ABSTRACT
The bluefin tuna heart remains at ambient water temperature (Ta) but must supply blood to warm regions of the body served by countercurrent vascular heat exchangers. Despite this unusual physiology, inherent difficulties have precluded an understanding of the cardiovascular responses to Ta in free-swimming bluefin tunas. We measured the heart rate (f(H)) responses of two captive Pacific bluefin tunas (Thunnus orientalis; 9.7 and 13.3 kg) over a cumulative period of 40 days. Routine f(H) during fasting in the holding tank at a Ta of 20°C was 45.1±8.0 and 40.7±6.5 beats min(-1) for Tuna 1 and Tuna 2, respectively. f(H) decreased in each fish with a Q10 temperature coefficient of 2.6 (Tuna 1) and 3.1 (Tuna 2) as Ta in the tank was slowly decreased to 15°C (~0.4°C h(-1)), despite a gradual increase in swimming speed. The same thermal challenge during digestion revealed similar thermal dependence of f(H) and indicated that the rate of visceral cooling is not buffered by the heat increment of feeding. Acutely decreasing Ta from 20 to 10°C while Tuna 1 swam in a tunnel respirometer caused a progressive increase in tail-beat frequency and oxygen consumption rate (M(O2)). f(H) of this fish decreased with a Q10 of 2.7 as Ta decreased between 20 and 15°C, while further cooling to 10°C saw a general plateau in f(H) around 35 beats min(-1) with a Q10 of 1.3. A discussion of the relationships between f(H), and haemoglobin-oxygen binding sheds further light on how bluefin cardiorespiratory systems function in a changing thermal environment.
Subject(s)
Heart Rate , Oxygen Consumption , Tuna/physiology , Animals , Pacific Ocean , Swimming , TemperatureABSTRACT
Specific dynamic action (SDA) is defined as the energy expended during ingestion, digestion, absorption and assimilation of a meal. This study presents the first data on the SDA response of individual tunas of any species. Juvenile Pacific bluefin tunas (Thunnus orientalis; body mass 9.7-11.0 kg; N=7) were individually fed known quantities of food consisting primarily of squid and sardine (meal energy range 1680-8749 kJ, approximately 4-13% of tuna body mass). Oxygen consumption rates (M(O2)) were measured in a swim tunnel respirometer during the postprandial period at a swimming speed of 1 body length (BL) s(-1) and a water temperature of 20 degrees C. was markedly elevated above routine levels in all fish following meal consumption [routine metabolic rate (RMR)=174+/-9 mg kg(-1) h(-1)]. The peak M(O2) during the SDA process ranged from 250 to 440 mg kg(-1) h(-1) (1.5-2.3 times RMR) and was linearly related to meal energy content. The duration of the postprandial increment in M(O2) ranged from 21 h to 33 h depending upon meal energy content. Consequently, the total energy used in SDA increased linearly with meal energy and ranged from 170 kJ to 688 kJ, such that the SDA process accounted for 9.2+/-0.7% of ingested energy across all experiments. These values suggest rapid and efficient food conversion in T. orientalis in comparison with most other fishes. Implanted archival temperature tags recorded the increment in visceral temperature (T(V)) in association with SDA. M(O2) returned to routine levels at the end of the digestive period 2-3 h earlier than T(V). The qualitative patterns in M(O2) and T(V) during digestion were similar, strengthening the possibility that archival measurements of T(V) can provide new insight into the energetics and habitat utilization of free-swimming bluefin in the natural environment. Despite efficient food conversion, SDA is likely to represent a significant component of the daily energy budget of wild bluefin tunas due to a regular and high ingestion of forage.
