A krill looks like a small, semi-transparent shrimp, typically between one and six centimeters long depending on the species, with a segmented body divided into a forward section covered by a shell-like carapace and a muscular, jointed abdomen ending in a fan-shaped tail. But that shrimp comparison only gets you so far. Krill carry features you won’t find on any shrimp at a seafood counter, including rows of glowing light organs, an intricate feeding basket made of hair-fine bristles, and a body plan tuned for constant swimming in open water.
The Basic Body Plan
Like shrimps and lobsters, a krill’s body is built around two main regions: the cephalothorax and the abdomen. The cephalothorax is a fused head-and-chest section protected by a thin shell called the carapace, which extends roughly one third of the animal’s total length. Behind that sits the abdomen, made up of six distinct segments, each enclosed in its own ring of exoskeleton. The abdomen ends in a pointed structure called the telson, flanked on either side by two pairs of flat, paddle-like uropods that together form the tail fan.1Advances in Marine Biology. The Species of Krill That tail fan is one of the most visually distinctive features when you see a krill up close: it spreads wide like a Japanese hand fan, and the animal can snap it shut against the abdomen in a fraction of a second to rocket backward.
Perched on the front of the cephalothorax are two large, dark compound eyes on short stalks. These are proportionally bigger than you might expect for such a small animal, giving the head a slightly bug-eyed look. Trailing behind the eyes are two pairs of antennae, the longer pair often extending well beyond the body. The overall silhouette is elongated and slightly curved, with the abdomen curling gently downward so the animal looks a bit like a comma viewed from the side.
How Big Are They
Most of the roughly 85 recognized krill species fall between one and two centimeters in adult length. The species people think of most often, Antarctic krill (Euphausia superba), is one of the larger ones, with adults commonly reaching about five to six centimeters, roughly the length of your little finger. The largest known species, Thysanopoda spinicaudata, can stretch to around 15 centimeters, though encountering one that size in the wild is rare. At the other end of the scale, some tropical species top out at barely a centimeter. Weight follows a similar pattern: an adult Antarctic krill weighs about one to two grams, roughly the mass of a paper clip.
Size varies not just between species but within a single species depending on age, season, and food supply. Antarctic krill can actually shrink during winter when food is scarce, reabsorbing body mass to survive. This ability to shrink and then regrow is unusual among crustaceans and means that body length alone is not always a reliable indicator of age.
Color and Transparency
At first glance, a living krill looks glassy and almost colorless. The exoskeleton is thin enough to be largely transparent, and in many species you can see internal organs through it, including the dark digestive gland (sometimes called the hepatopancreas) sitting behind the head and the greenish or brownish gut running along the abdomen. This transparency is an adaptation for life in open water, where there is nowhere to hide and being see-through helps avoid the notice of predators.
The transparency is not total, though. Many krill species have a rosy or reddish tinge, concentrated in spots along the body. That color comes largely from astaxanthin, the same pigment that turns cooked shrimp and salmon flesh pink-orange. The pigment sits in specialized cells in the exoskeleton, and its intensity varies with diet: krill that have been feeding heavily on certain algae tend to look more vividly red. When krill swarm in dense aggregations, their collective pinkish color can be visible from the surface, and it is the reason that the feces and stomach contents of krill-eating whales are famously pink.
The Feeding Basket
One of the most remarkable anatomical features of krill is invisible to the naked eye unless you know where to look. Underneath the cephalothorax, krill have six pairs of thoracic limbs (thoracopods) that form a complex three-dimensional structure called the feeding basket. These limbs are fringed with multiple tiers of increasingly fine bristles, creating a mesh filter that can capture particles as small as about two micrometers in diameter, all the way up to items over two millimeters across.2PubMed Central. The production of ‘food boluses’ by Antarctic krill and implications for organic matter transport To put two micrometers in perspective, that is smaller than a single red blood cell.
The filtering apparatus works at multiple scales. The primary bristles are spaced tens of micrometers apart, while secondary bristles (called setules) between them narrow the gaps further. Even finer microsetules thicken the boundary layer of water around each bristle, helping trap the smallest particles.3Canadian Journal of Fisheries and Aquatic Sciences. Morphological Study of Sieve Efficiencies and Mandibular Surfaces in the Antarctic Krill, Euphausia superba The result is a filter fine enough to strain individual diatoms and bacteria out of seawater, yet sturdy enough to grab larger phytoplankton and even small zooplankton. When a krill is actively feeding, the thoracopods sweep rhythmically, pushing water through the basket and trapping food against the bristles. The collected material is then pushed forward toward the mandibles, which crush and process it before swallowing.
