The Magazine for Underwater Professionals

Jan/Feb 2018


Norway lobsters' appetite for jellyfish caught on camera

Norway lobsters’ secret love of jellyfish has been caught on camera by scientists for the first time, raising questions about whether jellyfish blooms are pests or potential food sources for the commercial fishing industry.

Jellyfish blooms, the vast swarms that are appearing with increasing frequency in waters around the world, are thought to have highly negative effects on marine life and commercial fisheries.  

But the scavenging that Heriot-Watt University, Scotland, scientists have caught on camera raises the question of whether the maligned gelatinous creature is actually a sustainable, rich food source for one of the Atlantic’s most commercially important invertebrate species: the Norway lobster.

As reported in Nature Scientific Reports, Dr Andrew Sweetman from Heriot-Watt University and colleagues Kathy Dunlop from Akvaplan-nive Norway and Daniel Jones from the National Oceanography Centre, UK, wanted to find out which deepwater species were most attracted by a jellyfish dinner.

Two underwater camera systems were lowered to depths of more than 250 metres in the Sognefjorden in western Norway, each baited with defrosted helmet jellyfish carcasses.

While the team had expected some of the usual scavengers to appear, like hagfish and amphipods, the team was surprised when 25-centimetre Norway lobsters rapidly located and decimated the jellyfish carcasses.

Sweetman, an associate professor of marine benthic ecology in the Lyell Centre at Heriot-Watt University, said: “We had no idea that Norway lobster, which is worth £78 million to Scotland alone, fed on the jellyfish carrion that sinks to the depths, so it was very exciting to capture this on camera.

“The Norway lobsters’ feasting was fast and furious. In both deployments, they located the jellyfish in under 25 minutes, scared the other scavengers away almost immediately and consumed over 50 per cent of the carcass.

“We looked at the nutritional value of the jellyfish, along with average Norway lobster energy intakes in the Firth of Clyde, and found that just one of these jellyfish could satisfy the lobster’s energy requirements for up to three months.

“Jellyfish have historically been considered a ‘dead end’ in the marine food chain, and it was only in 2012 that we discovered that anything was using it as a food source.

“To discover that it’s a potentially huge food source for one of the Atlantic and North Sea’s most commercially important catches is really interesting, and raises questions about how jellyfish could contribute to sustainable commercial fishing.”

Although the experiment took place in Norway, Sweetman and colleagues are confident that the Norway lobsters in UK waters will have a similar appetite to the fjord-dwelling populations.

Sweetman said: “There are various species of jellyfish in Scottish waters and lochs, and I’m confident any Norway lobsters in those areas will also be feasting on the carrion.

“An interesting next step would be to find out how the Norway lobster are using the energy from the jellyfish. New techniques mean we could label jellyfish tissue with an isotope and trace it in the lobster – so we could actually tell whether it was going into reproductive cells, or helping general growth. That would be really fascinating.”

The research was carried out as part of the Jelly Farm project, which is funded by the Norwegian Research Council and EFFECTS by the Fram Centre Fjord and Coast Flagship Programme.


NUS-developed underwater droid inspired by manta rays

Researchers from the National University of Singapore (NUS) have created MantaDroid, an aquatic robot that emulates the swimming locomotion of manta rays. The robotic manta ray, which swims at the speed of twice its body length per second and can operate for up to 10 hours, could potentially be employed for underwater surveillance in the future.

Manta rays are considered one of nature’s most graceful and efficient swimmers. Unlike most underwater species, manta rays possess a unique propulsion mechanism that enables them to cruise through turbulent seas by flapping their pectoral fins effortlessly. This distinctive feature has sparked great interests in understanding the science behind the mechanism and to incorporate similar mechanisms into autonomous underwater vehicles. 

Motivated to develop a bio-inspired AUV, a research team led by Associate Professor Chew Chee Meng and Associate Professor Yeo Khoon Seng from the Department of Mechanical Engineering at the NUS Faculty of Engineering developed MantaDroid, which resembles a juvenile manta ray. Measuring 35 centimetres in length, 63 centimetres in width and weighing only 0.7 kilograms, MantaDroid can swim at a speed of 0.7 metres per second.

MantaDroid was designed and optimised over two years after an in-depth study of fluid dynamics and multiple experiments which included testing of 40 different fin designs. The fins that were eventually installed on the robot are a pair of flexible pectoral fins made using PVC sheets. The fins achieved good manoeuvrability and swimming capability when tested in the pool, according to the researchers. 

“Unlike other flapping-based underwater robots that replicate manta rays’ flapping kinematics by using multiple motors to achieve active actuations throughout the fins, MantaDroid is powered by only one electric motor on each fin. We then let the passive flexibility of the fins interact naturally with the fluid dynamics of the water to propel the subsequent motions,” said Associate Professor Chew.

MantaDroid makes a promising alternative to traditional propeller-based thrusters used in conventional AUVs, and could potentially operate for a longer range. Like the real manta ray, MantaDroid also has a flat and wide body that can accommodate a range of sensors and be utilised for different purposes such as studying marine biodiversity, measuring hydrographic data and performing search operations.

The NUS team will be testing MantaDroid in sea environment next, to investigate its swimming capability at different depths and its ability to withstand underwater current. The team is also working to incorporate more modes of movement in the robot’s fin mechanism.

  • MantaDroid in the pool





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