Only a Handful of Fish Species are Warm-Blooded. Why?

Tuna, opah, and billfishes such as marlins and swordfish are among the 0.1% of fish species that are warm-blooded. Evidence by an international team of researchers led by UC San Diego’s Scripps Institution of Oceanography reveals that they developed that trait to keep up with new neighbors millions of years ago.The team’s new study, released June 26 in the journal Science Advances, employed an unprecedented exhaustive genetic analysis of the evolutionary tree of ray-finned fishes like tuna to understand when they became endothermic, or warm-blooded, and which specific species acquired it. “An evolutionary tree is like a family tree on steroids. It lets us trace how different species and their traits evolved over millions of years,” said lead author Fernando Melendez-Vazquez, a Scripps Oceanography PhD candidate. “By calibrating the tree with fossil data, we can estimate when certain species—and the traits they carry, like warm-bloodedness—first appeared.”The emergence of warm-bloodedness tracks with the first appearance of whales and other cetaceans in the oceans after they transitioned away from living on land. Fossil evidence shows that ray-finned fishes were in the same places at the same times as their large new competitors. 










Key genes linked to warm-blooded traits across marine animals. Different parts of the tuna’s body are connected to genes involved in muscle power, metabolism, nerve and bone development, and how the body responds to the environment. These genes evolved more rapidly in warm-blooded species—including tuna, whales, penguins, sea lions, and sea turtles—compared to their cold-blooded relatives. Illustration: Julie Johnson (Life Science Studios)

Warm-bloodedness gives the marine organisms that possess it an advantage in finding and catching prey. Maintaining a high and stable internal body temperature allows for faster muscle function, enhanced visual processing, and sustained high-speed swimming, warm-bloodedness gives marine organisms that possess it a significant edge in detecting, pursuing, and capturing agile prey, especially in colder or deeper waters where most fish would slow down.Previously, researchers had hypothesized that ray-finned fishes became warm-blooded based on their diet or on the range of depth to which they traversed the oceans. “This study reshapes our understanding of how endothermy evolved in ray-finned fishes,” said lead author Fernando Melendez-Vazquez, a Scripps Oceanography PhD candidate. “By integrating ecological, morphological, and genomic data, we’ve shown that competition with cetaceans, combined with specific physical and genetic adaptations, likely drove this rare trait’s emergence.”At a time of accelerating change in the ocean, the National Science Foundation-supported study helps illuminate how marine species have adapted to environmental pressures in the past, and how they might respond in the future. It also underscores the power of international collaboration to uncover nature’s hidden patterns and evolutionary surprises.“Our findings highlight the power of interdisciplinary approaches in uncovering the evolutionary forces behind complex adaptations,” said corresponding author Dahiana Arcila, a marine biologist at Scripps Oceanography and curator of Scripps’ Marine Vertebrate Collection. "This work opens new avenues for exploring how ecological interactions shape biodiversity.”The international team includes researchers from the Smithsonian National Museum of Natural History, Japan’s Chiba Natural History Museum and Institute, France’s Université Claude Bernard of Lyon, Australia’s Commonwealth Scientific and Industrial Research Organization (CSIRO), Italy’s Università di Torino, and Switzerland’s University of Basel Zoological Institute, among others.