26. Apr, 2017

Creatures from the abyss

The deep sea is the largest habitat on earth that remains largely unexplored. It has fascinated mankind since ancient times with its sea serpent legends, and brougth to live in 1870 in the vivid imagination of the French novelist Jules Verne. In his book Twenty Thousand Leagues Under the Seas Verne describes how the submarine of the brilliant and obsessed captain Nemo is attacked by a group of  giant squids.

Left: Giant amphipod. Right: Goblins shark jaw (Photo: Carl Moore/courtesy of NOAA.

Although Jules Verne was still unaware of the forces of water pressure, he was right about the enormous dimensions of some creatures that are lurking in the depths of the Oceans. He was of course also not aware of the existence of  smaller and very strange looking species. Fishes, molluscs and crabs that -although harmless to mankind-  would be described almost 150 years later as 'frightening'', ''nightmarish'' or  even  'terrifying' in the colourful jargon of some Internet sites.

The deeps  The area below the surface of the sea where sunlight can still penetrate is called epipelagic or photic zone. The deep sea begins below about  200 meters. From there to about 1000 meters, the mesopelagic or "twilight" zone, sunlight continues to decrease until it is gone altogether. This faint light is deep blue in color because all the other colors of light are absorbed at depth. The deepest ocean waters below 1,000 m (called bathypelagic) are as black as night as far as sunlight is concerned. Most of the deep sea floor consists of mud  consisting of very fine sediment particles and organic remains due to the accumulation of pelagic organisms that sink after they die.

The deepest point ever visited by man is the  Mariana trench. Here, in  the tradition  of captain Nemo, explorers Jacques Piccard and Don Walsh first reached a depth of 11.000 meters in their bathyscaphe Trieste on 23 January 1960.  On March 2012 a second manned solo descent was made by the deep-submergence vehicle Deepsea Challenger  to the the Challenger Deep in the trench.

The dark cold waters of much of the deep sea reach temperatures that fluctuate between about -1 to about + 4 Celsius. Cold water can contain more oxygen than warm water, and  since  oxygen-rich waters cool off so much it will become dense enough to sink to the bottom of the sea. Life in the deep sea must withstand total darkness, extreme cold, and great pressures. But despite the intense darkness of the seas  deep sea explorers descending  in their submersible vessels  are often mesmerized by all kinds of  floating, swirling, zooming flashes of light. These forms of  bioluminisence are  a chemical reaction of small glandular organs called photophores on the animals body. In some fish it is used to  lure prey.  For example the anglerfish and  viperfish are equipped with a long, thin modified dorsal fin on their heads tipped with a photophore lit with bioluminescence. The gulper eels (Eurypharynx pelecanoides) with their enormous beak  have a luminous organ at the end of its tail. Others like the lanternfish use forward-facing light organs as headlights, probably meant  to signal other species of their nature and presence.

Its inhabitants Some fishes have the deep seas as their regular habitat, while others species such as the Sperm whale and the Greenland shark only visit the great depths  occasionally. Fishes in the mesopelagic zones often have swim bladders and a tolerance for higher temperatures, allowing them to make vertical migrations to shallower depths during the day. An example is the giant oarfish (Regalecus glesne),  the longest bony fish alive, growing up to 11 m (36 ft) in length that  feeds on  krill and other small crustaceans, as well as small fish and squid. Sometimes its  dead body is found floating of the surface or stranded on a beach. Pictures of living deep sea creatures in general are very rare, most are taken taken from dead species. 

Many deep sea creatures of the deeper (bathypelagic) zones have strange even bizarre shapes that bear physical characteristics of ancestors who swam the seas in the time of the dinosaurs. The Blobfish (Psychrolutes marcidus) to start with, a gelatinous lump of whitish jelly without swim bladder often described as the ugliest fish in the world. Then we have the larger sharkish species: the Sixgill shark (Hexanchus griseus), the goblin shark (Mitsukurina owstoni: see picture above right)  and the  Camara,  or  ghost shark. Footages of the goblin shark with its jaw that shoots out of the head are very rare. Two other examples are  the Rhinochimaera, occasionally caught in deepwater research trawls, and a weird shaped living fossil called  the frilled shark. The only specimen ever caught  was transferred to a marine park where it  died some hours later. Sightings of living frilled sharks are rare, because the fish generally remain thousands of feet beneath the water's surface. Animals from the deep sea brought from great depth to the surface in nets and submersible boxes on submarines generally die.  In some deep-sea fishes, their gas-filled swim bladder expands to a deadly size when surfacing. Deep-sea speciesfrom the bathypelagic zone do not have expanding  air pockets, but still collaps during surfacing because their biomolecules stop functioning properly with the rapid pressure as well as temperature changes. However some clever capture devices have been developed that permit submarines to bring deep sea fishes to the surface in a pressurized environment during the ascent.

Some adaptations Apart from bioluminiscence as an adaptation to extreme darkness,  species that live in the meso- and bathypelagic zones need special adaptations. Some have very large eyes to capture what little light exists. Other animals are essentially blind and rely on other enhanced senses including smell, touch and vibration. Deep-sea pelagic fish such as the gulper eel have very large mouths, huge hinged jaws and large and expandable stomachs to engulf and process large quantities of scarce food. This holds  also for a legendary deep water shark called Megamouth: like the other planktivorous sharks it swims with its enormous mouth wide open. Other  specimens like the fangtooth fish have extremely long fang-like teeth that point inward, ensuring  that any prey captured has little chance of escaping. These fishes are less mobile and have a much slower metabolism than the fishes from the mesoplelagic zones, and often wait in ambush to catch their prey. Two other characteristics of certain deep sea fishes are gigantism and long-evity. Examples of the first are the enormous giant squid, and the giant amphipod (see picure above, left). Many deep-sea organisms have also been found to live for decades or even centuries. One example is the Greenland shark. Other smaller species like the orange roughy can become more  than 100 years  old. The roughies are targets of deep sea fisheries and reproduce and grow to maturity very slowly, such that populations may take decades to recover after being overfished.