Authors: Julius Nielsen and Martin Nielsen

The deep sea begins 200 meters beneath the water surface. At this depth the amount of sunlight is limited and can be compared with the light on an evening sky, hence it is called the twilight zone. Below depths of around one kilometer there is no sunlight left, and in this world of darkness, food is scarce. The inhabitants of the deep sea are therefore always searching for something to eat.

Every year the research vessel RV Pâmiut, owned by the Institute of Natural Resources in Greenland, travels the Davis Strait (western Greenland) and Denmark Strait (eastern Greenland) to survey the commercial stocks of fish and shrimps. Besides gathering information regarding age and size distribution of redfish (Sebastes sp.) atlantic cod (Gadus morhua), greenland halibut (Reinhardtius hippoglossoides) and deep sea prawn (Pandalus borealis), the biologists on board encounters a wildlife evolved under extreme conditions. To an imaginative mind, life at these depths resembles what life might look like on another planet. In this article we will dive into the deep sea and encounter this extreme habitat and the wildlife within. We will dive into the Greenlandic Deep Sea. 

As for life in all ecosystems, the same fundamental rule applies for all organisms in the deep sea: it is all about finding food without getting eaten yourself. Those who accomplish this have the greatest chance of surviving and passing on their genes to future generations. In the darkness of the deep sea it is a great advantage to be able to produce light, and life at these depths has evolved, so most fish posses this ability. Some use light when hunting, while others use it for communication or camouflage.

One of the most fearsome predators are the females of a deep sea angler fishes – a fish with an appearance, strategy of reproduction and hunting, that reflects the extreme conditions of the deep sea. The male, which is only a few centimeters long when fully grown, has one goal in life; to find a female whom he can bite and thereby attach himself to. He is then absorbed by the much larger female and reduced to a mere sperm sack connected to the metabolism of the female. The female uses the sperm provided to fertilize her eggs whenever she is ready to spawn. The gain from this destructive behavior of the male ensures his paternity of the next generation. At the same time the female receives a small portion of nutrition and will not have to use valuable energy searching for a suitable partner. Opposite the male, the female is armed with sharp teeth in a large mouth. With prey being scarce, this is to ensure that all sizes of prey can be hunted and eaten, even prey larger than herself. Furthermore, the first dorsal fin ray has evolved to function as a fishing rod with a light organ emitting bioluminescence (light produced by bacteria situated in the light organ) to lure prey closer to the deadly teeth.

Light is also used by some fish as camouflage. Examples of such are fish hunting for crustaceans in the pelagic zone. These fish are at great risk of being detected by predatory fish from beneath. To avoid this many fish from the order of dragonfish and allies (Stomiiformes) have photoreceptors along their ventral side. These photoreceptors emit light in well-defined blue wavelengths equivalent to the light intensity above the fish. This counter-shading effect makes the silhouette invisible from beneath and combined with reflecting sides, thin body and light-sensitive eyes, hatchet fish and silvery lightfish (Maurolicus muelleri) are perfectly adapted to life in the pelagic of the deep sea.

Other groups of animals similarly use light as camouflage. One example is the warty bobtail squid (Rossia palpebrosa), which hides on the sea bottom during the day while growing light-producing bacteria. At night when the bacteria reach a certain density they begin to emit light serving as camouflage. During daytime the squid releases the majority of bacteria which lowers their density, stopping the light-production.

One of the most advanced predatory fish in the deep sea is the northern stoplight loose-jaw (Malocosteus niger). The English name originates from the fish’s ability to open its jaws in an angle of 180°, and because it has a red and a yellow light organ on the head emitting light. The majority of animals in the deep sea are only capable of seeing light at blue wavelengths, and therefore the stoplight loose-jaw has a great advantage as it searches for prey with a ‘flashlight’ the prey cannot see. Some prey animals have however developed eyes capable of seeing light at red and yellow wavelengths to, and scientists refer to this kind of evolution (as in many others) as an arms race, where prey and predators continuously develop and adapt abilities to outsmart each other.

As mentioned, the majority of animals in the deep sea have learned the advantages of being able to produce light. This is also the case for one of the giants in the deep sea - namely the Greenland shark (Somniosus microcephalus). This shark can reach a length of seven meters and weigh more than 1100 kg. This makes it one of the largest fish in the world. The scientific knowledge of this mysterious shark is limited, but theories of its way of life are many. In the eyes of the shark are often found parasitic copepods which impair their vision. But since the copepods emit light, it is possible that the relationship is in fact mutualistic (instead of parasitic). A mutualistic relationship implies that both species benefit from the interaction, and the scientists believe that the light in the eyes of the Greenland shark could function to attract prey, in the same way as the deep sea angler fish uses its fishing rod. When a potential prey gets close, the shark can locate it precisely with other senses than vision, such as the electrical sense characteristic of many elasmobranches. This beneficial role of the copepod has not been scientifically proved yet, since this mysterious shark live in an environment difficult to study. Beside the electrical sense, the Greenland shark has developed a sensitive olfactory sense, which it uses searching for carcasses on the sea floor. Beside the eyes, the age of the Greenland sharks is also a mystery to the scientists. The only re-catch of a tagged shark revealed a growth of eight centimeters in 16 years. This combined with the cold water and the low metabolism makes many scientists believe that these giants can become extremely old. Measurements indicate an age of around 200 years for the largest individuals, possibly making Greenland sharks the longest living vertebrates on Earth.

The adaptations, fish and cephalopods have developed and evolved in order to survive in the extreme environment in the deep sea, are many more than mentioned in this article. 80 % of the surface of Earth is covered by water and the average depth is four kilometers, making the deep sea the most unexplored habitat on Earth. Research vessels, such as RV Pâmiut, have an important role in the exploration of the arctic deep sea and are a major contributor to our knowledge of this spectacular warzone in the deep.