A copepod in a microscope
The Copepod is a millimetre big predator that eats phytoplankton. The animal send out chemical cues in the water which trigger the phytoplanktons´ defensive mechanisms.

Copepod chemical signals impact marine food chains


When copepods release chemical cues, they reveal themselves to potential prey, phytoplankton. The cues trigger defensive mechanisms in the phytoplankton, which seems to have an impact on the entire marine food web. This is revealed by new research from the University of Gothenburg.

Phytoplankton provide nearly half of the world’s total primary production and are a food source for copepods and other animals. Phytoplankton may be microscopic unicellular organisms, but their defence against predators is ingenious. Some species of phytoplankton scare away copepods through bioluminescence on contact, what is known as marine phosphorescence. Other plankton species produce toxins or split from long strings of cells to single cells to make it more difficult for copepods to find them.

The smell triggers defense mechanisms

“Copepods send out chemical cues like smells in the water. When the phytoplankton detect this smell, this triggers their defensive mechanisms. I’ve seen that the amount of chemical signal released by the copepods determines how the phytoplankton defend themselves,” says Kristie Rigby, doctoral student in marine sciences at the University of Gothenburg.

Her dissertation is a piece of the puzzle for mapping marine food webs as one of the Earth’s most important carbon traps. Kristie Rigby was able to isolate the chemical cues, known as copepodamides, from the copepods and use them in laboratory experiments. By conducting experiments with multiple and single species phytoplankton, she explored the effects of copepodamides on species composition, bioluminescence, toxin production and colony plasticity for various diatoms and dinoflagellates.

Consequences farther up in the food chain

The studies showed that the plankton species are impacted in different ways depending on the presence of copepods and the chemical cues they release. Some species benefit at the cost of others when copepods release their cues into the sea. The balance in the marine ecosystem is connected with how much phytoplankton is available as food for zooplankton, such as copepods. This in turn can lead to consequences farther up in the food chain for species that eat copepods.

Portrait of Kristie Rigby.
Kristie Rigby.

“The trigger of defensive mechanisms caused by copepods’ chemical cues have an impact on the food web. We have seen species composition change depending on how much chemical cues are in the water. This plays just a big a role, if not greater, than the actual behaviour of the copepods, which is important to understand.”

Thesis title: Signals in the Sea: predatory induced defences in marine phytoplankton

Contact: Kristie Rigby, doctoral student at the Department of Marine Sciences, University of Gothenburg

Phone: +46 (0)766-18 34 76, email:

Supervisor: Erik Selander, Senior Lecturer