Nekton is the grouping of living organisms that live in the water column of the ocean and freshwater lakes

Oceans 11 – Dalesandro

Notes – The Mesopelagic Zone
-This zone extends from the bottom of the epipelagic zone (200 m deep) down

to the edge of the bathypelagic zone (1000 m deep).

• 
  The mesopelagic is also called the twilight zone.
  

-Since photosynthesis is impossible here, marine snow is an important source of

food in the mesopelagic.

• 
  Only about 20% of the food created in the epipelagic zone makes it down this far, so many creatures have to spend some of their time in the epipelagic zone to feed.
  

• 
  Most mesopelagic creatures who rise into the epipelagic for food do so at night, to avoid predators.
  

-Organisms living in this zone have to deal with increasing water pressures and

much cooler water temperatures.
-Most species also have adaptations to deal with the lack of light, such as very

large eyes.

• 
  Many mesopelagic animals possess bioluminescence. This means they can produce their own light.
  

• 
  Bioluminescent organs inside theses organisms are called photophores.
  

• 
  Some fish, shrimp and squid have photophores on their undersides that match the surface brightness. They can reduce the light during cloudy days, and boost it when it is sunny. The photophores may be turned off at night to make the animal almost invisible.
  

• 
  Bioluminescence can also be used to attract a mate or to stun a predator while the light-producing animal makes a getaway.
  

-Fish who call the mesopelagic zone home include hatchetfish, lanternfish,

bristlemouths, coelacanth, and anglerfish.

• 
  They are joined by species of krill, jellyfish, octopus, squid, worms, and shrimp.
  

-In general, the mesopelagic zone has much less life than the epipelagic and is very

empty by comparison.

Mesopelagic Zone – Organism Fact Sheet 13 – Skipjack Tuna


-Skipjack tuna grow up

to a length of 1m and

they can weigh up to

18kg. Similar to other

tunas, skipjack tuna has

a spindle-shaped body.

-Oddly, the skipjack has

very few scales. Some

specimens are almost

scaleless.

-They are a long-lived ish, with a life span of about 12 years.

- Skipjack tuna are a medium-sized fish species, bluish with black stripes. They

inhabit the entire World Ocean, from the arctic to the tropics.

-Skipjack are a very important food species for humans. Each year, human

fisheries take over 100 000 tons of skipjack for canning or eating fresh (cooked

and as sushi).
-These tuna eat smaller fish, crustaceans (crabs, shrimp), and some molluscs

(snails, whelks, etc).

-Skipjacks breed incredibly fast. Despite the fact that humans take huge amounts

of them every year, their current population is stable, not declining.

-Predators who eat skipjack tuna include yellowtail tuna, billfish, and sharks.
-Skipjacks are very cannibalistic. They will readily attack and eat smaller

members of their own species.

Recreational fishing of skipjack. Human fisheries take thousands of tons of

skipjack each year. Luckily, they reproduce

very quickly.