Routine Dives Research Articles

Cardiorespiratory adaptations in small cetaceans and marine mammals.

The dive response, or the 'master switch of life', is probably the most studied physiological trait in marine mammals and is thought to conserve the available O2 for the heart and brain. Although generally thought to be an autonomic reflex, several studies indicate that the cardiovascular changes during diving are anticipatory and can be conditioned. The respiratory adaptations, where the aquatic breathing pattern resembles intermittent breathing in land mammals, with expiratory flow exceeding 160litres s-1 has been measured in cetaceans, and where exposure to extreme pressures results in alveolar collapse (atelectasis) and recruitment upon ascent. Cardiorespiratory coupling, where breathing results in changes in heart rate, has been proposed to improve gas exchange. Cardiorespiratory coupling has also been reported in marine mammals, and in the bottlenose dolphin, where it alters both heart rate and stroke volume. When accounting for this respiratory dependence on cardiac function, several studies have reported an absence of a diving-related bradycardia except during dives that exceed the duration that is fuelled by aerobic metabolism. This review summarizes what is known about the respiratory physiology in marine mammals, with a special focus on cetaceans. The cardiorespiratory coupling is reviewed, and the selective gas exchange hypothesis is summarized, which provides a testable mechanism for how breath-hold diving vertebrates may actively prevent uptake of N2 during routine dives, and how stress results in failure of this mechanism, which results in diving-related gas emboli.

High heart rates in hunting harbour porpoises.

The impressive breath-hold capabilities of marine mammals are facilitated by both enhanced O2 stores and reductions in the rate of O2 consumption via peripheral vasoconstriction and bradycardia, called the dive response. Many studies have focused on the extreme role of the dive response in maximizing dive duration in marine mammals, but few have addressed how these adjustments may compromise the capability to hunt, digest and thermoregulate during routine dives. Here, we use DTAGs, which record heart rate together with foraging and movement behaviour, to investigate how O2 management is balanced between the need to dive and forage in five wild harbour porpoises that hunt thousands of small prey daily during continuous shallow diving. Dive heart rates were moderate (median minimum 47-69 bpm) and relatively stable across dive types, dive duration (0.5-3.3 min) and activity. A moderate dive response, allowing for some perfusion of peripheral tissues, may be essential for fuelling the high field metabolic rates required to maintain body temperature and support digestion during diving in these small, continuously feeding cetaceans. Thus, despite having the capacity to prolong dives via a strong dive response, for these shallow-diving cetaceans, it appears to be more efficient to maintain circulation while diving: extreme heart rate gymnastics are for deep dives and emergencies, not everyday use.

A Baseline Model For Estimating the Risk of Gas Embolism in Sea Turtles During Routine Dives.

Sea turtles, like other air-breathing diving vertebrates, commonly experience significant gas embolism (GE) when incidentally caught at depth in fishing gear and brought to the surface. To better understand why sea turtles develop GE, we built a mathematical model to estimate partial pressures of N2 (PN2), O2 (PO2), and CO2 (PCO2) in the major body-compartments of diving loggerheads (Caretta caretta), leatherbacks (Dermochelys coriacea), and green turtles (Chelonia mydas). This model was adapted from a published model for estimating gas dynamics in marine mammals and penguins. To parameterize the sea turtle model, we used values gleaned from previously published literature and 22 necropsies. Next, we applied this model to data collected from free-roaming individuals of the three study species. Finally, we varied body-condition and cardiac output within the model to see how these factors affected the risk of GE. Our model suggests that cardiac output likely plays a significant role in the modulation of GE, especially in the deeper diving leatherback turtles. This baseline model also indicates that even during routine diving behavior, sea turtles are at high risk of GE. This likely means that turtles have additional behavioral, anatomical, and/or physiologic adaptions that serve to reduce the probability of GE but were not incorporated in this model. Identifying these adaptations and incorporating them into future iterations of this model will further reveal the factors driving GE in sea turtles.

