Why Do We Know So Little About One of the Most Abundant Animals on the Planet?
Krill are ecologically and economically important, yet they have long been misunderstood and misrepresented
Adapted excerpt from The Curious Life of Krill
What are krill? Ask the average person on the street to describe a krill, and the most frequent response is a blank look. On rare occasions the response is “krill are the tiny shrimp-like crustaceans that whales feed on.” Many imagine that krill are microscopic, like water fleas or phytoplankton. Few appreciate their real size, which is far from microscopic. If all the animal inhabitants of the ocean from the largest whale to the most minuscule invertebrate are lined up based on size or weight, krill fall in the middle of the pack. Next to their seafaring brethren, they are average in size — not large but not microscopic. Not even tiny.
photo by Stephen Nicol
Antarctic krill, one of 85 krill species, were first noticed by whalers in the Southern Ocean. The bellies of the giant whales they hunted were filled almost exclusively with what whalers variously described as shrimps, squillae, animalcules, and insects. Krill had identity issues from the start. Whalers had known of the existence of krill from their observations in the North Atlantic, where several species are abundant and were frequently sighted on the feeding grounds of the great whales.
The word krill is understood to mean “young fish” in Norwegian, but I was informed by a Norwegian colleague that the term is actually an onomatopoeia, a word formed to replicate the sound of millions of krill pattering on the water as they jumped clear of the surface — behavior I had witnessed firsthand in the Bay of Fundy. This surface swarming behavior, exhibited by many species of krill, was another indication of their existence. Since krill are generally only found deep in offshore waters, they are usually seen only by fishermen and whalers and others who venture far from land.
Krill go by many names in the languages of maritime nations. Gaelic fishermen knew them as suil dhu, which means “black eyes.” In Japan they are known as esada or okiami, and fishermen in the English-speaking world often refer to them as red bait. But most people have no need of a word for animals they never see and rarely hear of.
In the language of science, however, krill belong to the taxonomic order of Euphausiids, and several of their 85 species are abundant in oceanic and some coastal areas around the world. Not surprisingly, Antarctic krill, first scientifically described in 1855, are found in the waters around the frozen continent. It is, without doubt, the most abundant, ecologically and economically important, and best-studied Euphausiid species. From here on, I will use the term krill to refer to Antarctic krill.
Krill are mostly transparent, when alive, with splashes of contrasting red and green, large spherical black eyes, and an elongated, streamlined shape that tapers to a pointed feathery tail. The green comes from the algae they eat and the red from special pigmentation spots that can expand and contract, making their shells a lighter or darker shade of red. When viewed en masse krill can turn the ocean blood-red.
All species of krill also have electric-blue light-emitting organs that dot the body, providing a spectacular light show, particularly when freshly collected. The overall effect is startling, and those who see living krill for the first time are often struck by their translucent beauty. When viewed closely, they are undoubtedly crustaceans, distant cousins to the prawns and shrimps that are so familiar in the displays in fish markets.
The Australian Antarctic Division in Tasmania established a research aquarium especially for the study of Antarctic krill. The aquarium quickly became a favorite of tour groups who grew to appreciate the beauty of free-swimming krill, including visitors as diverse as schoolchildren, politicians, and wildlife celebrities, such as David Attenborough and Jacques Cousteau. This was often the first time they had seen living krill. Inevitably, comments began with, “I didn’t realize they were so big!” This has led to my lifetime quest: to quash the misconception that krill are microscopic organisms. Adult Antarctic krill can reach a length of over six centimeters (roughly two and a half inches) and can weigh up to two grams (0,07 ounces).
Photo Dr David Demer, NOAA/NMFS/SWFSC/AMLR.
Over the years, frustrated by constant references to the microscopic size of krill, I made what many have considered an unwise decision. I had a krill tattooed on my left arm as a handy illustration of the appearance and actual size of krill that I could flourish when needed. That was the theory anyway. Unfortunately, my tattoo artist, though undoubtedly talented, was unable to render my favorite crustacean at the requisite size, and he took a few liberties with its anatomy too. As a result, when I roll up my left sleeve I display a rather terrifying lobster-like creature about twice the size of a krill while declaring “krill look something like this. They are quite a bit smaller but are definitely not microscopic!” Not quite what I had in mind, but it’s still a guaranteed conversation starter.
