Northern krill

File:Meganyctiphanes_norvegica_162191972.pngs and two antenna]]

File:Meganyctiphanes_norvegica_162191940.pngous exoskeleton.]]

Similar to all krill species, northern krill are crustaceans that possess a transparent chitinous exoskeleton. Northern krill can grow up to 4-6 centimeters long in adult hood, although on average they are 1.5–2 centimeters.{{Cite journal |last1=Silva |first1=Teresa |last2=Gislason |first2=Astthor |last3=Astthorsson |first3=Olafur S. |last4=Marteinsdóttir |first4=Gudrún |date=2017 |title=Distribution, maturity and population structure of Meganyctiphanes norvegica and Thysanoessa inermis around Iceland in spring |journal=PLOS ONE |volume=12 |issue=11 |pages=e0187360 |doi=10.1371/journal.pone.0187360 |doi-access=free |issn=1932-6203 |pmc=5675434 |pmid=29112989|bibcode=2017PLoSO..1287360S }} Their anatomy is a decapod consisting of three main sections: the cephalothorax, the abdomen, and the tail fan. The cephalothorax is the krill's head which houses two antenna and compound eyes. Northern krill utilize their antenna to sense their surroundings and aiding in eating feed. In addition to the northern krill's antenna they also have maxillipeds which aid them in handling food to their mandible.{{Citation |last=Schmidt |first=Katrin |title=Food and Feeding in Northern Krill (Meganyctiphanes norvegica Sars) |date=2010-01-01 |series=Advances in Marine Biology |volume=57 |pages=127–171 |editor-last=Tarling |editor-first=Geraint A. |url=https://www.sciencedirect.com/science/article/abs/pii/B9780123813084000054 |access-date=2025-03-02 |publisher=Academic Press|doi=10.1016/B978-0-12-381308-4.00005-4 |pmid=20955891 |isbn=978-0-12-381308-4 |url-access=subscription }} Found in insects, krill also operate through compound eyes which allows them to see a large array of area, detect rapid movement, and see in deep sea light conditions.{{Cite journal |last1=Tarling |first1=Geraint A. |last2=Ensor |first2=Natalie S. |last3=Fregin |first3=Torsten |last4=Goodall-Copestake |first4=William P. |last5=Fretwell |first5=Peter |date=2010 |title=An introduction to the biology of Northern krill (Meganyctiphanes norvegica Sars) |url=https://pubmed.ncbi.nlm.nih.gov/20955887/ |journal=Advances in Marine Biology |volume=57 |pages=1–40 |doi=10.1016/B978-0-12-381308-4.00001-7 |issn=0065-2881 |pmid=20955887|isbn=978-0-12-381308-4 }} Northern krill also possess mandibles and maxillipeds which aid them in eating

Attached to the cephalothorax is the abdomen which contains ten appendages to aid with swimming. These appendages are five pairs of forked non walking legs referred to as pleopods or "swimmerets". Unique to a standard decapod krill use their pleopods for more than just swimming incorporating them into how they groom and eat. Their tail fan assists the northern krill to propel forward when swimming.{{Cite journal |last=Fregin |first=Torsten |date=2010-01-01 |title=An Introduction to the Biology of Northern Krill ( Meganyctiphanes norvegica Sars) |url=https://www.academia.edu/10765201 |journal=Advances in Marine Biology - ADVAN MAR BIOL|series=Advances in Marine Biology |volume=57 |pages=1–40 |doi=10.1016/B978-0-12-381308-4.00001-7 |pmid=20955887 |isbn=978-0-12-381308-4 }}

Northern krill are adapted for survival in the pelagic zone of the ocean, enabling them to feed, escape predators, and reproduce.{{Cite web |last=University |first=Uppsala |title=Krill provide insights into how marine species can adapt to warmer waters |url=https://phys.org/news/2024-08-krill-insights-marine-species-warmer.html |access-date=2025-03-02 |website=phys.org |language=en}}

The northern krill's digestive system starts at their maxillipeds which files food into their mouths which leads to the esophagus. The esophagus is connected to their stomach which consists of a cardiac stomach and a pyloric stomach. Their cardiac stomach is the initial part of the stomach which helps grind the krill's food. The pyloric stomach is located in the lower stomach region which directs movement of their food into the small intestine. The northern krill's pyloric stomach allows any bacteria to be passed right through the stomach due to the fast turnover rate. The food moves through the intestines located through the abdomen, leading to the anus to discrete the food particles.

