The largest salmon on Earth has tusks like a humped pig

The largest salmon species ever discovered, Oncorhynchus rastrosus, can use its tusk-like teeth to compete with rivals, defend itself from predators, and dig nests.

5 million years ago, Oncorhynchus rastrosus, a prehistoric salmon up to 2.7m long and weighing more than 180kg, lived throughout the northwest sea of ​​North America. They often ambushed their prey in the seas and rivers during the Mesozoic. They are nearly twice as long and three times heavier than the largest salmon in the world today, the Chinook salmon. O. rastrosus also has a pair of scary front teeth, sticking out on both sides of its mouth like tusks, but not fangs. Researchers described the O. rastrosus salmon in detail in the journal PLOS ONE on April 24, according to Popular Science .

Specimen of salmon Oncorhynchus rastrosus with backward teeth. (Photo: University of Oregon)

O. rastrosus was first described in 1972. At that time, scientists thought that its oversized teeth grew backwards like fangs. The largest member of the Salmon family, they are often called "sword-toothed salmon" due to the position of their distinctive pair of teeth. However, the results of computed tomography of some O. rastrosus fossils and analysis in the study confirmed that two 5 cm long curved teeth resemble warty pig tusks.

The team isn't sure what these distinctive tusks are used for, but think they mainly help fight other salmon or predators. This may also be a way for female fish to dig nests to lay eggs or help both species swim upstream to reproduce. "When they swim upstream, they can hook their tusks somewhere to rest without wasting energy, similar to the way you hold onto a handrail at the edge of a swimming pool," said paleontologist Edward Davis at the University of Oregon. , explain.

According to fish specialist professor Brian Sidlauskas at the University of Oregon, this salmon most likely does not use its teeth to catch prey. They may be water filters that feed on plankton. That may be one of the reasons they reach such giant sizes. Their relatives the sockeye salmon as well as baleen whales and basking sharks all have gill rakers to filter oxygen and microorganisms from the water. According to Davis, this giant salmon species has an unusually large number of gill rakers. Foraging by filtering water with gill rakers can help them grow by consuming more organisms and obtaining more nutrients.

O. rastrosus also lives in environments with large sources of water and food to support its body. This giant salmon lived during the late Mesozoic, when the world's oceans were much warmer than today. Global carbon dioxide levels are also close to what the Earth will experience by the year 2100. Like today's salmon, O. rastrosus hatches in freshwater, swims to the ocean, then returns to freshwater to spawn. and die.

O. rastrosus became extinct when the Earth began to cool. This climate change is likely to eliminate the resources that giant salmon need to maintain their large bodies. In the future, Davis and his colleagues plan to further analyze some O. rastrosus specimens and explore how their tusks are used.