The Whalefishes
Have you wondered how does a deep sea creature deals with the scarce availablity of food down there? No? Then the Whalefishes that live in some of the deepest waters has a intriguing answer for you!
The ocean depths are divided roughly into 5 - Epipelagic, Mesopelagic, Bathypalagic, Abyssopelagic and Hadopelagic zone. The whalefish resides in the Bathypalagic or Midnight zone, which is the largest zone being 1000 to 4000m deep.
Don't get misled by the name! A Whalefish is nowhere near the size of a whale. Infact it's rather small fish being max to max 40cm long. Then why the name? It is given so because of its superficial morphological (body structure) similarities with (Baleen) Whales.
Whalefishes were discovered in 1895 by 2 Smithsonian institute scientists. Over the years, the researchers found out that scales, pelvic fin and swim bladders were absent in them.Above all, the strangest thing they found out was, that all of the specimens they found were females.
In 1956, a group of scientists discovered a fish called as 'Tapetails'. Now these tapetails, unlike whalefishes were found near the ocean surface feeding on the copepods. The name tapetail is because of its tapering streamerlike tail.
These fishes had a up-turned mouth with which they gorged onto the smaller shellfishes to fullfill thier nutritional needs. The absurd thing they found was, that all these tapetails were in larval stage and were only a few inches long.
Another such wierd sea fish they found was the 'Bignose fish', this fish was covered with scales and had a large large nose like bulge on its snout with highly sensitive sense of small.
They had a immovable jaw and were devoid of a stomach, gut and even a oesophagus. In the digestive system all they had was a absurdly large liver. Again,all the specimens of this fish ever found were all males!
In 2003 a group of Japanese scientists after studying DNA samples of Tapetails and Whalefishes found out that, even if they looked completely different, thier genetic material was almost identical in one specific gene!
Furthermore, some specimens of tailfish were found to be in process of becoming big nose fish. Unbelievable but true, these 3 completely different looking fishes are one and the same fish in 3 different forms - male, female and larva (Bignose fish, whalefish and Tapetails)
How does this happen?
'Exteme levels of Sexual dimorphism'. Not just fishes and for the fact even humans exhibit sexual dimorphism. It is the change in morphology or structure of body because of belonging to that particular sex (male or female) in the same species.
Levels of sexual dimorphism changes from species to species but are mostly minute differences.
On the other hand, in Flabby whalefish ('Gyrinomimus grahami'), we get to see that in such a huge extent, one big reason why the fish might do this could be the need to cope up with low availablity of food in the deep waters.
How does the fish live >2000m deep but have larvae at the surface?
Larva is a premature stage and needs loads of nutrition to mature. In the deep, as the larvae would never have enough nutrition many deep sea fishes have 'pelagic larvae'.
These larvae migrate from thier original habitat to regions where they would get more food and safety from predators. In this case, the Whalefish larvae migrate to surface to feed on copepods.
How does a larva mature?
Male Whalefish (Bignose fish)
During larval stage the fish eats crustaceans and gives nutrition to its liver to grow in size. After a point, the fish's upper jaw bone starts to fuse with its nasal bone.
As soon as this process begins or just before this (we're not sure yet), the fish enters a frenzy eating phase when they eat as many as shelled creatures as possible, along with thier shells.
After the bones completely fuse, the fish practically has its mouth closed and stops eating altogether until it dies. Even if it could open the jaw,the space would be so tiny that it won't be able to take in any food.
A fully grown adult male Whalefish are tiny animals. They only grow upto 3-4 cms long.
While the upper jaw fuses, on the inside, its stomach, oesophagus, gut etc also shrivel and disappear only leaving behind a large sized liver. This liver then remains the only source of food for the fish, it utilizes the previously eaten shells to form food tissues.
This nutrition is then used by the fish in its activities, but most importantly in providing energy to sperms.(method of copulation and reproduction are still unknown)
It grows a strong sense of smell which is useful for it to find a female to mate. But, if the liver runs out of the food tissue,the fish most probably starves to death.
Female Whalefish:
Unlike the males, female whalefishes are bightly coloured and are quite large, they grow upto 1.3 feet! The name whalefish originally is derived from the female whalefish because the juvenile stage of this fish grows to attain whale like shape of the body.
In contrast to what happens with males, female Whalefish's jaw-size increases in great proportions and widens as well. This enables it to open the mouth wider so as to consume a prey that's larger than its own body!
To be able to house a prey this big, the female whalefishes has a very distensible and large stomach. Although just like males, they also don't have spines in thier fins, swim bladder and functional eyes (vestigial and very small).
Research suggests that, the fish might have lost eyesight over years because of the extreme darkness in the depth where having eyesight isn't much of a help. To cope with this, whalefishes have a 'lateral line'
Lateral line is a line of segmented sensory pores running laterally along the fish's body. This helps it in identifying even the most minute changes occurring in its environment.
How is sexual dimorphism related to coping up with less food availablity?
These changes we see from sexual dimorphism could be interpreted by one as modifications cause by evolution in order to adjust with the amount of food availablity.
The males can't consume food after the jaw seals, hence they don't grow in size to minimise energy loss. For the same, they even lose most of thier internal organs of the digestive system.
In females, to keep up with energy needs of thier relatively large body sizes, they have a wider jaw and large distensible stomach that can hold more food for sustainability.
Over last 34 years, only 18 whalefishes have been spotted. Although pretty amazingly, just a few weeks ago, the researchers spotted a female whalefish 2,013m deep, off the Monterey Bay, California, USA. Atleast for me, This is one of the most clearest whalefish videos until now! :-
The female whalefishes are bright red or orange coloured. But it is deceptive to think that they'll be easy to find. In 2000m deep water, the light with longer wavelength cannot pentrate, which also means that these lights aren't visible here.
Red, Orange and such lights come under these.
Hence, as whalefish are red or orange, they camouflage in this environment and are almost invisible to the human eye. It is an astonishing fact that a fish that's so brightly coloured is invisible in the deep water!
It is often very intriguing to know about the modifications deep sea fishes exhibit for survival in such harsh living conditions. This species of whalefish was one such example, and this is still when we know only a few things about the fish!