The Mesopelagic Zone
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
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.
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
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.
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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