Penguin Food: How 'superswarms' of krill gather

Krill swarm from above
An extraordinary gathering


Antarctic krill (Euphausia superba)


How 'superswarms' of krill gather
By Matt Walker
Editor, Earth News


When krill come together, they form some of the largest gatherings of life on the planet.

Now scientists have discovered just how these small marine crustaceans do it.

Huge 'superswarms' containing trillions of krill are formed by juveniles not adults, and these swarms are even denser than experts supposed.

That suggests that all krill in the Southern Ocean are more vulnerable to overfishing than previously thought, the scientists warn.

Krill are small shrimp-like crustaceans that gather in huge numbers.

Previous research has found that some gatherings of Antarctic krill (Euphausia superba) can stretch for tens of kilometres.

It was astonishing how much biomass could be concentrated into such a small area
British Antarctic Survey scientist Dr Geraint Tarling

But while huge swarms are known to exist, scientists did not really understand why some swarms are bigger than others, and what drives krill to gather in this way.

So researchers working for the British Antarctic Survey (BAS) decided to investigate the phenomenon.

Led by Dr Geraint Tarling, a BAS researcher based in Cambridge, UK, the research team studied the composition and structure of 4525 separate krill swarms in the Scotia Sea, a vast expanse of water in the Southern Ocean.

The team used echo-sounding equipment, which works much like underwater radar, to find the krill across an area of water equivalent to the eastern half of the Atlantic Ocean.

What they found surprised them.

Krill tend to gather into two distinct types of swarm.

Some krill gather into smaller swarms, no longer than 50m long and up to 4m deep.

These swarms are not very tightly packed, with just ten individual krill per cubic metre, on average.
Antarctic krill (Euphausia superba)
A lone shrimp

However, other much bigger swarms also occur.

Dubbed "superswarms", these are an order of magnitude larger in area, often stretching over one kilometre in length, and averaging almost 30m deep.

What is more, these superswarms are much more densely packed, containing up to ten times greater density of animals.

"I was coming at it thinking there might be small swarms tightly packed, and then large swarms that were a bit more diffuse," says Dr Tarling.

"But what we actually found was the opposite. There were small swarms that were quite diffuse and large swarms that were tightly packed."

That means that the majority of krill living in the Antarctic Ocean at any one time will exist within a few, huge superswarms.

"We talking trillions of krill in one aggregation," explains Dr Tarling.

"Ten or 12 swarms could explain 60 or 70% of the biomass in an area the size of the eastern Atlantic."

"It was astonishing how much biomass could be concentrated into such a small area."

Youthful gathering

The scientists then searched for reasons why such superswarms form.

Certain factors made superswarms more likely.

"The factors we identified included whether there was more likely to be a lot of food around or not, and when there wasn't that much food around, they tended to form larger swarms," says Dr Tarling.
Acoustic image of krill swarm
A superswarm of krill located by echosounder (swarm shown in red)

The small, diffuse swarms are usually formed by mature, adult krill, the researchers discovered.

However, the huge superswarms are formed by juvenile krill.

"Where the animals were less mature, they were more likely to form the larger swarms," says Dr Tarling.

"Why they do that I don't know."

Nightime mystery

One possible explanation could be that swarming together offers individual krill protection against marine predators such as whales or seals.

"All types of swarms are probably to a greater or lesser extent an antipredator response. There is safety in numbers, the predator confusion affect," Dr Tarling says.

But swarming comes at a cost, as each individual shrimp has to compete with millions of others for food.

Adult krill are quite negatively buoyant, and have to keep swimming to stay afloat. That takes a lot of energy, which must be supplied by food, so adult krill likely want to avoid competing with millions of others for their next meal.

But juvenile krill are more buoyant, and need to eat less. So they can afford to gather into huge superswarms for protection.

Another reason could be that researchers have previously shown it is more energetically efficient to be in swarm than be isolated.

"For a juvenile that wants to grow very quickly, saving energy could be a bonus for them," says Dr Tarling.

One mystery to emerge from the research, which is published in the journal Deep Sea Research I, is that superswarms are more likely to gather at night.

"That is more puzzling for us to explain," says Dr Tarling.

"Up until this point, most polar biologists believed that the swarms dispersed [at night], because that's the time they feed."

"When daylight comes they get back into the swarm again for the antipredator benefit. But we found the opposite to that."

Vulnerable to overfishing

The discovery that most krill in the Southern Ocean can be found gathered into just a few superswarms has significant implications for how the animals are fished, Dr Tarling warns.

Fishing fleets can efficiently locate and scoop up whole swarms of krill.

But by fishing out just a few huge superswarms, they may be removing the majority of krill living in the entire ocean.

"Focusing on large swarms can have a much larger effect on the environment than you would predict."

Source:
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8299000/8299690.stm

Archaeopteryx Was Not Very Bird-like

Archaeopteryx Was Not Very Bird-like: Inside The First Bird, Surprising Signs Of A Dinosaur

ScienceDaily (Oct. 9, 2009) — The raptor-like Archaeopteryx has long been viewed as the archetypal first bird, but new research reveals that it was actually a lot less "bird-like" than scientists had believed.

In fact, the landmark study led by paleobiologist Gregory M. Erickson of The Florida State University has upended the iconic first-known-bird image of Archaeopteryx (from the Greek for "ancient wing"), which lived 150 million years ago during the Late Jurassic period in what is now Germany. Instead, the animal has been recast as more of a feathered dinosaur -- bird on the outside, dinosaur on the inside.

