Krillion Game

How deep do krill live?

Updated

Krill are tiny, shrimp-like crustaceans, and there are a staggering number of them. Our game’s name is a nod to that (“one in a krillion”), and our dives sink through the ocean’s depth zones. So it is fair to ask: how deep do real krill actually go?

The short answer: most live near the surface, but some have been seen thousands of metres down. Here is what the research says.

The short answer

  • Usually: the top 150 metres. According to the British Antarctic Survey (BAS), the bulk of adult Antarctic krill live in the upper 150 m of the ocean.
  • Every day: up and down. The Australian Antarctic Division (AAD) says krill usually feed at the surface at night and sink deeper during the day.
  • Eggs: down to about 1,000 metres. Antarctic krill eggs sink for about seven days and hatch around 1,000 m down.
  • Record: about 3,000 metres. In 2008, BAS reported krill feeding on the sea floor at around 3,000 m. Some were seen at 3,500 m.

What krill are

“Krill” is not one animal. The AAD describes the word as a general term for about 86 species of crustaceans found in the open ocean, all belonging to a group called euphausiids. They look like small prawns: mostly transparent, with large black eyes, and often with a green gut full of the microscopic plants they have eaten.

Five species live in the Southern Ocean around Antarctica. The dominant one, and the one most studied, is Antarctic krill (Euphausia superba). Adults grow to about 6 cm and weigh just over a gram. Sizes across the group range from tropical species under a centimetre long to one deep-sea, bottom-living species that can reach 14 cm.

Where they spend most of their time

Krill eat phytoplankton, the microscopic algae that need sunlight to grow. Phytoplankton live in the sunlight zone, the top 200 metres of the ocean where photosynthesis is possible. So it makes sense that most adult Antarctic krill stay within about 150 m of the surface. That is the top layer of the five ocean zones we cover in ocean zones by depth.

Within that layer they keep moving. The AAD notes that krill usually feed at the surface at night and sink deeper in the day. This daily vertical migration is common among small ocean animals. The Woods Hole Oceanographic Institution describes the nightly rise and morning descent of fish, squid and plankton as the largest animal migration on Earth. Feeding in the dark makes them harder for predators to see.

They also gather in extraordinary densities. The AAD records swarms with as many as 30,000 krill in a single cubic metre of water. Some are dense enough to colour the sea reddish-brown.

The 3,000-metre surprise

For a long time, Antarctic krill were thought of as surface animals. That changed in 2008. Scientists from BAS and the National Oceanography Centre, Southampton, were filming the deep sea floor in the waters around the Antarctic Peninsula with Isis, a remotely operated vehicle. They found krill living and feeding near the bottom, at depths down to around 3,000 metres.

The paper, published in Current Biology, reported that the krill were actively feeding: they nosedived into the soft sediment to pick up food from it. Some were seen as deep as 3,500 m. Among them were adult females that appeared ready to spawn. As Professor Andrew Clarke of BAS put it, the findings “revise significantly our understanding of the depth distribution and ecology of Antarctic krill”. BAS said the discovery “completely changes scientists’ understanding” of an animal that is a major food source for fish, squid, penguins, seals and whales.

On the ocean-zone scale, 3,000 m is deep in the midnight zone: completely dark, about 4°C, and under roughly 300 atmospheres of water pressure. For a sense of scale, it is close to the depth of the Titanic wreck (about 3,800 m).

Eggs that sink 1,000 metres

Krill’s link to the deep starts before they hatch. In summer, female Antarctic krill lay several broods of up to 10,000 eggs at a time, each about half a millimetre across. The AAD explains that the eggs sink from near the surface for seven days, down to about 1,000 metres, where they hatch.

The newly hatched larvae, called nauplii, then begin the long swim back up to the light. Young krill feed on algae growing on the underside of sea ice and change shape many times over several months before reaching adulthood.

How many krill are there?

Estimates vary, but all of them are huge. The AAD puts the total at about 400 million tonnes of Antarctic krill in the Southern Ocean. CCAMLR, the international commission that manages Antarctic fisheries, gives an estimate of about 379 million tonnes and notes that more than half of it is eaten each year by whales, seals, penguins, fish and squid.

Krill sit at the centre of the Antarctic food web. The AAD says most of the larger Antarctic animals depend on them, directly or indirectly.

How long krill live

Krill are long-lived for such small animals. The AAD says adults can live for more than five years in the open ocean, with lifespans estimated at five to ten years. CCAMLR reports that krill have lived up to eight years in aquaria, but probably live three to four years in the wild. They are also remarkably tough: the AAD notes they can survive around 200 days without food, shrinking in length as they starve.

Why we named a game after them

Krillion is a word game, not a biology lesson. The name plays on “one in a krillion”, the kind of answer almost nobody else would think of. Krill fit that idea: individually tiny, collectively astronomical. Our scoring borrows the ocean’s depth, so a rare answer takes your diver deeper, and the rarest earn the line “Practically one in a krillion.”

The ocean is fair game in the prompts too. In Name a sea, Mediterranean is a Surface answer while the Scotia Sea and Weddell Sea, both in krill country, score much deeper. In Name a bird that can’t fly, “Penguin” is Surface, but naming a krill-eating species such as the Adélie or gentoo penguin earns a Midnight Zone answer. Try a dive in Krillion Unlimited and see how deep you get.

Sources

Frequently asked questions

How deep do krill usually live?

Most adult Antarctic krill are found in the top 150 metres of the ocean, in the sunlight zone where their food grows. They tend to feed near the surface at night and sink deeper during the day.

What is the deepest krill have been found?

In 2008, British Antarctic Survey scientists reported Antarctic krill feeding on the sea floor at around 3,000 metres, with some seen as deep as 3,500 metres, using the deep-diving robot vehicle Isis.

Why do krill eggs sink so deep?

Antarctic krill eggs sink from near the surface for about seven days and hatch around 1,000 metres down. The larvae then swim back up to the surface to feed, a journey sometimes called the developmental ascent.

How many species of krill are there?

The Australian Antarctic Division says the word krill covers about 86 species of crustaceans called euphausiids. Five of them live in the Southern Ocean, and Antarctic krill (Euphausia superba) is the dominant one.

Why is the game called Krillion?

It is a play on 'one in a krillion': the rarest answers are the ones almost nobody else gives. The deep-sea theme comes from krill, tiny animals that live in astronomically large numbers.

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