For the first time scientists have been able to determine the age of Antarctic krill by counting the growth bands in their eyestalks.

Krill biologist Dr So Kawaguchi investigates Antarctic krill. They are one of the most abundant and successful animal species on Earth. Scientists estimate there are about 500 million tonnes of Antarctic krill in the Southern Ocean. (Photo: Australian Antarctic Division)
Krill biologist Dr So Kawaguchi investigates Antarctic krill. They are one of the most abundant and successful animal species on Earth. Scientists estimate there are about 500 million tonnes of Antarctic krill in the Southern Ocean. (Photo: Australian Antarctic Division)

The research found that krill grow annual bands in their eyestalks, much like growth rings in trees, and these correlate directly with their age. Australian Antarctic Division krill biologist, Dr So Kawaguchi, said it’s a remarkable finding. “Despite more than 50 years of research, until now it’s been impossible to accurately assess the longevity of krill and the age structure of their populations,” Dr Kawaguchi said. “Krill don’t have any hard parts, such as ear bones, shells or scales, so we can’t determine age using these calcified structures.” “Additionally, there’s almost no size difference in krill beyond two years of age, and their regular moult means they can actually shrink in size, depending on the time of year and food availability.”

Except for their large black eyes, krill are mostly transparent, although their shells have a bright red tinge from small pigment spots. Adult Antarctic krill are approximately six centimetres in length and weigh over a gram. Pictured here is one of their compound eyes (Photo: PLOS ONE/Australian Antarctic Division)
Except for their large black eyes, krill are mostly transparent, although their shells have a bright red tinge from small pigment spots. Adult Antarctic krill are approximately six centimetres in length and weigh over a gram. Pictured here is one of their compound eyes (Photo: PLOS ONE/Australian Antarctic Division)

The new method, pioneered on larger crustaceans such as lobsters and crabs, involves looking at the eyestalks under a microscope. “We look at a longitudinal section of the eyestalk to identify the light and dark growth bands and count exactly how many years the specimen has been alive.” The aging technique has also been successfully used on formalin-preserved samples, which means we are now able to accurately determine the age of preserved krill from the early 1900s. “The ability to retrospectively age krill allows us to compare length-at-age over time and across environments to examine changes in the Southern Ocean ecosystem,” Dr Kawaguchi said.

A cross section of a known-age Antarctic krill eye stalk showing the annual bands indicated by dots by which age can be determined. Green and white arrows indicate the epicuticle (the outermost portion of the exoskeleton) and endocuticle (the inner, elastic layer of the exoskeleton) respectively.
A cross section of a known-age Antarctic krill eye stalk showing the annual bands indicated by dots by which age can be determined. Green and white arrows indicate the epicuticle (the outermost portion of the exoskeleton) and endocuticle (the inner, elastic layer of the exoskeleton) respectively.

The scientists studied both wild and known-age captive krill, bred in the Australian Antarctic Division krill aquarium and the Japanese Port of Nagoya Public Aquarium. The age-based assessment methods will provide information on stock structure to assist with catch limits and management options for the krill fishery through the Commission for the Conservation of Antarctic Marine Living Resources. “Krill are a keystone species in the Southern Ocean, predated by penguins, seals, flying seabirds and whales, so any fishery needs to be carefully managed.”

“The Southern Ocean is also undergoing major changes in the sea-ice zone, in primary production and through ocean acidification, so better understanding how long they live will help us more accurately predict the potential impacts of climate change on krill.”

Scientists at the Australian Antarctic Division study krill to provide a better understanding of krill life cycles so that the fishery can be better managed, and to understand the impact of environmental changes such as ocean acidification on the ecosystem. (Photo: Australian Antarctic Division)
Scientists at the Australian Antarctic Division study krill to provide a better understanding of krill life cycles so that the fishery can be better managed, and to understand the impact of environmental changes such as ocean acidification on the ecosystem. (Photo: Australian Antarctic Division)

Source: Australian Antarctic Division (AAD)