This feeding basket also has an unexpected side effect: when krill eat, they sometimes produce compacted “food boluses,” clumps of captured material that can detach and sink, carrying organic matter toward the ocean floor. It is one of many ways that krill play a surprisingly outsized role in moving carbon through the ocean.
Glowing in the Dark
Krill are one of the few crustacean groups that produce their own light, and they do it with dedicated organs called photophores scattered across the body. A typical krill has ten photophores in total: one pair on the eyestalks, two pairs on the underside of the thorax, and four unpaired ones running along the ventral surface of the abdomen.4Arthropod Structure & Development. The eyestalk photophore of Northern krill Meganyctiphanes norvegica M. Sars (Euphausiacea) re-investigated: Innervation by specialized ommatidia of the compound eye Each photophore is a small, disc-shaped organ a fraction of a millimeter across, complete with its own reflector and lens structure, and it generates blue-green light through a chemical reaction involving molecules called luciferin and luciferase, the same type of chemistry that fireflies use.
What makes the photophores especially interesting is that eight of the ten sit on short muscular stalks and can be actively aimed. The krill rotates them to direct their glow away from any source of ambient light coming from above, which effectively makes the animal match the dim light filtering down through the water.5Journal of Experimental Biology. Eye/Photophore Coordination and Light-Following in Krill, Euphausia Superba This is a form of counter-illumination camouflage: a predator looking upward from below would otherwise see the krill’s silhouette as a dark spot against the lighter surface water. By shining light downward to match that background glow, the krill erases its own shadow. It is an elegant trick, and the photophores adjust their orientation in real time as the animal moves or as ambient light shifts.
The glow itself is visible to the human eye in darkened conditions. If you scoop krill out of the ocean at night and dim the lights, you can sometimes see pinpoints of cold blue-green light flickering along the body. It is one of the most striking things about live krill, and it is easy to see why early naturalists were fascinated by them. The order name Euphausiacea comes from the Greek for “shining brightly.”
Swimming Legs and Constant Motion
Below the abdomen, krill have five pairs of paddle-like swimming legs called pleopods. These are distinct from the thoracopods used for feeding: pleopods are broad, flat, and fringed with fine hairs, built for pushing water rather than filtering it. Krill swim by beating these five pairs in a coordinated wave that ripples from back to front, a pattern known as metachronal swimming.6Marine Biology. Hydrodynamics of active metachronal swimming modes in Antarctic krill, Euphausia superba If you watch a krill from below, the pleopods look like a line of tiny oars rowing in quick succession, each one beginning its stroke just as the one behind it is partway through.
This back-to-front wave is not just visually distinctive; it turns out to be hydrodynamically efficient. The sequential timing generates a smoother, more continuous thrust than if all five pairs beat in unison. Models comparing the two patterns show that the metachronal wave produces a higher average swimming speed for the same leg effort.7PubMed Central. Coordination of multiple appendages in drag-based swimming That efficiency matters because krill are denser than seawater and must swim continuously just to stay at their preferred depth. If a krill stops beating its pleopods, it sinks.8Limnology and Oceanography: Fluids and Environments. The hydrodynamics of hovering in Antarctic krill This makes krill fundamentally different from many small planktonic animals that can drift passively. A krill’s default state is active swimming, and the body reflects that, with a muscular abdomen making up the bulk of its mass.
The Tail Flip Escape
When a predator gets too close, krill abandon their normal forward swimming and execute a violent backward escape. The animal rapidly flexes its abdomen, snapping the tail fan shut against the body, which launches it backward and away from the threat. This maneuver is sometimes called a caridoid escape reaction, and it is shared with shrimps and other similar crustaceans. In Antarctic krill, a single tail flip can accelerate the animal to a peak speed of about 57 centimeters per second, which translates to roughly 17 body lengths per second, over a burst lasting just 42 milliseconds.9Nature Publishing Group. Hydrodynamics of the fast-start caridoid escape response in Antarctic krill, Euphausia superba
The tail fan’s shape is central to why this works. During the escape flex, the uropods and telson stay spread at maximum width for as long as possible, maximizing the surface area pushing against the water. Only after the main acceleration phase is complete does the tail fold in.9Nature Publishing Group. Hydrodynamics of the fast-start caridoid escape response in Antarctic krill, Euphausia superba The entire sequence is reflexive: it happens too fast for deliberate decision-making, driven by a rapid nerve signal that triggers simultaneous contraction of the abdominal muscles. If you have ever startled a krill in an aquarium, you have seen them vanish backward in a blur, sometimes several in rapid succession if they are in a group.
