Shrinking of fish under warmer temperatures decrease dispersal abilities and speciation rates

There is an ongoing debate as to whether fish body size will decrease with global warming and how body size changes may impact dispersal abilities and speciation rates. Although theory predicts that, when fish face warmer temperatures, they grow to smaller adult sizes, see a reduction in their ability to move, and increase their probability of speciation, evaluations of such predictions are hampered owing to the lack of empirical data spanning both wide temporal and geographical scales. Here, using phylogenetic methods, temperature, and 21,895 occurrences for 158 worldwide-distributed species of fish, we show that smaller fish have occurred in warmer waters for over 150 million years and across marine and freshwater realms. Smaller fish have historically moved the shortest distances and at low speeds. In addition, small fish display the lowest probability of giving rise to new species. Further, we found that species of fish that displayed high speeds of geographical movement and rates of size evolution experienced higher rates of temperature change in their lineage. Taking these results together, global warming predicts a future where smaller fish that have reduced ability to move over aquatic systems will be more prevalent, in turn, this will result in fewer species contributing global biodiversity.