Species Spotlight: Greenland Shark

The marine environment is incredibly diverse, with so many creatures unlike anything you will see on land. One such creature is the Greenland shark (Somniosus microcephalus), which is one of nature's oddballs, with poisonous flesh, an astounding life expectancy, and the ability to thrive in one of Earth's harshest environments.

Description

The Greenland shark is a species of sleeper shark belonging to the family Somniosidae. They are the largest Arctic fish and one of the largest shark species, with a length of around 6.4 to 7.3 m (21 to 24 ft) and a weight of around 1,000 to 1,400 kg (1.1 to 1.5 US tons). They live a long life of near solitude in the deep sea, but the darkness down there doesn't bother them as almost all Greenland sharks are blind.

Greenland shark at the floe edge of the Admiralty Inlet, Nunavut

It is a very unusual looking shark with a short, blunt snout, small eyes, and small fins relative to its body length. Even the gill openings are incredibly small for such a large shark. Unlike most other sharks, they don't have countershading and, instead, the body is uniform in colour, usually a blackish-brown with a mottled appearance.

Even more strangely, a Greenland shark's flesh is extremely poisonous. It contains high levels of urea and trimethylamine N-oxide (TMAO) which, when ingested, is metabolised into trimethylamine (TMA). This toxic compound causes unpleasant intestinal issues, as well as effects similar to extreme drunkenness. Eating too much Greenland shark flesh can even be fatal. However, TMAO is helpful to the shark. It increases buoyancy, acts as a natural antifreeze, and counteracts deep water pressure.

Habitat

Greenland Sharks are native to the Arctic and North Atlantic oceans. They live at incredible depths and have been observed as far down as 2,200 m (1.4 miles). However, the more common depth range for these sharks is between 0 and 1,500 m.

They also migrate each year and their preferred depth will vary with the temperature and the season. In the winter, Greenland sharks can be found congregating in shallower waters in the far north. On the other hand, in the summer, they move to deeper waters towards the south of their natural range.

Diet

Like most sharks, the Greenland shark is an apex predator. They mostly feed on fish, such as cod and herring, though other prey items recorded in the past have included smaller sharks, skates, eels, and squid. It has also been theorised that they may occasionally ambush sleeping seals and other marine mammals.

Their teeth are highly specialised and are designed to cut plugs out of flesh. The upper teeth are narrow and pointed and are used for gripping. Meanwhile, while the lower teeth are broad and they curve to the side. By swinging its head in a circular motion, a Greenland shark is able to cut out a round plug from its prey.

Greenland shark jaws with pointed teeth on the upper jaw and flat on the lower jaw (from Chernova, et al. 2015)

Life Cycle

Greenland sharks can live for up to 400 years, making them the longest living vertebrate in the world. They mature at around 150 years of age, so it is a long time before they are capable of reproducing.

Females are ovoviviparous, meaning that, instead of laying eggs, the eggs remain in the mother’s body as the embryos develop and she will then give birth to live young. The gestation period of a Greenland shark is not currently known.

Around 10 pups are born per litter, with each pup measuring around 38 to 42 cm (15 to 16.5 inches) in length. The sharks then grow at a rate of just 0.5 to 1 cm a year, meaning it takes them a very long time for them to reach their adult size.

How fast are they?

As Arctic natives, Greenland sharks live in very cold waters that can range from -2 to 12°C (28 to 54°F). As a result, they have a slow metabolism and are a very slow-moving species. In fact, they are the slowest moving of all known shark species and they have both the lowest swim speed and frequency of tail beats for their size across all fish species.

They have a cruising speed of just 1.22 km/h (0.76 mph) and a maximum speed of 2.5 km/h (1.6 mph), which they are only capable of maintaining for short bursts. This sluggish nature is how the sleeper shark family gets its name.

Why are they blind?

The vast majority of Greenland Sharks are blind. This is because they become colonised by a parasitic copepod, called Ommatokoita elongata, which attaches to the shark’s eye and feeds on the surface of the cornea. It has been suggested that the copepod may be bioluminescent, which would attract prey for the shark and make this a mutually beneficial arrangement. However, this has not yet been proven.

Ommatokoita elongata attached to the eye of a Pacific sleeper shark, Somniosus pacificus (from Benz, et al. 2002)

Luckily, in the dark ocean depths, Greenland sharks do not rely on their vision for survival.

Biggest Threats

Greenland sharks are currently listed as Near Threatened on the IUCN Redlist of Threatened Species. Fishing and the harvesting of aquatic resources cause the biggest impact to Greenland shark populations.

Despite the fact that their flesh is toxic, Greenland sharks are hunted, or are purchased as bycatch from fishing ships, and used for food as the meat can be treated to make it safe for consumption. This is done by boiling it in several water changes or by drying the meat.

