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Marine Life & Conservation

How Do Marine Mammals Avoid the Bends?

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Study offers new hypothesis and highlights role that sonar plays in strandings

Deep-diving whales and other marine mammals can get the bends—the same painful and potentially life-threatening decompression sickness that strikes scuba divers who surface too quickly. A new study offers a hypothesis of how marine mammals generally avoid getting the bends and how they can succumb under stressful conditions.

The key is the unusual lung architecture of whales, dolphins and porpoises (and possibly other breath-holding diving vertebrates), which creates two different pulmonary regions under deep-sea pressure, say researchers at the Woods Hole Oceanographic Institution (WHOI) and the Fundacion Oceanografic in Spain. Their study was published April 25, 2018, in the journal Proceedings of the Royal Society B.

Photo by Lance Wills, © Woods Hole Oceanographic Institution

How some marine mammals and turtles can repeatedly dive as deep and as long as they do has perplexed scientists for a very long time,” says Michael Moore, director of the Marine Mammal Center at WHOI and co-author of the study. “This paper opens a window through which we can take a new perspective on the question.

When air-breathing mammals dive to high-pressure depths, their lungs compress. That collapses their alveoli—the tiny sacs at the end of the airways where gas exchange occurs. Nitrogen bubbles build up in the animals’ bloodstream and tissue. If they ascend slowly, the nitrogen can return to the lungs and be exhaled. But if they ascend too fast, the nitrogen bubbles don’t have time to diffuse back into the lungs. Under less pressure at shallower depths, the nitrogen bubbles expand in the bloodstream and tissue, causing pain and damage.

Marine mammals’ chest structure allows their lungs to compress. Scientists have assumed that this passive compression was marine mammals’ main adaptation to avoid taking up excessive nitrogen at depth and getting the bends.

In their study, the researchers took CT images of a deceased dolphin, seal, and a domestic pig pressurized in a hyperbaric chamber. The team was able to see how the marine mammals’ lung architecture creates two pulmonary regions: one air-filled and the other collapsed. The researchers believe that blood flows mainly through the collapsed region of the lungs. That causes what is called a ventilation-perfusion mismatch, which allows some oxygen and carbon dioxide to be absorbed by the animal’s bloodstream, while minimizing or preventing the exchange of nitrogen. This is possible because each gas has a different solubility in the blood. The terrestrial pig did not show that structural adaptation.

This mechanism would protect cetaceans from taking up excessive amounts of nitrogen and thus minimize risk of the bends, says lead author Daniel García-Parraga of the Fundacion Oceanografic. However, he said, “Excessive stress, as may occur during exposure to human-made sound, may cause the system to fail and increase blood to flow to the air-filled regions. This would enhance gas exchange, and nitrogen would increase in the blood and tissues as the pressure decreases during ascent.

Scientists once thought that diving marine mammals were immune from decompression sickness, but a 2002 stranding event linked to navy sonar exercises revealed that 14 whales that died after beaching off the Canary Islands had gas bubbles in their tissues—a sign of the bends. The researchers say the paper’s findings could support previous implications of decompression sickness in some cetacean mass strandings associated with navy sonar exercises.

The team says further research will require the development of tools to analyze how lung blood flow and ventilation patterns change with various stressors during diving.

This work was supported by funding from the Fundacion Oceanografic and the Office of Naval Research.


The Woods Hole Oceanographic Institution is a private, non-profit organization on Cape Cod, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the ocean and its interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment.

For more information, please visit www.whoi.edu.

Marine Life & Conservation

New report shows simple measures needed to cut beach plastic pollution

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The Marine Conservation Society has launched its annual State of our Beaches 2023 report, highlighting the need for less single-use plastic items and more refillable options.

The charity’s beach cleaning programme, now in its 30th year, asks volunteers to record all litter that they find within a 100-metre stretch of beach. By gathering vital data from across the UK and Channel Islands, the charity creates change for cleaner seas and a healthier planet through policy and industry change. Last year, over 148 kilometres of beach were surveyed by volunteers, who removed 17,208kgs of litter from our coast.

Using data collected by over 14,000 volunteers, the State of our Beaches 2023 report shows that half of all litter collected came from public sources – either dumped, blown or washed onto our beaches.

Drinks-related litter, such as plastic bottles, caps and cans were one of the most found items, with an average of 16 found per 100 metres of beach surveyed. This figure is up 14% compared to the charity’s 2022 report.

Plastics caps and lids were recorded on 89% of surveys, with over 30,000 found in total. Plastic bottles were also commonly collected, with 73% of surveys finding this single-use item – a 4% increase compared to 2022. The charity’s inland litter pick survey, Source to Sea, also reported finding plastic bottles on 92% of cleans.

However, the charity’s State of Our Beaches 2023 report also indicates that some types of litter are on a downward trend. Although still in the top 10 litter items, disposable plastic cutlery, trays and straws were found on 5% fewer beaches across the UK, showing that bans on these plastic items between 2022 – 2023 could be working. The presence of plastic cotton bud sticks also dropped by 14%.

