Mechanical challenges to freshwater residency in sharks and rays

Abstract

Major transitions between marine and freshwater habitats are relatively infrequent, primarily as a result of major physiological and ecological challenges. Few species of cartilaginous fish have evolved to occupy freshwater habitats. Current thought suggests that the metabolic physiology of sharks has remained a barrier to the diversification of this taxon in freshwater ecosystems. Here, we demonstrate that the physical properties of water provide an additional constraint for this species-rich group to occupy freshwater systems. Using hydromechanical modeling, we show that occurrence in fresh water results in a two-to three-fold increase in negative buoyancy for sharks and rays. This carries the energetic cost of lift production and results in increased buoyancy-dependent mechanical power requirements for swimming and increased optimal swim speeds. The primary source of buoyancy, the lipidrich liver, offers only limited compensation for increased negative buoyancy as a result of decreasing water density; maintaining the same submerged weight would involve increasing the liver volume by very large amounts: 3-to 4-fold in scenarios where liver density is also reduced to currently observed minimal levels and 8-fold without any changes in liver density. The first data on body density from two species of elasmobranch occurring in freshwater (the bull shark Carcharhinus leucas, Mü ller and Henle 1839, and the largetooth sawfish Pristis pristis, Linnaeus 1758) support this hypothesis, showing similar liver sizes as marine forms but lower liver densities, but the greatest negative buoyancies of any elasmobranch studied to date. Our data suggest that the mechanical challenges associated with buoyancy control may have hampered the invasion of freshwater habitats in elasmobranchs, highlighting an additional key factor that may govern the predisposition of marine organisms to successfully establish in freshwater habitats.

Conservation challenges of sharks with continental scale migrations

“…Adult movement away from inshore regions may be the result of changes in diet or resource needs, or could be a mechanism to reduce competition or avoid predation on kin. Recent research by Gleiss et al (2015) has also suggested liver density may play a role in use of freshwater regions by elasmobranch species. It is thus possible that previously unconsidered biological drivers are also at work.…”

A novel method for determining post-release mortality, behavior, and recovery period using acceleration data loggers

“…Additionally, in the data collected here, ascent TBAA is much higher than descent TBAA, and ascent TBC is shorter duration than descent TBC, showing that sharks beat their tail harder and faster while ascending than while descending. This is likely due to their lack of a swim bladder and negative buoyancy (Baldridge, 1970;Gleiss et al, 2015), which makes ascending more work than descending (Gleiss et al, 2011;Nakamura et al, 2011;Weihs, 1977). Thus individuals chose to exert additional energy performing low-cost behaviors (such as descents) during recovery, possibly to increase water flow over the gills, but not in high-cost behaviors (such as ascents; Gleiss et al, 2011).…”

Effect of weight and frontal area of external telemetry packages on the kinematics, activity levels and swimming performance of small‐bodied sharks

“…While the majority of tag validation studies to date have cited tag mass as a key determinant of tag suitability, studies ought to move away from this paradigm and provide systematic assessments of hydrodynamic drag and alterations to buoyancy, which are arguably more relevant to aquatic animals. Furthermore, buoyancy control represents a substantial energetic investment for elasmobranchs, including juveniles that are typically less buoyant than adults (Gleiss et al, 2015;Iosilevskii & Papastamatiou, 2016). Therefore, small juvenile N. brevirostris experienced alterations in activity levels, swimming kinematics and swimming performance by carrying external telemetry packages.…”