Sensory perception of copepods
Copepods are ubiquitous in both saltwater and freshwater environments, and many species migrate up and down the water column at different times of day to gather food and avoid predators. We spoke to Professor Christophe Eloy about his research into how these copepods use the information they acquire and how this influences their behaviour.
A type of extremely small crustacean, copepods are highly abundant in both saltwater and freshwater environments and are found in a variety of different habitats, from surface waters right down to the ocean floor. A wide variability in swimming behaviour has also been observed among copepods, with some species that essentially drift or are carried on ocean currents, while others have very good swimming ability. “Some copepods are very good swimmers for their size. The bottom of their body is a bit like a shrimp. They have six legs, which they use effectively as oars,” outlines Christophe Eloy, Professor of Fluid Mechanics at Centrale Marseille. As the Principal Investigator of the C0PEP0D project, Professor Eloy is investigating the behaviour of these strong-swimming crustaceans, looking to understand how they sense the surrounding environment with only very limited visual perception abilities. “We want to understand their behaviour, so we want to understand how they use the sensations or signals they acquire and how they behave,” he explains.
Copepod behaviour
These copepods have only a very primitive eye, so rely on other sources of information to sense their environment. Copepods have very long antennae covered with sensors, which they use to measure the flow of water relative to their bodies. “This is how they reconstruct their surroundings,” says Professor Eloy. These sensors enable copepods to detect when a predator or their prey is nearby for example, as well as to pinpoint their location in space, yet the underlying mechanisms behind this are not clear. “We have very little idea how the copepods do this,” acknowledges Professor Eloy.
This topic lies at the heart of the project’s work, with researchers developing hypotheses on what optimal or ideal behaviour would look like for copepods. These hypotheses can then be tested in a virtual environment closely resembling the conditions that copepods face in marine environments. “We are trying to infer what the copepods should do, if they wanted to
function in something close to an optimal way, then we look to assess whether this hypothesis is correct. We try to simulate the information or signals that copepods may encounter, then we essentially listen and learn. We try to simulate what a certain behaviour would achieve,” says Professor Eloy. “For example, I recently co-authored a paper titled Surfing on turbulence, which explored whether using hydrodynamical signals would help copepods to increase their vertical swimming speed in turbulent waters. We assumed copepods can measure the gradients of the flow, and then we investigated whether copepods can swim more efficiently by choosing their swimming direction based on this information.”
The project’s agenda includes research into how copepods effectively process this information. Copepods are highly resilient organisms, evolving to become a ubiquitous feature of marine environments, now researchers are looking at the reasons behind this success. “Some members of my team are developing algorithms to basically find behaviours. We use reinforcement learning, a branch of machine learning, to train a neural network and find the optimal or ‘good’ behaviour,” outlines Professor Eloy. The project is still at a relatively early stage, with the team conducting experiments on real copepods to verify that their hypotheses match reality, while Professor Eloy also hopes to broaden the scope of the research in future.
We are trying to infer what the Copepods should do, if they wanted to function in an optimal way, then we look to assess whether this hypothesis is correct. We try
Researchers showed that the behaviour they had hypothesised would prove to be very efficient, with copepods’ ability to adapt to the extent of the flow helping them reach average mean velocities which were much higher than would otherwise have been possible. Another aspect of Professor Eloy’s research centres around investigating how males follow the pheromone trails left by females. “There is evidence that males change behaviour and swim faster when they encounter a path that was previously followed by a female. It is probable that this is based on chemical sensing, so they sense this pheromone,” he outlines. This sensing may be disrupted in turbulent waters, another topic of interest in the project. “The natural habitat of Copepods is not completely quiet, and a pheromone trail will be stirred and stretched by turbulent flow,” points out Professor Eloy. “Does this chemical sensing approach still work in turbulent flow? If it does, can the copepods exploit the fact that they can measure both the chemical concentration and the flow?”
areas. “We are collaborating with biologists who are providing us with the copepods to do the experiments, as well as the algae they eat,” he continues. “We wanted to understand the differences between copepod species, and the variations of behaviour that could be observed.”
The wider aim in the project is to essentially reverse-engineer the algorithms by which copepods process the information available to them about their surroundings, which could then inform the ongoing development of machine learning. The project’s inter-
disciplinary nature, bringing together biology, fluid mechanics and artificial intelligence, is a distinguishing feature. “Marine biologists observe these organisms, while my field is fluid mechanics. My focus is on trying to understand the flow and the signals,” explains Professor Eloy. “Our research also holds relevance to machine learning. People trying to build robots for detecting different things without vision are basically trying to solve the same problem that we are addressing. Our project very much lies at the crossroads between these different topics.”
C0PEP0D
Life and death of a virtual copepod in turbulence
Project Objectives
The objective of C0PEP0D is to decipher how copepods exploit hydrodynamic and chemical sensing to track targets in turbulent flows. We address three questions:
• Q1: Mating. How do male copepods follow the pheromone trail left by females?
• Q2: Finding. How do copepods use hydrodynamic signals to “see”?
• Q3: Feeding. What are the best feeding strategies in turbulent flow?
We hypothesize that reinforcement learning can help reverse-engineer the algorithms used by copepods. To test this hypothesis, we are building a virtual environment, where copepods are trained: virtual copepods sense flow velocity and chemical concentration and this sensing information is processed by a neural network trained by reinforcement learning. This theoretical and numerical approach is complemented by experiments on real copepods with the goal of measuring how copepods reacts in turbulent flow.
Project Funding
This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 834238
Project Partners https://c0pep0d.github.io/team/
Contact Details
Project Coordinator, Professor Christophe Eloy
“We hope to see whether this approach could be extended to other planktonic organisms, while it would also be very interesting to look at predator-prey relationships,” he says.
“We sometimes see that prey adapt when a predator evolves a certain strategy, so there’s a kind of evolutionary arms race going on.”
Predator-prey relationships
A prime example is the way that certain predators approach copepods in the water. Some species suck in water with their mouths as they move through the water, counteracting the flow their bodies generate, which makes them more difficult to detect.
“That makes them almost invisible to what is in front of them, in terms of the flow generated, while prey have also evolved to make themselves less visible to predators,” says Professor Eloy. The project’s agenda crosses several disciplinary boundaries, and while Professor Eloy’s background is in fluid mechanics, he is collaborating with other projects and researchers in complementary
Christophe Eloy is a Professor of Fluid Mechanics at Centrale Méditerranée. His research interests span several fundamental problems in solid and fluid mechanics, including fluid-structure interactions, hydrodynamic instabilities and animal locomotion.
to simulate the information or signals that the Copepods may encounter, then we essentially listen and learn.