Chapter 13: Synapses

Synapses fish combat constant water loss and salt gain by drinking seawater, actively secreting excess ions via gill chloride cells, and producing small volumes of highly concentrated urine. In contrast, freshwater animals—like fish and amphibians—face the inverse problem: excessive water influx and ion loss. They do not drink, produce copious dilute urine, and actively absorb ions across their gills or skin to maintain homeostasis. The authors then explore strategies used by euryhaline species, which tolerate wide salinity ranges. Fish such as salmon undergo complex physiological remodeling—modifying gill ion transporter expression, altering hormone levels (e.g., cortisol and prolactin), and changing kidney function—when transitioning between freshwater and marine environments. Elasmobranchs (sharks and rays) maintain osmotic equilibrium by accumulating urea and trimethylamine oxide (TMAO), a strategy called urea retention. The chapter explains how these solutes stabilize proteins and maintain internal osmolarity close to that of seawater, minimizing osmotic stress. Terrestrial animals face the challenge of minimizing water loss while regulating salt intake. The chapter outlines adaptations such as behavioral avoidance of heat, nocturnal activity, efficient kidneys that concentrate urine, and specialized nasal passages that reduce evaporative loss. Kangaroo rats exemplify extreme water conservation by surviving entirely on metabolic water from food oxidation. Reptiles and birds often rely on extrarenal salt glands to excrete excess ions, allowing them to inhabit arid or saline environments. The final section covers animals in hyper-saline and dehydrating habitats, including desert insects, brine shrimp, and camels. These organisms exhibit unique physiological tricks—like hygroscopic rectal pads, tolerance of body water fluctuations, and selective storage of electrolytes—to manage severe osmotic stress. Overall, this chapter presents a wide array of evolutionary innovations that animals employ to maintain water and salt balance across the full spectrum of Earth’s habitats.