Today, oceans host a large variety of fast, resilient marine predators, including tunas, mackerels and various other fish species. Many of these fish belong to a lineage known as Scombridae, whose members are characterized by fins supported by thin, bony structures known as rays.
Some members of Scombridae are also endothermic, which means they can produce and retain body heat. This advantageous capability allows them to keep some parts of their bodies warmer than the surrounding water, which in turn improves their efficiency in hunting and exploring marine environments.
Researchers at Yale University, Virginia Polytechnic Institute and State University, the Santa Barbara Museum of Natural History and other institutions in the U.S. recently tried to retrace the evolutionary history of tunas and other scombrids to better understand how they evolved into the species that inhabit today's marine environments.
Their paper, published in the Proceedings of the Royal Society B: Biological Sciences, suggests that the evolution of these fishes' distinctive physiological features may have been slower and more gradual than earlier research predicted.
"Tunas and mackerels are some of the most economically important fishes in the ocean and act as top predators in pelagic ecosystems," Chase D. Brownstein, first author of the paper, told Phys.org. "These fishes also appear to have been part of the initial set of radiations of pelagic fishes that took place after the Cretaceous-Paleogene mass extinction 66.02 million years ago, which induced the disappearance of many lineages of large-bodied marine vertebrates."
Tracing the evolution of tunas and their ancestors
Brownstein and his colleagues wanted to better understand precisely when tunas and mackerels first appeared. In addition, they wanted to reconstruct the origins of their physiology and life history, as well as how they evolved to have the morphological features observed today.
"We were particularly interested in the origins of endothermy, commonly known as 'warm-bloodedness,' across tunas," said Brownstein.
To trace the evolution of tunas and other Scombridae, the researchers examined both fossil evidence and molecular data obtained from contemporary fish. In total, they analyzed approximately 1,000 different fish DNA sequences.
They also compared the traits of different fish species, focusing on the presence or absence of endothermy, body sizes and life spans. From the results of their analyses, the team inferred the evolutionary relationships between different species and created a tentative evolutionary tree.
"We calibrated the resulting tree in time using fossils and an idea called the molecular clock, which assumes that differences in DNA between species correspond to how long ago they shared a common ancestor," explained Brownstein. "The molecular clock allows us to reconstruct the relative ages of different splits among species, and the fossils allow us to place these relative ages in absolute time. Finally, we reconstructed the evolution of several features, including endothermy, along the tree."
Implications for the ocean's slow transformation
One key landmark in the evolution of many species was the Cretaceous-Paleogene (K-Pg) mass extinction, which occurred around 66 million years ago. This event took place after an asteroid hit Earth, prompting environmental changes that caused the extinction of an estimated 75% of animal species on Earth, including dinosaurs.
The K-Pg mass extinction is also known to have prompted the disappearance of many open-ocean (i.e., pelagic) predatory fish species. The researchers' analyses suggest that the ancestors of existing tunas and mackerels diversified into several distinct species relatively quickly following the K-Pg event.
"At first glance, our findings would seem to indicate that tunas and mackerels were radiating in response to the opportunities available to invade ecological positions previously occupied by species that the extinction had killed off, but our analyses suggest this was not 100% the case," said Brownstein.
"Interestingly, we found that at least three different lineages in the tuna-mackerel clade have independently evolved endothermy. For a while, endothermy was thought to be important to the story of how fishes like tunas became so big, but we only find very loose associations between endothermy origins and giant body sizes."
Many previous studies proposed that tunas and mackerels rapidly grew in size after the K-Pg mass extinction and linked the emergence of endothermy to this size change. While the evolutionary tree created by Brownstein and his colleagues also suggests that these fish got bigger right after the K-Pg, the researchers showed that they did not become endothermic until several million years later.
"The origins of endothermy occur way later and are staggered across something like 30 million years of time," explained Brownstein. "This suggests a very indirect connection, if any, between endothermy origins and the K-Pg, or subsequent events such as bouts of extreme global warming starting ~56 million years ago. Most broadly, this provides a cautionary tale for the overinterpretation of how trait evolution has occurred in response to ecosystem change."
This study offers new insight into how some of the most well-known ocean predators evolved after the K-Pg event, gradually evolving into the fish that inhabit Earth's oceans and seas today. The team's newly compiled evolutionary tree could soon be refined further or expanded to include an even wider variety of fish species.
"Understanding the relationship between ecological change and trait evolution is at the forefront of our team's research agenda," added Brownstein. "I am excited to continue understanding this relationship in our future work."