Animal blood salinity matches ocean water and evidences evolution
The claim that animal blood salinity matches ocean water and evidences evolution is scientifically inaccurate. Physiological studies show that animals actively regulate their internal osmotic pressure and ion concentrations, and that body fluid regulation evolves dynamically when species transition between marine and freshwater environments.
The claim relies on a common misconception (often associated with older misinterpretations of physiological continuity) that animal blood perfectly mirrors ancient or modern ocean salinity. In reality, marine and freshwater animals maintain strict osmoregulatory control, and evolutionary transitions involve shifts in hemolymph osmolality rather than passive matching. Papers 1 and 2 both refute the premise by documenting active physiological regulation and evolutionary adaptation of body fluid concentrations.
Carol Eunmi Lee, Marijan Posavi, Guy Charmantier. Rapid evolution of body fluid regulation following independent invasions into freshwater habitats.. 2012. https://doi.org/10.1111/j.1420-9101.2012.02459.x
Paper 1 demonstrates that animal body fluid regulation evolves rapidly when populations invade freshwater habitats, showing that fluid salinity is not rigidly tied to ocean water.
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Tatsuya Sakamoto, Satoshi Ogawa, Yudai Nishiyama, Chiaki Akada, Hideya Takahashi, Taro Watanabe, Hiroyuki Minakata, Hirotaka Sakamoto. Osmotic/ionic status of body fluids in the euryhaline cephalopod suggest possible parallel evolution of osmoregulation.. 2015. https://doi.org/10.1038/srep14469
Paper 2 shows that euryhaline marine organisms actively regulate their hemolymph osmotic and ionic status rather than passively matching ocean salinity.
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