What? Krill are not shrimp?

Antarctic krill (scientific name: Euphausia superba), also known as giant krill or Antarctic krill, is 4-6 cm long and weighs about 2 grams. Its body is relatively transparent with reddish-brown spots. It lives in the relatively cold waters around Antarctica [1].

Don't be fooled by its name, Antarctic krill is not a real shrimp, but a shrimp-like zooplankton. So what are the differences between Antarctic krill and our common shrimp?

1. From a taxonomic perspective

Antarctic krill and all other krill genera belong to the phylum Arthropoda, class Malacora, order Euphausiaceae, while the shrimps we often talk about, such as prawns and lobsters, belong to the phylum Arthropoda, class Malacora, order Decapoda. They can be said to be in the same class but different orders , one is the order Euphausiaceae and the other is the order Decapoda[2].

2. From the perspective of body structure

Shrimp have 8 pairs of thoracic limbs, including 3 pairs of maxillopods in front and 5 pairs of walking legs. The maxillopods are located next to the mouth and can help with feeding. 5 pairs of walking legs and 10 legs are also the main characteristics of the decapods. Krill, on the other hand, have 19 pairs of appendages, and their thoracic limbs and abdominal limbs have feather-like bristles, which give them a strong swimming ability. However, their legs do not form maxillopods , which is different from the decapods [2].

And unlike shrimps whose gills are covered by carapace, the gills of Antarctic krill are almost visible to the naked eye.

Picture 1 Shrimp | hepgun.com; Picture 2 Krill | https://dbpost.com/

What is most special is that Antarctic krill have spherical light organs at the base of the eyestalk, on both sides of the head and thorax, and on the underside of the abdomen.

When stimulated by external factors, they can emit cold blue phosphorescence like fireflies, which is the origin of the name “krill” [1].

Antarctic krill have spherical light organs | futura-sciences.com

Krill like to move in groups, and high density of krill can sometimes cause the sea water to change color. [1]

Swarms of Krill | Frozen Planet

References:

[1] Magnificent Polar Regions, edited by Zhao Jinping, Ocean University of China Press

[2] Blue Animal Bio http://www.blueanimalbio.com/