Decompression vs. Decomposition: Distribution, Amount, and Gas Composition of Bubbles in Stranded Marine Mammals

Abstract

Gas embolic lesions linked to military sonar have been described in stranded cetaceans including beaked whales. These descriptions suggest that gas bubbles in marine mammal tissues may be more common than previously thought. In this study we have analyzed gas amount (by gas score) and gas composition within different decomposition codes using a standardized methodology. This broad study has allowed us to explore species-specific variability in bubble prevalence, amount, distribution, and composition, as well as masking of bubble content by putrefaction gases. Bubbles detected within the cardiovascular system and other tissues related to both pre- and port-mortem processes are a common finding on necropsy of stranded cetaceans. To minimize masking by putrefaction gases, necropsy, and gas sampling must be performed as soon as possible. Before 24 h post mortem is recommended but preferably within 12 h post mortem. At necropsy, amount of bubbles (gas score) in decomposition code 2 in stranded cetaceans was found to be more important than merely presence vs. absence of bubbles from a pathological point of view. Deep divers presented higher abundance of gas bubbles, mainly composed of 70% nitrogen and 30% CO2, suggesting a higher predisposition of these species to suffer from decompression-related gas embolism.

How man-made interference might cause gas bubble emboli in deep diving whales

“…Carbon dioxide, on the other hand, is produced by aerobic metabolism and may accumulate to high levels in metabolically active tissues. Local accumulation, and high diffusivity of CO 2 may form bubble precursors that continues to grow from N 2 diffusion (Harris et al, 1945; Behnke, 1951; Bernaldo De Quirós et al, 2012, 2013). Even though the carbonic anhydrase enzyme system might dissolve such bubble precursors quickly, they might still trigger bubble formation in N 2 -supersaturated tissue.…”

“…Once the dissolved tissue gas tension (P tiss ) exceeds the ambient pressure (P amb ), the tissue is supersaturated, and bubbles may form. The bubbles are believed to be the instigator for decompression sickness (DCS) symptoms seen in human divers, or gas bubble emboli found in marine mammals (Moore et al, 2009; Bernaldo De Quirós et al, 2012; Dennison et al, 2012). Scholander (1940) hypothesized that the unusual respiratory system in marine mammals, with a stiff trachea and rather compliant chest, allows the alveoli to collapse at shallow depths, thereby limiting the uptake of inert gas, reducing the level of N 2 that would be taken up during a dive and therefore reducing the likelihood of gas bubble emboli in breath-hold diving mammals.…”

“…However, necropsy results in stranded marine mammals have indicated lesions that are similar to those found in human divers with DCS symptoms (Jepson et al, 2003; Fernández et al, 2005). Also more recent work has suggested that marine mammals may experience inert gas bubbles more commonly than formerly thought (Bernaldo De Quirós et al, 2012; Dennison et al, 2012; Hooker et al, 2012). Consequently, cetaceans do not appear immune against gas bubble emboli.…”

“…Recent work by Bernaldo De Quirós et al (2012) showed that among stranded whales, deep diving species of whales, such as sperm and beaked whales, had a higher abundance of gas bubbles compared to shallow diving species. The bubbles were mainly composed of N 2 , confirming that the bubbles were related to DCS and not caused by putrefaction.…”

The physiological consequences of breath-hold diving in marine mammals; the Scholander legacy

“…However, necropsy reports from mass stranded whales indicated DCS-like symptoms (Jepson et al, 2003;Fernández et al, 2005). A more recent study has shown that the gas bubble composition in stranded whales is similar to that from land mammals suffering DCS in experimental dive models (Bernaldo De Quirós et al, 2012). Imaging work in both live and stranded marine mammals indicates that they live with elevated inert gas tensions that cause bubbles to form under certain circumstances (Dennison et al, 2012).…”