Lizzie Price, Beachwatch Manager at the Marine Conservation Society, said, “Half of the litter found on our beaches originates from public sources, underscoring the pressing issue of single-use plastic. Evidence shows that refillable alternatives and litter return schemes effectively reduce rubbish, as demonstrated by the decline our data has shown in items like plastic bags and disposable cutlery.

“Urgent and decisive action from UK governments is needed to accelerate our shift towards a circular economy, where we reuse, repair and recycle. The public needs greater access to refillable products and systems need to be implemented in which manufacturers are responsible for their waste.”

Governments across the UK have committed to delivering deposit return schemes from October 2025. However, the proposed scheme in England does not include glass items, despite glass being found on 52% of beaches in 2023. The charity is calling for governments across the UK to introduce compatible all inclusive deposit return schemes as soon as possible. This would see aluminium cans, and plastic and glass bottles have a 20p deposit which would be returned when recycled which would reduce drinks-related pollution.

Businesses are, however, starting to take refillable and reusable options into their own hands. Supermarket chain, Aldi, the largest corporate funder of the charity’s 2024 Beachwatch programme, is already leading the charge in refillable items by trialling refillable options for cereals and porridge oats in their stores over the past 5 months.

Plastics and Packaging Director, Luke Emery, from Aldi, who are running 4 beach cleans for staff and volunteers in 2024, said, We are continually working to reduce single-use plastics and packaging, and making unpackaged product options more commonplace for our customers is a key part of this. We are really pleased with the uptake of our refillable range so far by customers. Not only are we making sustainable shopping more accessible, but we also offer refill products for a better price than the packaged alternative, helping our customers save money.”

Marine litter is one of the biggest threats to our seas, with plastic breaking down over time into microplastics and threatening marine life which can ingest or become entangled in them. The Marine Conservation Society is urging the UK Government to move faster on its policies to cut down plastic waste and prevent items from reaching our seas.

To find out more about the charity’s beach cleans and how to get involved, please visit mcsuk.org/beach-cleans

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Guarding Against Coral Invaders

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Protecting (Dutch) Caribbean Reefs from Unomia stolonifera

Recent reports highlight the concerning spread of the invasive soft coral, Unomia stolonifera, currently devastating Venezuela’s marine ecosystems and detected in Cuba. With the potential threat of its expansion to the (Dutch) Caribbean islands, urgent action and awareness are essential to safeguard marine biodiversity and local economies from possible catastrophic consequences.

Invasive species are animals or plants from another region of the world that don’t belong in their new environment. These species can have major ecological effects by decimating native flora or fauna. They can also cause large economic losses and impact human health. Invasive species also pose a significant threat to marine ecosystems worldwide, including the Dutch Caribbean. Among these invaders is the octocoral species Unomia stolonifera or “Pulsing Xenia”, originally from the Indo-Pacific. With its rapid growth and lack of natural predators, this species can outcompete native species and disrupt fragile marine habitats such as seagrass beds and coral reefs.

Background

The invasive soft coral U. stolonifera was first identified in 2014, off the coast of Venezuela. It is believed to have been introduced via the illegal aquarium trade.  Since this species can reproduce sexually and asexually (or fragment), even small pieces can regenerate to spread.  Once introduced it quickly took over shallow reefs and hard substrate at depths of 0-50 meters, outcompeting local corals and seagrass for space.  Follow on surveys found that this coral species exhibited average percentage cover as high as 80%, vastly outcompeting native corals. In highly colonized areas, fish are disappearing due to loss of habitats.

In 2022, during a survey conducted in Cuba by the University of Havana, an unknown octocoral was discovered which was later identified as the invasive Unomia stolonifera. It is suspected that the coral larvae arrived in ballast water from fossil fuel ships originating from Venezuela, as nearby sites adjacent to Venezuelan ports have been heavily affected by the invasion.

How to help

Prevention through continuous monitoring, particularly in high-risk areas such as marine harbors and oil facilities, is paramount. Early detection plays a pivotal role in mitigating the threat posed by Unomia stolonifera.

The public’s involvement and awareness are also vital. Local communities, recreational divers, tourists, and all stakeholders are urged to participate in early detection efforts by reporting sightings (photo, location and date) of this invasive coral to their respective Protected Area Management Organization (PMO’s)- the Fundacion Parke Nacional Aruba (FPNA)STINAPA BonaireCARMABI Curaçao Saba Conservation Foundation (SCF)Nature Foundation St. Maarten (NFSXM) and St. Eustatius National Parks (STENAPA). If an invaded area is confirmed, follow the recommendations by the local PMO’s.

Keys to Success

Despite the challenges, early detection is key to mitigating the threat posed by Unomia stolonifera. With continued vigilance, research, and community engagement, there is hope for containing this potential issue before it becomes a major threat.

About the DCNA

The Dutch Caribbean Nature Alliance (DCNA) supports (science) communication and outreach in the Dutch Caribbean region by making nature related (scientific) information more widely available through amongst others the Dutch Caribbean Biodiversity Database, DCNA’s news platform BioNews and through the press. This article contains the results from several scientific studies but the studies themselves are not DCNA studies. No rights can be derived from the content. DCNA is not liable for the content and the in(direct) impacts resulting from publishing this article.

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