Working with colleagues at Stanford University, Dr Holly Shiels and her team have discovered how bluefin tuna keep their hearts pumping during temperatures that would stop a human’s heart from beating. The research answers important questions about how animals react to rapid temperature changes, knowledge which becomes more important as the earth warms.
Pacific bluefin tuna are top predators renowned for their epic migrations. They are unique among bony fish as they are warm-bodied and capable of elevating their core temperature up to 20°C above that of the water that surrounds them. They are also capable of diving down to the colder waters below 1000m, which affects their heart temperature. Dr Holly Shiels said:
“When tunas dive down to cold depths their body temperature stays warm but their heart temperature can fall by 15°C within minutes. The heart is chilled because it receives blood directly from the gills which mirrors water temperature. This clearly imposes stress upon the heart but it keeps beating, despite the temperature change. In most other animals the heart would stop.”
The team conducted their research at Stanford University’s Tuna Research and Conservation Center, one of the only places on the planet with live tuna for research. They used archival tags to track and monitor the fish in the wild, measuring the depth they swam to, their internal body temperature, and the ambient water temperature. They then used the data to set experimental conditions in the lab with single heart tuna cells, investigating how they beat. Dr Shiels explained the findings:
“We discovered that changes in the heart beat due to the temperate, coupled with the stimulation of adrenalin by diving, adjusts the electrical activity of the heart cells to maintain the constant calcium cycling needed to keep pumping. If we went through this temperature change our calcium cycling would be disrupted, our hearts would stop beating, and we would die.”
The next step for the team will be to test other fish species to see if this method of keeping the heart pumping at low temperatures is unique to bluefin tuna. Dr Shiels concluded:
“This research was about understanding how animals perform under dramatic environmental changes. This gives us a clear insight into how one species maintains its heart function over varying temperatures, something we will need to study further given recorded changes in the earth’s temperature.”