Voyage to the bottom of the deep sea (I): Adaptations to deep sea life

The publication of this week is a voyage to the bottom of the deep sea; where there are life in forms we aren’t generally used to. This post will be divided in two parts: in the first part we are talking about the adaptations and in the second part about some examples of the biodiversity. 

INTRODUCTION

Till recently, the maximum depth where fishes have been seen was about 7,700 meters, but a research of the University of Hawaii has overtake it. Now, the maximum depth is about 8,143 meters, in the Mariana Trench. This value is close to the theoretic limit of 8,200 meters that some scientist have calculated as the maximum depth where a fish can live. In this video you can see the fish, which is eel-like, while, translucent and blind:

ADAPTATIONS TO DEEP SEA LIFE

Deep-sea animals have developed a group of adaptations due to the lack of light, the extremely high pressures and a low temperature of the water (near to 4ºC).

Sense organs 

Most of them have developed very sensitive eyes, despite living in the darkness, to sense the bioluminescent animals and the environmental light coming from the surface. The eyes are tubular, which consist on a multi-layer retina and a big lens, what allows them to detect the maximum quantity of light in one direction. Some species have secondary lens in the laterals and a bigger lens to improve lateral vision. Other can distinguish between environmental and bioluminescent light thanks to a filters.

Anglerfish

Some species have specialized the olfactory sense to detect preys and other fellows.

Like other fish species, to detect the vibrations in the water, they present the lateral line system, though this system can be complemented, in some species, by complementary sense organs coming from the modification of the fins.

Colour

The colour of the deep-sea animals is a response to the necessity of become camouflaged of the predators and to take advantage of the environmental light. These animals usually have either a red or orange coloration to be camouflaged in the blue environmental light; or silvery to avoid the predators could see perfectly its outline; or colourless and transparent.

Shape

The shape of the deep-sea fishes is very different from those who live in the sea surface. They don’t usually have hydrodynamic shapes because they spend almost all the time suspended in the water waiting for a prey. They present big mouths with sharp teeth. Some fishes have long bodies, what has been associated for the necessity of enlarge the lateral line system to increase the sensitivity to detect preys. Other have globular shapes, like frogfishes; laterally compressed bodies…

Humpback anglerfish has a globular shape and bait appendix in the head to attract its preys.

Bioluminescence

Bioluminescence is the capacity for producing light without heat thanks to the protein luciferin in the presence of oxygen and the luciferase, normally inside of a specialized organ called photophore. However, there are some species that accumulates bioluminescent bacteria inside the photophore. Other present a gland that expel a bioluminescent fluid to distract predators. These animals use this capability to attract preys, distract predators and to communicate with fellows.

This fish has a photophore in the later part of the eye.

Some fishes can produce red light, so, they can see them preys without being seen. 

Feeding

Food of deep-sea species can be of three types:

• Big pieces: living preys and dead animals.
• Particles coming from the surface, smaller and less nutritive.
• Dissolved nutrients.

Benthic species (those which live on the sediment) depend on the accumulation of organic particles in the sea floor or on the organisms that lives in the upper part of the sediment, while pelagic ones (those which stay in the water column) are predators.

Predators usually have a bioluminescent bait, which is a illuminated prolongation that they use near to the mouth to attract preys. In addition, many ones can expand the jaws to swallow the whole animal.

Reproduction

To overcome the difficulty in finding a partner in the deep-sea is so big that they have developed different strategies: to produce light, sounds or pheromones to attract the partner; to be hermaphrodite; or to maintain long relationships.

An example of this last case are anglerfishes. Females grows till a length of 35 cm (without the fishing line), despite their ovaries are inactive; while males are tiny. Females produce pheromones to attract males and then they combine their veins and this stimulates gonads and finally the eggs are fertilized. Finally, the male’s body become a testis mass.

The fact that deep waters are more stable than shallow waters suppose an advantage: they lay less eggs, but they are bigger, have a shorter larvae life and survive almost all.

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