Krill have the same basic body plan as other crustaceans such as lobsters or shrimp. Their elongated cephalothorax bears up to 13 pairs of limbs, 6-8 of which form a net-like structure, with bristles adapted for sieving food from the water. An additional five pairs of paddle-like limbs called swim-merets or pleopods, used for propelling the krill through the water, are found on the segmented abdomen and tail. Unlike more advanced crustaceans, however, euphausiids have exposed gills, which lie below the carapace.
Krill have two pairs of antennae, prominent compound eyes, and transparent skin with red spots of pigment. Mycosporine-like amino acids in their tissues, derived from algae they consume, absorb UV light and help to prevent sun damage. The gut is visible through the skin and may appear variously colored depending on their diet. Krill species range from less than 0.5 in (1.25 cm) to several inches (centimeters) in length.
Krill are sometimes called "light-shrimp," a name deriving from bioluminescent organs on their eyestalks and body, called photophores, that produce a yellow-green or blue light. These may be used for mating displays or to confuse predators. The bioluminescent protein, luciferin, is believed to be obtained through consumption of dinoflagellates.
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