Deep sea extends to about one half of the entire globe and includes all water below depth of 200 meters which is the limit of effective sunlight and green plant life. It is divisible into two regions; from a depth of 200 to 1000 meters it is known as archibenthal and below 1000 meters as abyssal benthal.
The characteristics of the deep sea are intense cold, utter absence of movement, profound darkness and enormous pressure. These characteristics gradually increase with depth and hence, there is naturally a transitional zone.
The deep sea forms are adapted migrants from the pelagic realm and show convergent evolution as follows:
I fragile and Weak Body
ADVERTISEMENTS:
The animals are fragile and weak having this and feebly flesh and .with very little lime in their bones that is why the skeletons are weak and fragile They possess relatively great jaws and cruel mouth ornaments eg. Silver sharks (Gastrostomus). The other examples are glass sponges *(HexactineUida), long legged crab (Kacmpfferia), fishes (chimaera) etc.
2. Colouration
It is more or less uniform. As a rule colours are simple, and common being brown or gray. In Macrurus the belly is black and the top is silvery.
3. Tectile Organs
ADVERTISEMENTS:
Long feelers, slender attenuations of the fins are present.
4. Photoreceptors
In archibenthos, which is a transitional zone the eyes are enlarged e.g., in Pandalus, Cyrtisome longipinnis and Regalecus (fish).
Next below this zone (dimlight the eyes though telescopic show enlargement of the lens; examples being the fished Argyropelecus and Gignatura. Here the retina of the eye possesses specialized rods and no cones. In abyssal benthos, eyeless animals and those with reduced eyes are numerous. Examples of eyeless forms are Eulima and Ipnops.
ADVERTISEMENTS:
5. Presence of Luminescent Organs
It is common among the curstaceans, cephalopods and fishes. Luminescent organs are found either over the entire body or in the belly or localized on highly modified organs. Teeth and mouth may also be luminous. The function of bioluminiscence may be of attractions of prey in the benthic forms. It may be possible that it helps in sex differentation. Examples are Ipnops Oneirodes, Photostomias etc.
6. Loss of Power Mastication
The power of mastication is lost in many deep-sea fishes, e.g., Cetomimus.
ADVERTISEMENTS:
7. Parental Care
Parental care is shown by many deep-sea fishes while some others produce Youngs in large number.