The Mesopelagic Zone

The mesopelagic zone is sometimes called the "twilight zone" of the ocean. This area is bordered by the photic zone above, and the darkness below. It's in this zone where you start to see bioluminescence on all sorts of animals. From this point on down, food becomes something of a scarcity and some animals migrate up to the surface at night to feed. The rest rely on food that falls down from above, as well as eating each other. Because sometimes the only things to eat may be bigger than the hunter, many animals have developed long sharp teeth, and expandable jaws and stomachs. If you scroll down, you'll see a few of the animals from these depths. 

This fish, for fairly obvious reasons, is called the bigscale. It's head is covered in bony plates, and it has very large scales on it's body. It's found from 200m - 2000m. Many fish at this depth lack the streamlined form found in surface fish because most deepwater fish are ambush predators and rely more on surprise or lures to catch prey than on prolonged chases. This helps conserve energy in a place where food is often scarce.

 

 

 

This animals is a ctenophore (the "c" is silent), and is related to jellyfish. Ctenophores are found from the surface to great depths. They swim by pulsing eight rows of tiny hairs (cilia), and capture prey with the sticky tentacles seen trailing behind the animal. They feed on small crustaceans and fish, and many emit bioluminescent flashes to deter predators. Their cilia, when illuminated, become iridescent bands.

 

Firefly squid are one example of the many kinds of squid and octopus that live in the open ocean. The firefly squid has taken bioluminescence to another level from the ctenophore. It has three kinds of photophores, or light generating organs. The first is found on the body surface, on the siphon and on the head, the second kind are around the eyes, and the third are on the tips of the third and fourth tentacles. Although people aren't sure of the exact reasons that different animals glow, there are lots of ideas. It can be used to frighten predators, communicate with and attract others of the same species, attract prey, or illuminate the darkness to see potential prey. One cool theory is that the photophores on the underside of the animals can mimic the exact hue of the light from above in more shallow water, and thus hides the outline of the animals from view from below. This is like the way surface fish are light on the bottom and dark on top.

 

The hatchet fish is another example of a bio-luminescent animal. They have photophores under their eyes, and along the underside of their bodies. You can see them on this picture along the bottom quarter of the animal. These photophores are probably used for counter-illumination, as mentioned above. The eyes of some species point upwards, possibly to see prey that are silhouetted against the lighter water above. Hatchet fish are found from 200m to 1500m and eat small fish and crustaceans. These fish are only a few inches long.

 

These two monsters are examples of the adaptations that active hunters have undergone in the deep sea. The fish on top is the viperfish. You can easily see the large eyes, huge teeth, and wide jaw. When the jaw is open, some of the teeth point outward, so that prey is speared if it doesn't end up right in the hunter's mouth. Viperfish have a specially adapted hinged skull which is why their jaws can open so wide. They have a long dorsal fin with a lure-like photophore on the end, and have been seen hanging motionless in the water with the lure in front of their mouths waiting for prey. This way they don't have to actively hunt, and can conserve precious energy. Although they are often thought of as deep sea fish, they have been found at depths of only 80m, presumable coming up to feed at night.

 

This dragonfish shares many of the characteristics of the viperfish, namely sharp long teeth, large eyes, and a huge mouth. Both fish have very large stomachs to holds the big meals they are capable of eating. Both fish are also generally darker in colour than these specimens. One problem with bringing up fish from the deep is the damage that they have to endure from the trawls. In this picture, you can see the photophore behind the eye as a silvery circle. This placement is generally thought to help vision, and possibly to attract mates. Dragonfish are found at depths of 30m to 600m.

 

Snipe eels are another pretty crazy looking fish. When they reach their full length of up to 1.2m, they resemble a long piece of ribbon or string with a bulbous little head on the end. A curious thing about this fish is that their upper and lower jaws curve away from each other at the tips. The reason for this is that when they hunt their favorite prey, shrimp, the antennae can slip between the fish's jaws and are caught by the back-facing teeth of the eel. The meal is then worked inwards towards the mouth, and eaten.

 

Siphonophores are colonies of animals related to jellyfish, each one of which is specialized to fulfill a particular function. There are individuals that are specialists at capturing food, at digesting food, and occasionally at forming a gas filled float. The best known sipohnophore is the Portuguese Man-of War. Siphonophores with the structure of the one on the left may grow up to 10m long, and catch their prey with paralyzing stinging cells.

 


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