Time Domains of Hypoxia Adaptation-Elephant Seals Stand Out Among Divers.

In this Opinion, adaptations to hypoxia are examined during the short time domains of breath holds from three accomplished diving animals: northern elephant seals (Mirounga angustirostris), California sea lions (Zalophus californianus), and emperor penguins (Aptenodytes forsteri). Review of dive behavior, oxygen (O2) storage, and arterial blood O2 profiles during dives reveals that the elephant seal undergoes the most frequent and extreme hypoxemia. Exceptional breath hold durations, routine hypoxemia, established research protocols, and accessibility to the animals make the elephant seal stand out for physiological investigation and evaluation of biochemical/molecular adaptations in hypoxemic tolerance, protection against re-perfusion injury, and O2 transport during dives. The northern elephant seal and southern elephant seal (M. leonina) are the premier pinniped divers (Le Boeuf et al., 1988; Hindell et al., 1991, 1992; Hassrick et al., 2010; Robinson et al., 2012). During several month-long trips to sea, these animals spend 80–90% of their time underwater, perform routine dives of 20–30 min duration to average depths >400 m, have short inter-dive surface intervals that average two min, and typically gain about one kg d−1 in body mass. In contrast to elephant seals, California sea lions only spend about 30% of their time at sea diving (Feldkamp et al., 1989). Most dives are < 100 m in depth and between 1 and 4 min in duration (McDonald and Ponganis, 2013; Tift et al., 2017). However, dependent on geographic location, climate variability, and prey distribution, these sea lions can regularly perform 10-min dives to 400–500 m, with the current longest reported dive of 16 min (Melin et al., 2008; McHuron et al., 2016, 2018). Emperor penguins are the premier avian divers; they exploit the entire water column to depths >500 m. Shallower dives are up to 5–6 min in duration while deep dives are about 10 min (Kooyman and Kooyman, 1995; Kirkwood and Robertson, 1997; Sato et al., 2011). The longest dive documented by a continuous dive profile in an emperor penguin is 27.6 min. During foraging trips to sea, emperor penguins spent about 31% of their time resting on the sea ice (Watanabe et al., 2012).

Penguin lungs and air sacs: implications for baroprotection, oxygen stores and buoyancy.

The anatomy and volume of the penguin respiratory system contribute significantly to pulmonary baroprotection, the body O2 store, buoyancy and hence the overall diving physiology of penguins. Therefore, three-dimensional reconstructions from computerized tomographic (CT) scans of live penguins were utilized to measure lung volumes, air sac volumes, tracheobronchial volumes and total body volumes at different inflation pressures in three species with different dive capacities [Adélie (Pygoscelis adeliae), king (Aptenodytes patagonicus) and emperor (A. forsteri) penguins]. Lung volumes scaled to body mass according to published avian allometrics. Air sac volumes at 30 cm H2O (2.94 kPa) inflation pressure, the assumed maximum volume possible prior to deep dives, were two to three times allometric air sac predictions and also two to three times previously determined end-of-dive total air volumes. Although it is unknown whether penguins inhale to such high volumes prior to dives, these values were supported by (a) body density/buoyancy calculations, (b) prior air volume measurements in free-diving ducks and (c) previous suggestions that penguins may exhale air prior to the final portions of deep dives. Based upon air capillary volumes, parabronchial volumes and tracheobronchial volumes estimated from the measured lung/airway volumes and the only available morphometry study of a penguin lung, the presumed maximum air sac volumes resulted in air sac volume to air capillary/parabronchial/tracheobronchial volume ratios that were not large enough to prevent barotrauma to the non-collapsing, rigid air capillaries during the deepest dives of all three species, and during many routine dives of king and emperor penguins. We conclude that volume reduction of airways and lung air spaces, via compression, constriction or blood engorgement, must occur to provide pulmonary baroprotection at depth. It is also possible that relative air capillary and parabronchial volumes are smaller in these deeper-diving species than in the spheniscid penguin of the morphometry study. If penguins do inhale to this maximum air sac volume prior to their deepest dives, the magnitude and distribution of the body O2 store would change considerably. In emperor penguins, total body O2 would increase by 75%, and the respiratory fraction would increase from 33% to 61%. We emphasize that the maximum pre-dive respiratory air volume is still unknown in penguins. However, even lesser increases in air sac volume prior to a dive would still significantly increase the O2 store. More refined evaluations of the respiratory O2 store and baroprotective mechanisms in penguins await further investigation of species-specific lung morphometry, start-of-dive air volumes and body buoyancy, and the possibility of air exhalation during dives.