Living krill are hard to find, and good images of live krill are scarce. An Internet search for krill more often than not turns up images of withered brown specimens practically reeking of formalin through the computer screen. Even worse, many depict species of other crustaceans that are mistakenly labeled as krill. I was once handed a brochure about the need to conserve and protect krill, but the sponsoring conservation organization had used a cover photograph of a swarm of squat lobsters, which have only a passing resemblance to krill. It is difficult to convince the public to conserve a species you can’t correctly identify.
Krill are certainly crustaceans with a shrimplike appearance. They are differentiated from actual common shrimps and prawns in that they don’t perch on the seafloor, but rather swim in the open ocean for their entire life—a pelagic, or “free-swimming,” lifestyle. To swim constantly requires enormous inputs of energy as krill are heavier than seawater. This apparent drawback has not impeded krill from an evolutionary perspective, and they are among the planet’s most successful inhabitants by any measure.
Krill also differ from other crustaceans in subtle ways, most of which could be spotted only by ardent crustacean scientists. For example, their gills are found outside their carapace (the largest part of the shell that covers their head and bodily organs). Sensory organs are strategically located on the front end of the body and consist of two pairs of antennae and large eyes. Krill also possess ten pairs of feathery feeding limbs, which they use to comb the water in a not-very- discriminating search for food particles, living and dead, animal and plant. This makes them highly versatile omnivores with a constant supply of food that allows them to swim, grow, and reproduce. Their tail is their powerhouse, almost pure muscle, which activates their five pairs of swimming legs to propel them forward, and it provides thrust for rapid backward escapes, often referred to as lobstering.
Krill rarely swim alone; they are most often found in swarms or schools containing astronomical numbers of companions, which, as anyone who comes from a large family can attest to, has both costs and benefits. Because they are average-sized, abundant, and aggregated, krill are a premium food source for organisms higher up the food chain. Entire marine ecosystems depend on krill to eat the truly tiny organisms, and in turn to provide themselves as concentrated protein for bigger animals. Krill swarms can be so extensive and visually striking that they can be seen from space.
It is difficult for krill scientists like me to avoid superlatives when describing Antarctic krill, because they are truly among the world’s most astonishing animals. Krill possess distinctive features that truly set them apart from the humdrum life in the oceans. Here are seven remarkable attributes of krill.
Krill are possibly the most abundant animal on the planet. It is a bold assertion that Antarctic krill could be the world’s most abundant multicelled animal (metazoan), and I should clarify that (1) I refer to Antarctic krill as a single species, not a general group, such as ants or beetles; (2) I use biomass as the unit of measure, meaning the number of individuals of a species multiplied by their average weight; and (3) I make the (rather shaky) assumption that we can accurately quantify the biomass of any species.
With those caveats in mind, an analysis by the World Wildlife Fund in 2011 suggested that cattle have a biomass of 520 million metric tons, human biomass is around 350 million metric tons, and krill are a rather distant third, with 150 million metric tons. A more recent assessment of krill biomass has put the figure at around 400 million metric tons, so krill are still in the running for global biomass domination. Without a doubt, however, krill have the greatest biomass of any marine metazoan. Because the oceans occupy seventy percent of the area of our planet, this fact alone makes them globally important.
photo courtesy by NOAA NMFS SWFSC Antarctic Marine Living Resources (AMLR) Program
Krill inhabit an entire ocean. Estimates of the area of the Southern Ocean inhabited by krill range from 19 to 32 million square kilometers (7,336,000 to 12,355,000 square miles), which is 10 percent of the planet’s ocean area, or roughly four times the land area of Australia. It is now apparent that Antarctic krill can also be found in all water depths — from the surface to the sea floor at 4,500 meters below. Living in this immense volume of water has extreme implications for krill, ranging from how they maintain connectivity between widely distant populations, to what physiological processes they require to cope with the daily pressure changes at different depths.