The northern krill's circulatory system is open. An open circulatory system is where the krill's form of blood hemolymph soaks the organs directly. The tissue fluid is mixed with the hemolymph. Northern krill have a small heart located on the back or dorsal side. This dorsal heart pumps the blood through the krill's arteries.{{Cite journal |last1=Benkort |first1=Déborah |last2=Lavoie |first2=Diane |last3=Plourde |first3=Stéphane |last4=Dufresne |first4=Christiane |last5=Maps |first5=Frédéric |date=2020-03-01 |title=Arctic and Nordic krill circuits of production revealed by the interactions between their physiology, swimming behaviour and circulation |url=https://www.sciencedirect.com/science/article/abs/pii/S0079661120300082 |journal=Progress in Oceanography |volume=182 |pages=102270 |doi=10.1016/j.pocean.2020.102270 |bibcode=2020PrOce.18202270B |issn=0079-6611|url-access=subscription }}

Northern krill do not breathe oxygen from the atmosphere they breathe oxygen present in water. Krill use their gills located on their pleopod appendages attached to their abdomen to exhale and respire. Once krill breathe in the oxygen can be seen circulating throughout their body via respiratory pigment. Respiratory pigment is the term used when referring to a protein that transports oxygen into the blood and tissues.{{Cite web |title=Respiratory pigment {{!}} biochemistry {{!}} Britannica |url=https://www.britannica.com/science/respiratory-pigment |access-date=2025-03-03 |website=www.britannica.com |language=en}} In northern krill, the respiratory pigment is called hemocyanin.{{Cite web |title=Hemocyanin {{!}} biochemistry {{!}} Britannica |url=https://www.britannica.com/science/hemocyanin |access-date=2025-03-03 |website=www.britannica.com |language=en}} Unlike hemoglobin, hemocyanin is susceptible to change due to temperature and has a weak affinity for oxygen. Due to their low affinity for oxygen, krill's respiration rate changes in response to temperature changes. This makes northern krill more susceptible to death with fluctuations in temperature and oxygenation levels.{{Cite journal |last1=Michael |first1=Katharina |last2=Suberg |first2=Lavinia A. |last3=Wessels |first3=Wiebke |last4=Kawaguchi |first4=So |last5=Meyer |first5=Bettina |date=2021-06-01 |title=Facing Southern Ocean warming: Temperature effects on whole animal performance of Antarctic krill (Euphausia superba) |url=https://www.sciencedirect.com/science/article/abs/pii/S0944200621000180 |journal=Zoology |volume=146 |pages=125910 |doi=10.1016/j.zool.2021.125910 |pmid=33735797 |bibcode=2021Zool..14625910M |issn=0944-2006}}

Not only do northern krill possess a transparent chitinous exoskeleton, but they also have bioluminescent organs. Found in many fish these bioluminescent organs otherwise known as photophores are hypothesized to help the krill camouflage and communicate. Photophores are organs that emit light. On krill, they are located on the eyestalks, ventral thorax, and abdomen. They are often attributed to aiding in luring prey and male intrasexual competition. Their transparent body and bioluminescent organs help northern krill evade predators.{{Cite journal |last1=Krieger |first1=Jakob |last2=Fregin |first2=Torsten |last3=Harzsch |first3=Steffen |date=November 2024 |title=The eyestalk photophore of Northern krill Meganyctiphanes norvegica (M. Sars) (Euphausiacea) re-investigated: Innervation by specialized ommatidia of the compound eye |url=https://pubmed.ncbi.nlm.nih.gov/39426244/ |journal=Arthropod Structure & Development |volume=83 |pages=101393 |doi=10.1016/j.asd.2024.101393 |issn=1873-5495 |pmid=39426244|bibcode=2024ArtSD..8301393K |doi-access=free }} These light-emitting organs work by being catalyzed by an enzyme luciferase. Krill can adjust and control the photophore to a certain extent.{{Cite web |title=Bioluminescence |url=https://education.nationalgeographic.org/resource/bioluminescence/ |access-date=2025-03-06 |website=education.nationalgeographic.org |language=en}}