Blood Oxygen Depletion Is Independent of Dive Function in a Deep Diving Vertebrate, the Northern Elephant Seal

Although energetics is fundamental to animal ecology, traditional methods of determining metabolic rate are neither direct nor instantaneous. Recently, continuous blood oxygen (O2) measurements were used to assess energy expenditure in diving elephant seals (Mirounga angustirostris), demonstrating that an exceptional hypoxemic tolerance and exquisite management of blood O2 stores underlie the extraordinary diving capability of this consummate diver. As the detailed relationship of energy expenditure and dive behavior remains unknown, we integrated behavior, ecology, and physiology to characterize the costs of different types of dives of elephant seals. Elephant seal dive profiles were analyzed and O2 utilization was classified according to dive type (overall function of dive: transit, foraging, food processing/rest). This is the first account linking behavior at this level with in vivo blood O2 measurements in an animal freely diving at sea, allowing us to assess patterns of O2 utilization and energy expenditure between various behaviors and activities in an animal in the wild. In routine dives of elephant seals, the blood O2 store was significantly depleted to a similar range irrespective of dive function, suggesting that all dive types have equal costs in terms of blood O2 depletion. Here, we present the first physiological evidence that all dive types have similarly high blood O2 demands, supporting an energy balance strategy achieved by devoting one major task to a given dive, thereby separating dive functions into distinct dive types. This strategy may optimize O2 store utilization and recovery, consequently maximizing time underwater and allowing these animals to take full advantage of their underwater resources. This approach may be important to optimizing energy expenditure throughout a dive bout or at-sea foraging trip and is well suited to the lifestyle of an elephant seal, which spends > 90% of its time at sea submerged making diving its most “natural” state.

Blood Temperature Profiles of Diving Elephant Seals

Hypothermia-induced reductions in metabolic rate have been proposed to suppress metabolism and prolong the duration of aerobic metabolism during dives of marine mammals and birds. To determine whether core hypothermia might contribute to the repetitive long-duration dives of the northern elephant seal Mirounga angustirostris, blood temperature profiles were obtained in translocated juvenile elephant seals equipped with a thermistor and backpack recorder. Representative temperature (the y-intercept of the mean temperature vs. dive duration relationship) was 37.2 degrees C +/- 0.6 degrees C (n=3 seals) in the extradural vein, 38.1 degrees C +/- 0.7 degrees C (n = 4 seals) in the hepatic sinus, and 38.8 degrees +/- 1.6 degrees C (n = 6 deals) in the aorta. Mean temperature was significantly though weakly negatively related to dive duration in all but one seal. Mean venous temperatures of all dives of individual seals ranged between 36 degrees and 38 degrees C, while mean arterial temperatures ranged between 35 degrees and 39 degrees C. Transient decreases in venous and arterial temperatures to as low as 30 degrees -33 degrees C occurred in some dives >30 min (0.1% of dives in the study). The lack of significant core hypothermia during routine dives (10-30 min) and only a weak negative correlation of mean temperature with dive duration do not support the hypothesis that a cold-induced Q(10) effect contributes to metabolic suppression of central tissues during dives. The wide range of arterial temperatures while diving and the transient declines in temperature during long dives suggest that alterations in blood flow patterns and peripheral heat loss contribute to thermoregulation during diving.

