Crustacean migration investigation
Studying the daily movements of krill in the Southern Ocean
Research led by a University of Leicester scientist is providing new insight into the behaviour of Antarctic krill, the tiny crustaceans which underpin numerous oceanic food chains.
Every evening, vast shoals of krill across the Southern Ocean rise to the surface where they feed on phytoplankton (microscopic plants) before descending to a depth of 50-100m during the day. This ‘diel vertical migration’ (DVM) reduces the susceptibility of krill to their many predators, including fish, birds and marine mammals such as whales. Crucially, it also has an effect on atmospheric CO2 levels as carbon captured by the plankton at the surface is released by the respiring krill deep within the ocean.
The question examined by Dr Ezio Rosato of the University of Leicester’s Department of Genetics and his colleagues was simply: how do the krill know when to rise and when to swim back down again? Although krill can probably detect light at 100m, cloudy weather or large blooms of plankton would negate this so there must be an internal factor too.
“We want to know how the rhythm of behaviour in krill is controlled, because it is very important in terms of how they go up and down the water column, and also in assessing the ecological impact of that vertical migration,” says co-investigator Dr Geraint Tarling from the British Antarctic Survey. “There are various ways to monitor this behaviour, but no real attempt has yet been made to understand the mechanisms behind the DVM: what controls it, how the krill know what time of day it is, and what other indicators they might use to estimate the time.”
Dr Rosato is no stranger to the mysteries of time regulation in arthropods; for more than twenty years he has been studying the genetic basis of the ‘circadian clock’ in the Drosophila fruit fly. A combination of genetic and environmental factors, circadian clocks have been observed in many plants and animals, including humans. They work through a ‘negative feedback loop’ of producing proteins which degrade at a steady rate. But no-one was certain whether such behaviour existed in krill.
“There are differences in the molecular components of circadian clocks between different organisms,” he explains. “This suggests clock mechanisms have evolved independently in different animals and plants and have been retained, as it is likely they provide a selective advantage. We find the same molecular components of the clock in krill, fruit flies and humans but the number of clock genes and proteins is different.”
Krill collected from the Scotia Sea in the Southern Ocean were placed in a series of vertical tubes with infra-red barriers near the top and bottom to detect vertical migration, with light sources initially set to the day/night timings of the latitude where the krill were found. The same krill were subsequently placed in constant darkness to examine their migratory behaviour without light cues.
“Circadian clocks are self-sustaining and entrainable, meaning they set the phase of their rhythm according to the rhythm in the external environment,” says Dr Rosato. “We are interested in understanding how clock proteins come together to regulate their rhythmic expression.”
Unlike Drosophila, a laboratory staple which is extremely easy to study, krill are far from perfect test subjects. “They are not ideal experimental animals,” admits Dr Rosato. “They are difficult and expensive to catch and experiment on and there is only one facility in the world where they can be studied over long periods – which is in Tasmania. But krill are a keystone species for the Southern Ocean. They are as important to the marine life there as crops are to humans.”
In the future, Dr Rosato and his colleagues hope to study other aspects of krill behaviour. The invertebrates have a 20-day reproductive cycle but during the winter they enter a state of quiescence and stop reproducing until spring. This ‘circannual clock’ may be triggered by day-length – ‘photoperiodism’ – but no-one knows for sure.
“One of the major problems of climate change is that a change in temporal rhythm can then adversely affect circannual clocks,” explains Dr Rosato. “For example, warmer weather means winters are shorter so behaviour triggered by warm weather, such as algal blooms, happen earlier. But if the krill’s circannual clock is timed to release them from quiescence just as the algae bloom, climate change means the blooms won’t be there when the krill need them. The krill will starve and die and this will have knock-on effect on fish, whales, birds etc.”
Dr Rosato and Dr Tarling’s colleagues on the project, ‘Gene Function in Antarctic Krill: determining the role of clock-genes in synchronised behavioural patterns’ are Dr Ted Gaten from the Department of Biology and Professor Charalambos Kyriacou from the Department of Genetics.