# Northern krill
## Northern Krill

Northern krill (Meganyctiphanes norvegica) is a species of krill that lives in the North Atlantic Ocean including the Norwegian Sea, North Sea, and parts of the Mediterranean. It is an important component of the zooplankton, providing food for whales, seals, fish, and birds. In the Southern Ocean, Antarctic krill Euphausia superba fills a similar role. M. norvegica is the only species recognized in the genus Meganyctiphanes.
Northern krill supply a crucial link in the North Atlantic Ocean food web, transmitting energy from primary producers to high-level predators. Krill in the North Atlantic Oceans feed on phytoplankton, copepods, and other zooplankton.

## Anatomy and morphology
### External Morphology

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. 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. 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. 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.

### Internal Anatomy

Northern krill are adapted for survival in the pelagic zone of the ocean, enabling them to feed, escape predators, and reproduce.

#### Digestive System

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.

#### Circulatory System

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.

#### Respiratory System

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. In humans respiratory pigment is hemoglobin. In northern krill, the respiratory pigment is called hemocyanin. 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.

#### Photophores

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 intrasexual competition. Their transparent body and bioluminescent organs help northern krill evade predators. These light-emitting organs work by being catalyzed by an enzyme luciferase. Krill can adjust and control the photophore to a certain extent.

#### Nervous System

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. 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.
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.

## Reproduction
### Reproduction

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. 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.