Extreme hypoxemic tolerance and blood oxygen depletion in diving elephant seals

Species that maintain aerobic metabolism when the oxygen (O(2)) supply is limited represent ideal models to examine the mechanisms underlying tolerance to hypoxia. The repetitive, long dives of northern elephant seals (Mirounga angustirostris) have remained a physiological enigma as O(2) stores appear inadequate to maintain aerobic metabolism. We evaluated hypoxemic tolerance and blood O(2) depletion by 1) measuring arterial and venous O(2) partial pressure (Po(2)) during dives with a Po(2)/temperature recorder on elephant seals, 2) characterizing the O(2)-hemoglobin (O(2)-Hb) dissociation curve of this species, 3) applying the dissociation curve to Po(2) profiles to obtain %Hb saturation (So(2)), and 4) calculating blood O(2) store depletion during diving. Optimization of O(2) stores was achieved by high venous O(2) loading and almost complete depletion of blood O(2) stores during dives, with net O(2) content depletion values up to 91% (arterial) and 100% (venous). In routine dives (>10 min) Pv(O(2)) and Pa(O(2)) values reached 2-10 and 12-23 mmHg, respectively. This corresponds to So(2) of 1-26% and O(2) contents of 0.3 (venous) and 2.7 ml O(2)/dl blood (arterial), demonstrating remarkable hypoxemic tolerance as Pa(O(2)) is nearly equivalent to the arterial hypoxemic threshold of seals. The contribution of the blood O(2) store alone to metabolic rate was nearly equivalent to resting metabolic rate, and mean temperature remained near 37 degrees C. These data suggest that elephant seals routinely tolerate extreme hypoxemia during dives to completely utilize the blood O(2) store and maximize aerobic dive duration.

Some like it hot: temperature profiles of diving elephant seals

To account for the repetitive, long duration dives of the northern elephant seal, Mirounga angustirostris, it has been hypothesized that this species must either become "hypometabolic", or, contrary to current concepts of aerobic dive limits, must resort to glycolysis and lactate accumulation during dives. As recent studies have revealed reductions in regional temperature during diving, it is possible that hypothermia may serve as a mechanism of reducing oxygen consumption and extending aerobic dive time. To document core body temperature during diving in this species, a thermistor was inserted into extradural vein, hepatic sinus, or aortic sites and a backpack recorder attached to translocated, juvenile elephant seals. Venous temperature during routine dives remained near 37°C. Arterial temperature profiles, however, often revealed increases during diving (similar to that of exercising mammals), even to greater than 40°C, with subsequent decreases during each surface interval. Decreases in venous and arterial temperature to as low as 31‐34°C did occur in long (&gt;30 min) dives, though these drops were transient and re‐warming occurred prior to the end of dive. These data suggest that hypothermia‐induced hypometabolism is an unlikely contributor to metabolic suppression while diving. Funding: NSF grant IOS‐0641801 &amp; SIO Halliday Student Field Research Award

Extreme blood oxygen depletion in diving elephant seals

The repetitive, long dives of elephant seals, Mirounga angustirostris, have remained a physiological enigma in that oxygen (O2) stores appear inadequate to maintain aerobic metabolism. To investigate blood O2 depletion in this species we 1) measured venous O2 partial pressure (PO2) continuously during dives with a backpack PO2/temperature recorder and, 2) characterized the O2‐hemoglobin (Hb) dissociation curve. Similar to other marine mammals, O2‐Hb dissociation curve results were: P50 (50% saturation) ~ 31 mmHg, Bohr effect (P50) ~ −0.6. End‐of‐dive PO2 routinely reached 2–10 mmHg in the extradural vein and hepatic sinus, demonstrating remarkable hypoxemic tolerance in this species. Based on prior studies, we hypothesize that arterial PO2 also reaches similarly low levels. These data allow calculation of % Hb‐saturation, the magnitude of blood O2 store depletion during dives, and the contribution to diving metabolic rate. Venous temperature during routine dives remained near 37°C, suggesting that hypothermia‐induced hypometabolism is an unlikely contributor to metabolic suppression while diving. We believe the elephant seal's extreme hypoxemic tolerance allows near complete depletion of its previously measured large blood O2 store, providing sufficient O2 to maintain metabolism during repetitive, long dives. Funding: NSF grant IOS‐0641801 &amp; SIO Halliday Student Field Research Award