Alternatively, it can be used to produce a traditional dish, Hákarl, which is considered a delicacy in Iceland. For this, the carcass is fermented in a pit for several weeks with stones pressing and draining the toxic fluids from the body. The meat is then hung up in strips and cured for several months.

Though demand for the species has decreased over time, it is estimated that around 1,300 are caught each year, mostly as accidental bycatch. The number of mature individuals continues to decline, which makes it harder for the population to recover with fewer reproductively active animals to help boost numbers. So, while the Greenland shark is not yet threatened, it's certainly something we have to keep an eye on if we want this fascinating creature to stick around.

Sea Level Rise Explained

Chances are you've already heard of rising sea levels in the media, but many people wonder whether it is as serious as it sounds or if the issue is being exaggerated. Sea level rise has been occurring to some degree for centuries but it has accelerated over the last few decades, causing more land to be submerged.

Read on to learn the evidence behind sea level rise and why it is such an important issue.

What is sea level rise?

Sea level rise is a global increase in the ocean’s surface levels and it is measured from the centre of the Earth. However, since the surface of the ocean is not flat, sea level rise can vary and it will not change at the same rate all over the world. As a result, local sea level rise in a specific location may be more or less than the global sea level rise, and both can be influenced by a number of factors. Sea levels vary greatly over time, from second to second or decade to decade. As a result, sea level rise can be difficult to measure. To keep it objective, sea level rise is defined as the average rise observed over a much longer period and is usually measured from several different coastal stations over a few centuries. With modern advancements, satellite measurements are now used alongside more traditional methods. What causes sea levels to rise? There are a number of factors that contribute to rising sea levels:

Ocean warming With each passing year, climate change is having a significant impact on our planet, and it is currently the leading cause of sea level rise. Burning fossil fuels, one of the biggest factors contributing to climate change, releases carbon dioxide into the atmosphere. The oceans then absorb this carbon dioxide and, because CO2 traps heat, the water becomes warmer and expands. This causes an overall rise in sea levels. Melting sea ice When glaciers and ice sheets melt, sea levels rise as the planet’s overall water mass increases.

There are large reserves of ice found in places like Greenland and Antarctica in the form of ice sheets. It is natural for them to melt during warmer months and refreeze in the colder months. But, as you can imagine, as global temperatures rise, these ice reserves are now melting at a faster rate. To make matters worse, it is no longer cold enough for them to refreeze in the winter. As a result, water that was previously stored as ice is released, causing a rise in sea levels. Movement of the Earth’s crust The Earth’s crust moves around a lot, hence why we have earthquakes. However, what many people don’t realise is, the crust is capable of moving vertically. This can create a shift in sea levels, causing them to rise. Isostatic rebound, in particular, contributes to rising sea levels. It is a vertical shift in the Earth’s crust that occurs in response to the removal of ice sheets that previously weighed it down. Changes in atmospheric pressure Regional changes in atmospheric pressure also have a significant impact on our oceans. They are linked to changes in the strength of ocean currents, as well as their distribution. These ocean-atmosphere interactions, combined with the changes in seawater density that occur as a result of increasing water temperatures and freshwater input from melting ice, contribute to a rise in sea levels. How much are sea levels rising? Between 1900 and 1990, sea levels rose at a rate of roughly 1.7 mm each year. However, sea level rise has accelerated over the last few decades and, between 1990 and 2009, sea levels began to rise approximately 2.8 mm each year. Most recently, in 2019, the estimated rate of sea level rise ranges between 3 and 4 mm a year. So, the issue continues to grow. There have been a number of models to try and predict future sea levels but this is not an easy task, especially considering we don’t know what future carbon emissions will be like. Regardless, all current models predict that the rate of sea level rise will continue to increase. What's the issue? Sea level rise has a number of significant impacts: It threatens coastal life all over the world As sea levels rise, it’s the areas closest to the sea that are most affected. Coastal areas can become damaged by floods and storm surges. Eventually, the shoreline will move further and further inland as coasts become submerged.

Since the coast tends to be where our most heavily populated areas are, this would have a massive impact on countless communities all over the world. Not only that, the coast is home to many fragile habitats that are rich in biodiversity, and these will become damaged as sea levels rise. People will lose their homes As sea levels rise further, claiming more land, more and more people will become displaced, especially in coastal communities. As a result, they will be forced to move further inland to try and build a new life for themselves. However, even life further inland is threatened by sea level rise. Rising seas can contaminate soil and groundwater with salt, which is very harmful. Salt contamination from seawater causes major issues When sea salt contaminates the soil, it leaves less land available for agriculture as nothing is able to grow in these soils. As a result, people lose their livelihoods and crops begin to fail as the issue spreads further. It can also affect drinking water as, for many of us, our water comes from fresh groundwater reservoirs. This supply of freshwater becomes contaminated with salt as sea water rises further. And, as all living things rely on water, life in such areas becomes incredibly difficult when there is less freshwater available.

