Titanichthys, an Ancient Fish that Fed Like a Basking Shark

Scientists have found that Titanichthys, a giant armoured fish (or placoderm) that lived 380 million years ago, used a feeding strategy similar to modern day basking sharks (Cetorhinus maximus), as published in Royal Society Open Science earlier this week. The research team was formed from a collaboration between paleontologists at the University of Bristol and University of Zurich as part of a post-graduate thesis.

Titanichthys, covered in its tough, armoured plates, was one of the largest animals of the Devonian period. It could reach an overall length of over 5 m (16.5 ft) and the length of its lower jaw exceeded 1 m (3 ft).

Artist's interpretation of Titanichthys. Image credit: Mark Witton

However, before now, there was no evidence to suggest how this massive fish fed. It's lower jaw was narrow and lacked dentition or sharp edges that would make it suitable for cutting. As a result, it has long been assumed that Titanichthys was a suspension feeder, filtering large amounts of plankton from the water column by swimming slowly with its mouth wide open. This is a technique known as continuous ram feeding.

But, to further complicate things, there is no fossil evidence to confirm this feeding strategy. Modern suspension feeders, such as basking sharks, have long projections covering their gills, known as gill rakers, to assist in filtering plankton, but no fossilised suspension feeding structures have ever been found for Titanichthys.

Instead, the research team focused on fossilised jaws collected from the Moroccan part of the Sahara Desert for their study. They used biomechanical analysis to compare the lower jaw of Titanichthys to those of other species, testing jaw resilience using a technique known as Finite Element Analysis (FEA). This allowed them to apply forces virtually to each jaw and assess how likely they were to break or bend.

Of their findings, lead author, Sam Coatham, said in a press release, "We have found that Titanichthys was very likely to have been a suspension feeder, showing that its lower jaw was considerably less mechanically robust than those of other placoderm species that fed on large or hard-shelled prey. Consequently, those feeding strategies (common amongst its relatives) would probably have not been available for Titanichthys."

The FEA revealed that the lower jaw of Titanichthys was far less resistant to stress and, therefore, more likely to break than the jaws of other placoderms, such as the better-known Dunkleosteus. As such, the jaws would not have been able to withstand feeding on larger prey as this would exert too much mechanical stress for them to handle.

This is similar to what is seen in both sharks and whales. The jaws of modern suspension feeders are less resistant to stress than their actively hunting relatives. Building on this finding, further analysis that compared the distribution of stress in the jaws showed similar patterns in both Titanichthys and the basking shark.

Basking shark suspension feeding at the water's surface

The team believes that several other extinct species would have also been suspension feeders, including other placoderms and even a species of plesiosaur. They have already identified promising areas for future research to better understand the development of suspension feeding.

"Our methods could be extended to identify other such species in the fossil record and investigate whether there were common factors driving the evolution and extinction of these species. We suggest a link between oceanic productivity and the evolution of Titanichthys, but this should be investigated in detail in the future. An established link could have implications for our understanding of the conservation of modern suspension feeders," Mr Coatham explained.

With so many of today's large suspension feeders being either vulnerable or endangered, the findings of this study, and of the future research it inspires, could prove useful in better protecting these species and minimising the factors that are currently impacting them.

Algal Blooms are Turning Antarctica Green

The beginnings of a new ecosystem is forming in Antarctica, with green algae blooming across the surface of melting snow, a study published in Nature Communications this week has reported. These blooms could spread further in the future as climate change causes more snow to melt, creating the slushy conditions needed for the algae to thrive. Already, in some areas, the algae is so dense that the bright green snow can be seen from space.

A green snow algae bloom. Image used under Creative Commons Attribution 4.0 International License.

The research team, made up of biologists from the University of Cambridge and the British Antarctic Survey, used both satellite data and ground observations to detect and measure green snow algae on the Antarctic Peninsula. For the satellite data, images were taken between 2017 and 2019 using the European Space Agency's Sentinel 2 satellite. Meanwhile, ground observations were conducted by the team at Ryder Bay, Adelaide Island and Fildes Peninsula, King George Island.

The final product of their hard work was the first ever large-scale algae map of the Antarctic Peninsula, which can now be used as a baseline to assess the rate at which algal blooms are forming across the continent due to climate change. The current green algal blooms could prove to be a source of nutrition for other species and they are already forming relationships with with fungal spores and bacteria.

"It's a community. This could potentially form new habitats. It's the beginning of a new ecosystem," explained Dr Matthew Davey, one of the lead researchers and a plant physiologist and chemical ecologist at the Department of Plant Sciences, University of Cambridge.

The research team identified 1,679 separate blooms of green snow algae, covering an astounding area of 1.9 sq km. They estimated that these blooms absorb roughly 479 tonnes of carbon dioxide a year, which is equivalent to the emissions of around 875,000 average UK car journeys. However, this is a relatively small amount on the global scale and is unlikely to make a significant impact on our carbon footprint.

Plus, the warming climate could have a negative impact on the algae regardless of the amount of atmospheric carbon it absorbs. "If it warms up a bit, you get a lot more blooms. If it warms up a lot, the whole system could crash completely because there's no snow," Dr Davey told New Scientist.

There are two factors that currently determine where the algal blooms are located. The temperature needs to be warm enough to turn the snow to slush. The largest blooms are located on the areas of the peninsula and surrounding islands that are warming the fastest and have an average temperature of just over 0°C. Also, there must be a nutrient source for the algae, such as penguin guano. Over 60% of the blooms found during the study were within 5 km of a penguin colony.

As useful as the team's findings are, green snow algae can only give a limited view of the bigger picture regarding the carbon cycle in Antarctica. Future studies will include red and orange algae, both of which were too difficult to detect for this initial study, and will measure blooms across the whole of Antarctica. This will give a better understanding of the total amount of carbon held in Antarctic snow algae, which could prove to be an effective carbon sink in the future for reducing carbon dioxide in the atmosphere.

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.