Respiratory function and mechanics in pinnipeds and cetaceans

In this Review, we focus on the functional properties of the respiratory system of pinnipeds and cetaceans, and briefly summarize the underlying anatomy; in doing so, we provide an overview of what is currently known about their respiratory physiology and mechanics. While exposure to high pressure is a common challenge among breath-hold divers, there is a large variation in respiratory anatomy, function and capacity between species – how are these traits adapted to allow the animals to withstand the physiological challenges faced during dives? The ultra-deep diving feats of some marine mammals defy our current understanding of respiratory physiology and lung mechanics. These animals cope daily with lung compression, alveolar collapse, transient hyperoxia and extreme hypoxia. By improving our understanding of respiratory physiology under these conditions, we will be better able to define the physiological constraints imposed on these animals, and how these limitations may affect the survival of marine mammals in a changing environment. Many of the respiratory traits to survive exposure to an extreme environment may inspire novel treatments for a variety of respiratory problems in humans.

Implanted Nanosensors in Marine Organisms for Physiological Biologging: Design, Feasibility, and Species Variability

In recent decades, biologists have sought to tag animals with various sensors to study aspects of their behavior otherwise inaccessible from controlled laboratory experiments. Despite this, chemical information, both environmental and physiological, remains challenging to collect despite its tremendous potential to elucidate a wide range of animal behaviors. In this work, we explore the design, feasibility, and data collection constraints of implantable, near-infrared fluorescent nanosensors based on DNA-wrapped single-wall carbon nanotubes (SWNT) embedded within a biocompatible poly(ethylene glycol) diacrylate (PEGDA) hydrogel. These sensors are enabled by Corona Phase Molecular Recognition (CoPhMoRe) to provide selective chemical detection for marine organism biologging. Riboflavin, a key nutrient in oxidative phosphorylation, is utilized as a model analyte in in vitro and ex vivo tissue measurements. Nine species of bony fish, sharks, eels, and turtles were utilized on site at Oceanogràfic in Valencia, Spain to investigate sensor design parameters, including implantation depth, sensor imaging and detection limits, fluence, and stability, as well as acute and long-term biocompatibility. Hydrogels were implanted subcutaneously and imaged using a customized, field-portable Raspberry Pi camera system. Hydrogels could be detected up to depths of 7 mm in the skin and muscle tissue of deceased teleost fish ( Sparus aurata and Stenotomus chrysops) and a deceased catshark ( Galeus melastomus). The effects of tissue heterogeneity on hydrogel delivery and fluorescence visibility were explored, with darker tissues masking hydrogel fluorescence. Hydrogels were implanted into a living eastern river cooter ( Pseudemys concinna), a European eel ( Anguilla anguilla), and a second species of catshark ( Scyliorhinus stellaris). The animals displayed no observable changes in movement and feeding patterns. Imaging by high-resolution ultrasound indicated no changes in tissue structure in the eel and catshark. In the turtle, some tissue reaction was detected upon dissection and histopathology. Analysis of movement patterns in sarasa comet goldfish ( Carassius auratus) indicated that the hydrogel implants did not affect swimming patterns. Taken together, these results indicate that this implantable form factor is a promising technique for biologging using aquatic vertebrates with further development. Future work will tune the sensor detection range to the physiological range of riboflavin, develop strategies to normalize sensor signal to account for the optical heterogeneity of animal tissues, and design a flexible, wearable device incorporating optoelectronic components that will enable sensor measurements in moving animals. This work advances the application of nanosensors to organisms beyond the commonly used rodent and zebrafish models and is an important step toward the physiological biologging of aquatic organisms.

Evaluating cardiac physiology through echocardiography in bottlenose dolphins: using stroke volume and cardiac output to estimate systolic left ventricular function during rest and following exercise

Heart-rate (fH) changes during diving and exercise are well documented for marine mammals, but changes in stroke volume (SV) and cardiac output (CO) are much less known. We hypothesized that both SV and CO are also modified following intense exercise. Using transthoracic ultrasound Doppler at the level of the aortic valve, we compared blood flow velocities in the left ventricle and cardiac frequencies during rest and at 1, 3 and 4 min after a bout of exercise in 13 adult bottlenose dolphins (Tursiops truncatus, six male and seven female, body mass range: 143-212 kg). Aortic cross sectional area and ventricle blood velocity at the aortic valve were used to calculate SV, which together with fH, provided estimates of left CO at rest and following exercise. The fH and SV stabilized approximately 4-7 sec following the post-respiratory tachycardia, so only data after the fH had stabilized were used for analysis and comparison. There were significant increases in fH, SV, and CO associated with each breath. At rest, fH, SV, and CO were uncorrelated with body mass, and averaged 41±8 beats min−1, 136±19 ml, and 5513±1182 l min−1, respectively. One minute following high intensity exercise, the cardiac variables had increased by 104±43%, 63±11%, and 234±84%, respectively. All variables remained significantly elevated in all animals for at least 4 min after the exercise. These baseline values provide the first data on stroke volume and cardiac output in awake and unrestrained cetaceans in water.

Defining risk variables causing gas embolism in loggerhead sea turtles (Caretta caretta) caught in trawls and gillnets

Incidental capture, or 'bycatch' in fishing gear is a major global threat to sea turtle populations. A recent study showed that underwater entrapment in fishing gear followed by rapid decompression may cause gas bubble formation within the blood stream (embolism) and tissues leading to organ injury, impairment, and even mortality in some bycaught individuals. We analyzed data from 128 capture events using logistic and ordinal regression to examine risk factors associated with gas embolism in sea turtles captured in trawls and gillnets. Likelihood of fatal decompression increases with increasing depth of gear deployment. A direct relationship was found between depth, risk and severity of embolism, which has not been previously demonstrated in any breath-hold diving species. For the trawl fishery in this study, an average trawl depth of 65 m was estimated to result in 50% mortality in by-caught turtles throughout the year. This finding is critical for a more accurate estimation of sea turtle mortality rates resulting from different fisheries and for devising efforts to avoid or minimize the harmful effects of capture.

New insights into risk variables associated with gas embolism in loggerhead sea turtles (Caretta caretta) caught in trawls and gillnets

Tissue and blood gas embolism (GE) associated with fisheries bycatch are likely a widespread, yet underestimated, cause of sea turtle mortality. Here, we evaluated risk factors associated with tissue and blood GE in loggerhead turtles caught incidentally by trawl and gillnet fisheries on the Valencian coastline of Spain. Of 413 turtles (303 caught by trawl, 110 by gillnet fisheries), 54% (n = 222) exhibited GE. For sea turtles caught in trawls, the probability and severity of GE increased with trawl depth and turtle body mass. In addition, trawl depth and the GE score together explained the probability of mortality (P[mortality]) following recompression therapy. Specifically, a turtle with a GE score of 3 caught in a trawl deployed at 110 m had a P[mortality] of ~50%. For turtles caught in gillnets, no risk variables were significantly correlated with either the P[GE] or GE score. However, gillnet depth or GE score, separately, explained P[mortality], and a turtle caught at 45 m or with a GE score between 3 and 4 had a P[mortality] of 50%. Differences in the fishery characteristics precluded direct comparison of GE risk and mortality between these gear types. Although P[mortality] is expected to be significantly higher in untreated turtles released at sea, our findings can improve estimates of sea turtle mortality associated with trawls and gillnets, and help guide associate conservation efforts.