How Larval Krill Look Different
A krill does not hatch looking like a miniature adult. It goes through a series of larval stages that are so different in form that early researchers sometimes mistook them for separate species. The eggs are tiny spheres, typically less than a millimeter across, and they sink as they develop. The first stage to hatch is the nauplius, a pear-shaped larva with no visible segments and only three pairs of simple appendages. It does not eat; it survives on yolk reserves. Over the next several weeks, the nauplius molts through stages that gradually add segments, limbs, and complexity.
In Antarctic krill raised in laboratory conditions, the nauplius hatches within about eight days of the egg being laid and reaches the first calyptopis stage, which has a recognizable carapace and functional eyes, by around 30 days. From that point, growth through the remaining larval stages (multiple calyptopis and furcilia stages) takes roughly another 97 days, for a total development time from egg to the final larval form of about 127 days at near-freezing water temperatures.10Journal of Experimental Marine Biology and Ecology. Development of the larvae of the Antarctic krill (Euphausia superba Dana) observed in the laboratory During the furcilia stages, the larva progressively gains its full set of pleopods. Only once all five pairs are developed and functional does it look and swim like the adults described above.
The larvae are also far more transparent than adults, essentially invisible in water without magnification. As they grow and develop pigment cells and photophores, they gradually acquire the faint rosy tinge and the glowing organs that mark the adult form.
Exposed Gills and an Unusual Anatomy
One subtle but important way krill differ from true shrimp is in their gills. In most shrimp, the gills are enclosed inside the carapace, hidden from view. In krill, the carapace is shorter and does not fully cover the thorax, leaving the gills partially exposed. If you look at a live krill from the side, you can see feathery gill structures protruding from beneath the edges of the carapace. This exposed arrangement is one of the defining features of the order Euphausiacea and is one of the easiest ways to tell a krill apart from a shrimp if you are sorting through a trawl net.
The exposed gills constantly contact flowing seawater as the krill swims, which may aid gas exchange, but it also means the gills are more vulnerable to fouling or parasites. The tradeoff seems to work for an animal that spends its entire life in open, well-oxygenated water, but it would be a liability in the cluttered, low-oxygen environments that some shrimp inhabit.
Why Krill Look Like Shrimp but Are Not
The resemblance between krill and shrimp is strong enough that many people use the terms interchangeably, and in some languages the word for krill is simply “small shrimp.” But krill belong to an entirely separate order (Euphausiacea) from true shrimp (Decapoda). Beyond the exposed gills, there are other anatomical differences. Krill have fewer thoracic legs specialized for walking; their thoracopods are primarily adapted for filter feeding rather than manipulating food or walking along the bottom. Krill also lack the large pincers (chelae) that many shrimp species carry. And of course, the photophores are unique to krill among crustaceans of their size range.
Despite these differences, the overall body plan convergence is striking. Both groups have a carapace, a segmented abdomen, a tail fan, compound eyes on stalks, and two pairs of antennae. The similarities reflect a shared crustacean heritage and the physical demands of life in water, not a close family relationship. Think of it like the way dolphins and sharks share a streamlined shape despite being only distantly related: the environment rewards certain body plans, and evolution arrives at them independently.
What a Swarm Looks Like
Individually, krill are small and easy to overlook. Collectively, they form some of the densest animal aggregations on Earth. Antarctic krill swarms can stretch for kilometers and contain billions of individuals, turning the water a murky reddish-brown. From above the surface, a dense swarm looks like an underwater cloud or a dark patch in the ocean. From underwater, being inside a krill swarm has been described by divers as like swimming through a living snowstorm, with thousands of tiny translucent bodies darting in every direction, each one flickering its pleopods.
Within a swarm, individual krill tend to orient the same direction and maintain a remarkably uniform spacing from their neighbors, typically a few body lengths apart. The swarm moves as a coordinated unit, shifting depth with the time of day: deeper during daylight hours, rising toward the surface at night. This daily vertical migration is one of the largest mass movements of biomass on the planet, and it is driven by individual animals each no bigger than your thumbnail, beating their pleopods in that efficient metachronal wave, hour after hour, in water just above freezing.