Krill school takes shape

Shape matters to Antarctic krill — it may affect where they swim in a school and could have implications for the estimation of krill biomass in the Southern Ocean.

Dr Luke Finley of the Australian Government Antarctic Division used digital photography and computer software to digitise the shapes of 220 male, female and juvenile krill. Using geometric morphometrics, a mathematical technique for visualising and quantifying shape, he generated a generic krill shape — the average shape of all the krill photographed.

By comparing individual krill shapes with the generic krill shape, Dr Finley found that mature female krill had a larger upper body (thorax) than males or juveniles, which they use to store their eggs. Male krill, however, appear to be built for speed with more streamlined bodies and longer more powerful tails. These differences are most apparent during the krill breeding season in the Antarctic summer.

‘Female krill breed near the continental shelf of Antarctica and then swim out into the open ocean to lay their eggs. They are more interested in conserving energy and investing in the production and safety of their eggs, than speed, and their shape reflects this behaviour,’ Dr Finley said.

‘Males, on the other hand, may sacrifice their safety and physical condition for speed and increasing their chance of mating with females — a live fast, die young strategy.’

Dr Finley used these shape measurements and other research to hypothesise what a school of krill might look like.

‘My theory is that a school is comprised of a dense core of female, sub-adult and juvenile krill, and that these individuals cycle through to the edge of this core to feed, defecate or release eggs. In contrast, males tend to stay on the outside of the school and use their superior speed to dart into the edges of the school to mate.’

This theory is difficult to prove as there are limits to how effectively a school can be sampled in the open ocean. However, trawl samples taken through a krill school by other researchers, found that krill caught near the edge of a school were predominantly males, while those caught in the middle of a school were predominantly females.

Dr Finley’s research could have implications for how precautionary krill catch limits are set in the Southern Ocean by the Commission for the Conservation of Antarctic Marine Living Resources.

Currently, acoustic scientists at the Antarctic Division are working on improving the procedure for the estimation of krill biomass. There are certain parameters used in the estimation that could be improved, and integrating the diversity of krill shapes observed during Dr Finley’s study may help this process.

‘It leads to the question of whether we should split krill into different shapes when we do these estimates, such as adult males and females, rather than using a generic krill shape,’ Dr Finley said.

‘Also, different geographic regions could be characterised by different krill shapes. For example, juvenile krill are more likely to be found inshore, while egg-bearing females are present in the open ocean. These are some of the questions we will be investigating to see if they have any effect on biomass estimates'.