Salinity tolerances and use of saline environments by freshwater turtles: implications of sea level rise

Abstract

The projected rise in global mean sea levels places many freshwater turtle species at risk of saltwater intrusion into freshwater habitats. Freshwater turtles are disproportionately more threatened than other taxa; thus, understanding the role of salinity in determining their contemporary distribution and evolution should be a research priority. Freshwater turtles are a slowly evolving lineage; however, they can adapt physiologically or behaviourally to various levels of salinity and, therefore, temporarily occur in marine or brackish environments. Here, we provide the first comprehensive global review on freshwater turtle use and tolerance of brackish water ecosystems. We link together current knowledge of geographic occurrence, salinity tolerance, phylogenetic relationships, and physiological and behavioural mechanisms to generate a baseline understanding of the response of freshwater turtles to changing saline environments. We also review the potential origins of salinity tolerance in freshwater turtles. Finally, we integrate 2100 sea level rise (SLR) projections, species distribution maps, literature gathered on brackish water use, and a phylogeny to predict the exposure of freshwater turtles to projected SLR globally. From our synthesis of published literature and available data, we build a framework for spatial and phylogenetic conservation prioritization of coastal freshwater turtles. Based on our literature review, 70 species (∼30% of coastal freshwater turtle species) from 10 of the 11 freshwater turtle families have been reported in brackish water ecosystems. Most anecdotal records, observations, and descriptions do not imply long-term salinity tolerance among freshwater turtles. Rather, experiments show that some species exhibit potential for adaptation and plasticity in physiological, behavioural, and life-history traits that enable them to endure varying periods (e.g. days or months) and levels of saltwater exposure. Species that specialize on brackish water habitats are likely to be vulnerable to SLR because of their exclusive coastal distributions and adaptations to a narrow range of salinities. Most species, however, have not been documented in brackish water habitats but may also be highly vulnerable to projected SLR. Our analysis suggests that approximately 90% of coastal freshwater turtle species assessed in our study will be affected by a 1-m increase in global mean SLR by 2100. Most at risk are freshwater turtles found in New Guinea, Southeast Asia, Australia, and North and South America that may lose more than 10% of their present geographic range. In addition, turtle species in the families Chelidae, Emydidae, and Trionychidae may experience the greatest exposure to projected SLR in their present geographic ranges. Better understanding of survival, growth, reproductive and population-level responses to SLR will improve region-specific population viability predictions of freshwater turtles that are increasingly exposed to SLR. Integrating phylogenetic, physiological, and spatial frameworks t...

Effects of temperature and salinity on body fluid dynamics and metabolism in the estuarine Diamond-backed Terrapin (Malaclemys terrapin)

“…Dunson and Mazzotti (1989) proposed that the evolution of fully marine reptiles from freshwater ancestors progressed through several transitional stages related to the capacity for osmoregulation. Initial exploration of brackish environments was facilitated by behavioral adjustments to avoid high salinities and reduce passive and active exchange of water and salts with the environment (Dunson and Mazzotti, 1989;Agha et al, 2018). Extrarenal glands to facilitate salt excretion evolved in more advanced forms, and subsequent specialization of salt glands to accommodate higher rates of salt intake are evident in fully marine species (Dunson, 1970).…”

“…Although terrapins possess lachrymal salt glands, they are still dependent on periodic access to freshwater or low salinity water in order to maintain osmotic balance and cannot survive in seawater indefinitely. This aspect of terrapin biology makes them particularly vulnerable to projected changes in coastal environments with climate change and sea level rises (Agha et al, 2018). Terrapins are a species of conservation concern (Hart and Lee, 2006), and additional studies to assess the implications of their osmoregulatory strategy for habitat utilization and resilience are warranted.…”

Physiological consequences of rising water salinity for a declining freshwater turtle

“…For reptiles and amphibians, even small increases in salinities equivalent to <1% seawater can dramatically affect physiological performance and lead to death (Hall et al , 2017; Hopkins and Brodie, 2015; Findlay and Kelly, 2011). While a few freshwater reptiles have adapted to brackish water environments along coastlines, maintaining osmoregulatory homoeostasis in chronically hypertonic environments can be physiologically challenging, especially given the limited tolerance of most freshwater reptiles to salinity (Agha et al , 2018; Dunson and Mazzotti, 1989).…”

Long-Term Variation in Survival of A Neotropical Freshwater Turtle: Habitat and Climatic Influences

“…Recapture probabilities varied by life stages; differences in the probability of recapture between size classes were associated with the high fidelity to territories by adults. The present increases in frequency and severity of El Niño and La Niña may exacerbate the consequences of climatic regimes on natural populations of turtles by increasing the mortality caused by drastic phenomena such as floods.Diversity 2019, 11, 97 2 of 12 scenarios [18][19][20]. The slow life history of turtles makes it necessary for long-term demographic research to generate information relevant to their conservation [21][22][23].…”

“…Diversity 2019, 11, 97 2 of 12 scenarios [18][19][20]. The slow life history of turtles makes it necessary for long-term demographic research to generate information relevant to their conservation [21][22][23].…”