The northern krill's nervous system controls its photophore organs and their compound eyes. The photophore organs are controlled by hormonal and neuron pathways. Hormones such as serotonin and nitric oxide manage the bioluminescence in their organs. Photophore operation is controlled by the northern krill's compound eyes by the eyes ommatidia. Ommatidia are the cells that make up their compound eyes.{{Cite journal |last=Wulff |first=Verner J. |date=2021 |title=Photoreception |url=https://www.accessscience.com/content/article/a511400 |language=en |doi=10.1036/1097-8542.511400|url-access=subscription }} Part of the ommatidia's job is to send nerve signals to the photophore located on the eyestalks. These axon sending nerve signals allow the krill to control the light from the photophore.{{Cite journal |last1=Krönström |first1=Jenny |last2=Dupont |first2=Samuel |last3=Mallefet |first3=Jerome |last4=Thorndyke |first4=Michael |last5=Holmgren |first5=Susanne |date=2007-09-15 |title=Serotonin and nitric oxide interaction in the control of bioluminescence in northern krill, Meganyctiphanes norvegica (M. Sars) |url=https://journals.biologists.com/jeb/article/210/18/3179/17031/Serotonin-and-nitric-oxide-interaction-in-the |journal=Journal of Experimental Biology |volume=210 |issue=18 |pages=3179–3187 |doi=10.1242/jeb.002394 |pmid=17766295 |bibcode=2007JExpB.210.3179K |issn=0022-0949|url-access=subscription }}

Another essential part of the northern krill's nervous system is the brain. While small it is still majorly important in processing sensory information and coordinating behavior. To control the northern krill's swimming and food behaviors their appendages such as their swimmerets are controlled by a chain of ganglia which make up a ventral nerve cord. This nerve cord controls the motor functions of the krill and their appendages.

Northern krill reproduce seasonally, their eggs hatching during the spring and summer months. Reproduction tends to mirror the krill's access to food. During spring and summer months their food source phytoplankton bloom and provide nutrition for their hatched larvae. Northern krill have sexual dimorphism, meaning that males and females have different reproductive structures.{{Citation |last=Cuzin-Roudy |first=Janine |title=Reproduction in Northern Krill (Meganyctiphanes norvegica Sars) |date=2010-01-01 |series=Advances in Marine Biology |volume=57 |pages=199–230 |editor-last=Tarling |editor-first=Geraint A. |url=https://www.sciencedirect.com/science/article/abs/pii/B9780123813084000078 |access-date=2025-03-10 |publisher=Academic Press|doi=10.1016/B978-0-12-381308-4.00007-8 |pmid=20955893 |isbn=978-0-12-381308-4 |url-access=subscription }} Females are usually smaller and have a chamber where fertilized eggs are stored and released, while males have modified pleopods specialized to transfer sperm to the female. With these appendages, males transfer packets of sperm (spermatophores). Up to 10,000 eggs can be oviparous at once. Females latch onto their eggs externally before releasing them near the surface. The eggs sink before hatching into swimming nauplius larvae.{{Cite web |title=Krill - Behavior, Diet, and Life Cycle |url=https://marinesanctuary.org/blog/krill-behavior-diet-lifecycle/#:~:text=Female%20krill%20can%20lay%20as,10-day%20period%20before%20hatching. |access-date=2025-03-10 |website=National Marine Sanctuary Foundation |language=en-US}}

{{Reflist}}

• {{cite web |url=http://nlbif.eti.uva.nl/bis/crustacea.php?menuentry=soorten&id=8 |title=Meganyctiphanes norvegica (M. Sars, 1857) |work=Macrobenthos of the North Sea: Crustacea |publisher=Universiteit van Amsterdam}}
• {{cite web |url=http://species-identification.org/species.php?species_group=euphausiids&id=47 |title=Meganyctiphanes norvegica (M. Sars, 1857) |work=Euphausiids of the World |publisher=Marine Species Identification Portal}}
• {{cite web |url=http://www.arcodiv.org/watercolumn/euphausiid/Meganyctiphanes_norvegica.html |title=Meganyctiphanes norvegica (M. Sars, 1857) |work=Arctic Ocean Diversity |author=Alexei Pinchuk & Russ Hopcroft |date=September 3, 2010}}
• {{Commons category-inline|Meganyctiphanes norvegica|Meganyctiphanes norvegica}}

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{{DEFAULTSORT:Northern Krill}}

Category:Krill

Category:Crustaceans of the Atlantic Ocean

Category:Crustaceans described in 1856