MYSTERIES OF ADAPTATION TO HYPOXIA AND PRESSURE IN MARINE MAMMALS The Kenneth S. Norris Lifetime Achievement Award Lecture

Presented on 12 December 2005 San Diego, CaliforniaAbstractThis paper reviews past and current work on diving behavior, effects of pressure, and the aerobic diving limit from the perspective of the Ken Norris Lifetime Achievement Award. Because of the influence of Norris to marine mammalogy in general, and to my career in particular, I want to emphasize the important tradition of mentors and colleagues as keystones to a successful career in science, and ultimately to the success of science itself. These two related activities are illustrated by studies on marine mammals that were conducted in an endeavor to understand: (1) the behavioral traits associated with deep diving, (2) the mechanical and physiological effects of pressure during routine dives to great depth, and (3) the degree of oxygen depletion that they routinely endure while diving. The search for answers has resulted in numerous physiological and ecological experiments, along with accompanying theoretical analyses. Currently it appears that some deep‐diving mammals may suffer from bends, and some may resort more often than what seems physiologically possible to anaerobic metabolism while diving. Above all, the way divers manage their nitrogen and oxygen stores remains a mystery.

Diving Heart Rate Development in Postnatal Harbour Seals,Phoca vitulina

Harbour seals, Phoca vitulina, dive from birth, providing a means of mapping the development of the diving response, and so our objective was to investigate the postpartum development of diving bradycardia. The study was conducted May-July 2000 and 2001 in the St. Lawrence River Estuary (48 degrees 41'N, 68 degrees 01'W). Both depth and heart rate (HR) were remotely recorded during 86,931 dives (ages 2-42 d, n = 15) and only depth for an additional 20,300 dives (combined data covered newborn to 60 d, n = 20). The mean dive depth and mean dive durations were conservative during nursing (2.1 +/- 0.1 m and 0.57 +/- 0.01 min, range = 0-30.9 m and 0-5.9 min, respectively). The HR of neonatal pups during submersion was bimodal, but as days passed, the milder of the two diving HRs disappeared from their diving HR record. By 15 d of age, most of the dive time was spent at the lower diving bradycardia rate. Additionally, this study shows that pups are born with the ability to maintain the lower, more fully developed dive bradycardia during focused diving but do not do so during shorter routine dives.

Short-term anoxia does not impair protein turnover in isolated perfused turtle heart

Protein turnover was studied under conditions of normoxia and anoxia in isolated perfused turtle (Trachemys (=Pseudemys) scripta elegans) hearts at 15 °C. Protein synthesis was measured by 3H-labelled phenylalanine incorporation into the protein pool. Protein degradation was studied using the protein synthesis inhibitor cycloheximide and measuring phenylalanine release from the heart. Heart rate, cardiac output and ventricle pressure development were unaffected by 2 h of anoxia. Protein synthesis rate was tenfold lower in turtle heart than in rat heart at their respective body temperatures. However, 1 h of anoxia did not affect the rate of protein synthesis in turle heat. RNA content of both ventricle and atria decreased after anoxic perfusion. Protein synthesis rates in atria were higher than those of ventricle under conditions of either anoxia or normoxia. Protein degradation rate did not change in cycloheximide-treated turtle hearts after 2 h of anoxia. These findings indicate that lack of oxygen per se does not affect protein turnover in the isolated perfused turtle heart. This feature presumably allows protein turnover to proceed in vivo during routine dives at elevated temperatures. Also, the turtle heart is a good model system to study the effects of anoxia on protein turnover without the potentially confounding factor of contractile failure. These hearts are very resistant to anoxia and well able to maintain contractility for extended periods of time.