Krill form some of the largest aggregations of animal life. Krill live in three-dimensional swarms, or schools, that can stretch for 20 kilometers and contain as many as three million metric tons of animals (roughly 30 trillion individuals). Not surprisingly, such swarms have been described as the largest aggregation of animal life on Earth by the arbiter of all big things — Guinness World Records. These vast dynamic clouds of krill are difficult to envision, though the film Happy Feet 2 did a wonderful job of portraying what a krill swarm might look like, even if the constituent members of these swarms were slightly modified in the animation. The aggregating habit, the density of the swarms, and their sheer size make krill ecologically critical and economically important.
Krill are much bigger than people think. As I have already mentioned, krill, with a length of 6 centimeters (roughly 2.5 inches), are far from being microscopic. Krill size has a number of consequences for their visibility as prey and how they are valued by humans. Conservation efforts have historically enjoyed more success for large animals, such as whales, elephants, and tuna; interest in saving species wanes for those on the smaller end of the spectrum. Thus depicting a small animal as being smaller than it actually is compounds the problem.
Krill have a unique biology. Recent studies have revealed that the krill genome (the amount of genetic information in each cell) is roughly 12 times the size of the human genome. We are not yet sure what the implications are, but it is an astonishing fact on its own. Krill can grow rapidly when food is abundant and shrink when food is in short supply. This is not simply a krill weight-loss exercise; krill grow and shrink by molting their shells every month or so, and when starved they must downsize with each molt. This trait complicates attempts to determine their age. We know from keeping krill in aquaria that they are long-lived, with life spans exceeding those of many of their predators. This means that at any point in time the population of krill is the product of many seasons, not just one or two, a discovery that has revolutionized the science of krill ecology and the management of its fishery.
Krill are a delicious, nutritious food source. The largest animals that have ever lived on our planet are blue whales, and the largest population of these giants used to feed exclusively in the Southern Ocean on one single food item: Antarctic krill. The population of blue whales was estimated at over 100,000 individuals before commercial whaling reduced their numbers by 99 percent in the early 1900s.
Blue whales feed in Antarctic waters for only six months of the year; the other six months they migrate thousands of kilometers north to breed, during which time it is believed that they do not feed at all. How do they manage this? The answer must surely be in the abundance of krill and its nutritional value. Other baleen whale species, such as fin whales (the second-largest animal to have lived on Earth), humpback whales, and uncounted numbers of minke whales, all follow the same pattern of feeding in Antarctic waters and migration northward in winter.
In addition, krill are a prized food source for tens of millions of penguins, flying seabirds, seals, fish, and squid. How could this ecosystem, this single krill species, have supported such a massive level of predation?
Antarctic krill have been the target of the largest fishery in the Southern Ocean for the past 40 years. Few people are aware that there are several active fisheries operating in the Antarctic region. The largest of these fisheries is for krill. After Antarctic seals and whales had been hunted to near extinction, the fishing industry turned its attention to krill. But after 40 years of development, the industry still struggles to produce marketable krill products that justify the massive expense of fishing in the Southern Ocean. The fishery’s slow development has allowed conservation efforts to expand, but the enigmatic nature of krill makes even their conservation difficult.
We know that krill are ecologically and economically important. We also acknowledge that they are fascinating, beautiful creatures in their own right. So why have they been misunderstood and misrepresented for so long? Living at the far end of the world under a layer of seawater and ice proved a reasonable strategy for krill to keep a low profile for several millennia. However, once humans found the whale-rich Southern Ocean they began rendering it considerably less whale-rich, and they also began noticing the rather shy but utterly essential food of the giants — krill.
We humans have a tendency to see a resource and immediately assess whether we could use it to allow us to access a higher standard of living. This happened to krill in the 1960s, and since then considerable effort has been expended on catching krill and trying to commercially exploit krill as a resource, but this exercise has not been particularly successful, and as a species it has remained off the wider public’s radar — until now.
Krill are at a crossroads. As the Southern Ocean changes and opens to more human exploration and use, krill are once more under scrutiny for their economic possibilities. Krill oil is now widely available in drug stores and supermarkets, and questions are being asked about the nature of krill and the sustainability of the fishery.
This is an adapted excerpt from The Curious Life of Krill by Stephen Nicol. Copyright @ 2018 by the author. Reproduced by permission of Island Press, Washington, D.C.