What evidence is there for rising sea levels? The estimated global sea level rise over the last several decades is based on data from tide gauge records, as well as satellite data and estimates of certain contributions, such as melting ice sheets. Church and White analysed data from these tide gauge and satellite records in 2011. They corrected the data to allow for vertical movement of the Earth’s crust and still found that sea levels rose at a rate of 1.7 ± 0.2 mm a year from 1900 to 1990. They also confirmed the increased rate of 2.8 ± 0.8 mm a year between 1990 and 2009. Two separate studies by Hansen et al. (2016), and Golledge et al. (2019) have created models that provide evidence that climate change increases the melting of the Antarctic ice sheet, resulting in sea level rise. From the models, they also predict a similar trend with the Greenland ice sheet, which would disrupt the North Atlantic Gulfstream and greatly affect the climate of northwestern Europe. Unfortunately, this will only exacerbate the issue of rising sea levels. Is there anything I can do? Since global warming is the biggest factor influencing sea level rise, the best way for you to do your part is to reduce your greenhouse gas emissions. If you avoid air travel as much as possible, this will have a positive impact as air travel has one of the largest carbon footprints. Where possible, use the train instead. It takes longer but it makes for a more pleasant journey. Closer to home, you could use your car less. Cars contribute a lot to greenhouse gas emissions, so cutting back on their use will make a real difference. Public transport or car pooling with multiple people can help. Or you could walk and get in your 10,000 steps for the day. Eating less meat, especially red meat, can also reduce your personal carbon footprint. The meat industry is a particularly high producer of greenhouse gas, but cutting back meat consumption can help. This doesn’t necessarily mean you have to go vegetarian; small lifestyle changes can make a world of difference. Many people already take part in Meat Free Mondays to do their bit for the planet. Reducing emissions will improve the planet’s overall health and reduce the impact that climate change is having on sea levels. But it’s a tricky issue. It will take time before we see any improvement and, even if greenhouse gas emissions cease, sea levels will continue to rise for a while. In fact, it’s estimated that, even with no emissions, sea levels will rise 0.7 to 1.2 m between 2000 and 2300.

Sea levels continue to rise at an ever increasing rate. But we shouldn't give up. Even though it’ll take some time and we won’t see the improvement for ourselves, it’s important that we act now for future generations and for the future of our planet as a whole. If not, the trend of sea level rise will continue unchallenged, and will inevitably have an even bigger impact.

Plastic Found in 4 out of 10 Seabird Nests

Four out of ten nests surveyed on the uninhabited Lady Isle in the Firth of Clyde, Scotland contain plastic, a study recently published in the Marine Pollution Bulletin has found. The survey, led by a team from the University of Glasgow, was carried out in May 2018. The researchers examined 1,597 seabird nests on the island and found that a whopping 625 contained plastic.

The amount of nests recorded with plastic debris varied between different seabird species. Shags (Phalacrocorax aristotelis) were the worst off, with 80% of nests containing plastic. Meanwhile, 53.5% of great black-backed gull (Larus marinus) nests, 35.6% of herring gull (Larus argentatus) nests, 25% of lesser black-gulled gull (Larus fuscus) nests, and 24.8% of cormorant (Phalacrocorax carbo) nests were found to contain plastic debris.

It is thought that these differences are likely due to the different nest-building behaviours that each species exhibits. For example, shags will reuse the same nest each year, which could allow plastic to accumulate in that nest exponentially.

Most of the plastic found in the seabird nests were identified as consumer waste discarded in urban areas. This was likely then washed to the island and collected by the birds. Dr Ruedi Nagar, a seabird ecologist at the University of Glasgow, explained further:

"They end up in seabird nests, not because seabirds actively pick them up in built-up areas and carry them to their nest, but because they are brought their passively by marine currents."

The research team's data showed that nests on the north side of the Lady Isle, the side closest to the outgoing tide from the mainland, were more likely to contain plastic debris. This, again, suggests that the plastic originated from the mainland and washed ashore, where the birds were then able to collect it from their nesting area.

Nesting herring gull

Danni Thompson, a researcher volunteering with Dr Nagar, focused more closely on herring gulls during the study. As the most abundant seabird species nesting on the island, it was thought that these results, in particular, could provide useful insights on how plastic debris is acquired by seabirds.

She explained, "As herring gulls often forage in landfills, we wanted to see if they were swallowing plastic whilst eating and then bringing it back to the nest." However, the team were able to confirm that the plastic debris most likely arrived by other means, not by ingestion.

While it is useful to have a clearer idea of how plastic makes it way into seabird nests, the next step is to find out the impact it could have, since plastic may negatively affect the eggs and chicks, and also increases the risk of entanglement. Further research is needed to identify the likelihood and extent of these impacts and how they affect seabird populations in the long-term. However, by identifying the source of the plastic debris, the team's findings will help conservationists plan appropriate action to combat this issue in the future.