Colonization of professional divers by toxigenicVibrio choleraeO1 andV. choleraenon-O1 at dive sites in the United States, Ukraine and Russia

Vibrio cholerae, recognized as the causative agent of epidemic cholera, was isolated from healthy professional divers and from water samples collected at dive sites in the United States, Ukraine and Russia. Swabs of nose, ear and throat of divers and their tank regulators, i.e. the divers and their diving gear, were taken before and after routine dives. Blood samples were collected before and 30-60 days after each dive to measure IgG and IgA titers against the whole cell antigen of V. cholerae O1. Nine strains of V. cholerae O1 and nine strains of V. cholerae non-O1 were isolated during this study. These isolates were identified by conventional biochemical tests and indirect fluorescent antibody staining methods, using fluorescein isothiocyanate-labeled monoclonal antibody, COLTA, prepared against the 'A' antigenic factor of the lipopolysaccharide of V. cholerae O1, and serotyped by slide agglutination. Seven of the nine strains of V. cholerae O1 isolated and successfully cultured during the studies, were toxigenic by enzyme-linked immunosorbent assay and polymerase chain reaction. Analyses of IgG and IgA antibodies of the divers showed that most of the divers had prior exposure to V. cholerae O1. V. cholerae serotype non-O1 strains isolated during the study were found to be non-toxigenic.

Colonization of professional divers by toxigenic Vibrio cholerae O1 and V. cholerae non-O1 at dive sites in the United States, Ukraine and Russia

Vibrio cholerae, recognized as the causative agent of epidemic cholera, was isolated from healthy professional divers and from water samples collected at dive sites in the United States, Ukraine and Russia. Swabs of nose, ear and throat of divers and their tank regulators, i.e. the divers and their diving gear, were taken before and after routine dives. Blood samples were collected before and 30–60 days after each dive to measure IgG and IgA titers against the whole cell antigen of V. cholerae O1. Nine strains of V. cholerae O1 and nine strains of V. cholerae non-O1 were isolated during this study. These isolates were identified by conventional biochemical tests and indirect fluorescent antibody staining methods, using fluorescein isothiocyanate-labeled monoclonal antibody, COLTA, prepared against the ‘A’ antigenic factor of the lipopolysaccharide of V. cholerae O1, and serotyped by slide agglutination. Seven of the nine strains of V. cholerae O1 isolated and successfully cultured during the studies, were toxigenic by enzyme-linked immunosorbent assay and polymerase chain reaction. Analyses of IgG and IgA antibodies of the divers showed that most of the divers had prior exposure to V. cholerae O1. V. cholerae serotype non-O1 strains isolated during the study were found to be non-toxigenic.

Heart rate during breath-holding and diving in the unrestrained harbor seal (Phoco vitulina richardi).

Heart rate and breathing movements have been monitored in three unrestrained harbor seals. On voluntary submergence heart rate fell markedly in all seals, and after 2–3 s submergence stabilized at about 40–50% of the predive level. Heart rate increased before the animals broke surface at the end of the dive and, when breathing began again, a postdive tachycardia was observed. Two of the three seals frequently showed anticipation of the dive as judged from their heart-rate response. Heart rate during feeding dives was generally more variable; in fact one seal exhibited no bradycardia in 20% of its feeding dives, although another seal showed a significantly greater bradycardia than was seen in routine dives. When breathing rate was low (less than five breaths min−1) respiratory variations in the heart rate occurred, although the onset of bradycardia was much more rapid during diving than during breath-holding. Because of the flexibility of the response it is concluded that the generation of diving bradycardia in the seal is a complex phenomenon which, aside from any responses set in train by peripheral receptors, must also involve some form of associative learning.