course of their development, the swimming efficiency of leptocephalus A. vulpes increased by 540% and 495% for the 2D and 3D simulations respectively. The specific cost of transport decreased by approximately 90% over this phase in both 2D and 3D simulations. The large ratio between the boundary layer thickness and the transverse radius of curvature caused significant discrepancies between the 2D and 3D results due to a strong three-dimensional influence on the skin friction. Although the 2D and 3D results differed, they both show rapid growth is beneficial as it allows the larvae to escape the high costs and inefficiencies of swimming at low Reynolds. However, this rapid lengthening is only effective to approximately the size at which they metamorphose to the subsequent stage. These results support the hypothesis that leptocephalus A. vulpes may disperse over long distances through persistent anguilliform swimming.