That's because new, microscopic images of the ancient cells and blood vessels inside the bones of the winged, feathered, claw-handed creature show unexpectedly slow growth and maturation that took years, similar to that found in dinosaurs, from which birds evolved. In contrast, living birds grow rapidly and mature in a matter of weeks.

Also groundbreaking is the finding that the rapid bone growth common to all living birds but surprisingly absent from the Archaeopteryx was not necessary for avian dinosaur flight.

The study is published in the Oct. 9, 2009, issue of the journal PLoS ONE. In addition to Erickson, an associate professor in Florida State's Department of Biological Science and a research associate at the American Museum of Natural History, co-authors include Florida State University biologist Brian D. Inouye and other U.S. scientists, as well as researchers from Germany and China.

"Living birds mature very quickly," Erickson said. "That's why we rarely see baby birds among flocks of invariably identical-size pigeons. Slow-growing animals such as Archaeopteryx would look foreign to contemporary bird-watchers."

Erickson said evidence already confirms that birds are, in fact, dinosaurs. "But just how dinosaur-like -- or even bird-like -- was the first bird?" he asked. "Almost nothing had been known of Archaeopteryx biology. There has been debate as to how well it flew, if at all. Some have suggested that early bird physiology may have been very different from living birds, but no one had tested fossils that were close to the base of bird ancestry."

Fossilized remains of Archaeopteryx were found in Germany in 1860, one year after Charles Darwin's "Origin of Species" was published. With its combination of bird-like features, including feathers and a wishbone, and reptilian ones -- teeth, three-fingered hands, a long bony tail -- the skeleton made evolutionary theory more credible. The 1860s evolutionist Thomas Henry Huxley saw the Archaeopteryx as a perfect transition between birds and reptiles. Erickson calls it "the poster child for evolution."

"For our study, which required tremendous collaboration, we set out to determine how Archaeopteryx grew and compare its growth to living birds, closely related non-avian dinosaurs, and other early birds that came after it," Erickson said. "I went to Munich with my colleague Mark Norell from the American Museum of Natural History, and we met with Oliver Rauhut, curator of the Bavarian State Collection for Palaeontology and Geology, which houses a small juvenile Archaeopteryx that is one of 10 specimens discovered to date. From that specimen, we extracted tiny bone chips and then examined them microscopically."

Surprisingly, the bones of the juvenile Archaeopteryx were not the highly vascularized, fast-growing type, as in other avian dinosaurs. Instead, Erickson found lizard-like, dense, nearly avascular bone.

"It led us to ask, 'Did Archaeopteryx grow in a unique way?'" he said.

To explain the strange bone type, the researchers also examined different-size species of dinosaurs that were close relatives of Archaeopteryx, including Deinonychosaurs, the raptors of "Jurassic Park" fame. They then looked to colleagues in China for specimens of two of the earliest birds: Jeholornis prima, a long-tailed creature, and the short-tailed Sapeornis chaochengensi, which had three fingers and teeth.

"In the smallest dinosaur specimens, and in an early bird, we found the same bone type as in the juvenile Archaopteryx specimen," Erickson said.

Next, the research team plugged bone formation rates into the sizes of the Archaeopteryx femora (thigh bones) to predict its rate of growth.

"We learned that the adult would have been raven-sized and taken about 970 days to mature," Erickson said. "Some same-size birds today can do likewise in eight or nine weeks. In contrast, maximal growth rates for Archaeopteryx resemble dinosaur rates, which are three times slower than living birds and four times faster than living reptiles.

"From these findings, we see that the physiological and metabolic transition into true birds occurred millions of years after Archaeopteryx," he said. "But, perhaps equally important, we've shown that avians were able to fly even with dinosaur physiology."

Inouye added, "Our data on dinosaur growth rates and survivorship are bringing modern physiology and population biology to a field that has historically focused more on finding and naming fossil species."

Funding for the study came from the National Science Foundation (NSF); Germany's Deutsche Forschungsgemeinschaft (DFG); and The Major Basic Research Projects of the Ministry of Science and Technology of China.

In addition to Gregory Erickson (first author) and Brian Inouye of Florida State University's Department of Biological Science in Tallahassee, Fla., co-authors of the PLoS ONE paper are Oliver W. M. Rauhut, Bavarian State Collection for Palaeontology and Geology, LMU Munich, Munich, Germany; Zhonghe Zhou, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; Alan Turner, Department of Anatomical Sciences, Stony Brook University, Stony Brook, N.Y.; Dongyu Hu, Paleontological Institute, Shenyang Normal University, Shenyang, China; and Mark Norell, Division of Paleontology, American Museum of Natural History, New York, N.Y.

Journal reference:

1. Erickson et al. Was Dinosaurian Physiology Inherited by Birds? Reconciling Slow Growth in Archaeopteryx. PLoS ONE, 2009; 4 (10): e7390 DOI: 10.1371/journal.pone.0007390

Adapted from materials provided by Florida State University, via EurekAlert!, a service of AAAS.

Florida State University. "Archaeopteryx Was Not Very Bird-like: Inside The First Bird, Surprising Signs Of A Dinosaur." ScienceDaily 9 October 2009. 10 October 2009 <http://www.sciencedaily.com­ /releases/2009/10/091009090436.htm>.