Researchers are trying to figure out how does Pacific bluefin tuna hearts maintain function as these fish dive from relatively warm waters of the sea surface into cooler waters in search of prey. Pacific bluefin tuna are unique because their bodies are almost endothermic, similar to that of humans. These fish are capable of elevating their core body temperatures up to 20 °C above that of the surrounding water. As these fish dive their heart temperature can drop to 15 °C within minutes, this is due to the fact that the heart is receiving blood directly from the gills which reflects water temperature. Professor Barbara Block at Stanford stated that, “Tunas are at a unique place in bony fish evolution. Their bodies are almost like ours — endothermic, but their heart is running as all fish at ambient temperatures. How the heart keeps pumping as the fish moves into the colder water is the key to their expanded global range.”
In order to solve this puzzle, Researchers from Manchester’s Medical and Human Sciences Faculty worked at the Tuna Research and Conservation Center at Stanford University. Here researchers used electronic tags to monitor wild bluefin tuna, as a result they were able to get important data on the fish range of depth , internal body temperature and the ambient water temperature (3.4 to 27 °C, this includes mean sea surface temperature of 17.36 °C ± 0.05). The data revealed that the tuna have a broad range in their thermal tolerance as they migrate from the western to eastern Pacific oceans.
The researchers noticed that the tagged fish showed interesting behaviors during their dives. For example when in search of prey these fish would display some sort of “ bounce’ dive swimming down into colder depths (500 or greater) and then quickly resurfacing into warmer surface waters. Here researchers were able to figure out that by activating adrenalin during these bounce dives was a way for the fish to adjust electrical activity of their heart cells to maintain constant calcium cycling in order to keep the heart going. For future studies the researchers from Manchester’s Medical and Human Sciences Faculty hope to monitor how bluefin tuna react to warmer temperatures, which is useful since the temperature of our planet is increasing.
Archival tag record of a juvenile Pacific bluefin tuna off the coast of California, 19—20 June 2012. Archival tag time series shows depth from pressure (blue), internal body temperature from position of archival tag in the peritoneal cavity (red) and ambient water temperature from an external sensor (green). Juvenile bluefin tuna demonstrate rapid bouts of diving associated with foraging behaviours that occur prior to a visceral heat increment of feeding event (peritoneal warming). This bounce diving, where time at depth is short, and interspersed regularly with time at the surface, may be due to cold-induced cardiac bradycardia and subsequent oxygen delivery limitations.
H. A. Shiels, G. L. J. Galli, B. A. Block. Cardiac function in an endothermic fish: cellular mechanisms for overcoming acute thermal challenges during diving. Proceedings of the Royal Society B: Biological Sciences, 2014; 282 (1800): 20141989 DOI: 10.1098/rspb.2014.1989
Manchester University. “How tuna stay warm with cold hearts.” ScienceDaily. ScienceDaily, 5 February 2015. <www.sciencedaily.com/releases/2015/02/150205083040.htm>.