Voyage to the bottom of the deep sea (II): Biodiversity in the deep sea

This week we are continuing our voyage to the bottom of the deep sea. While last week we focused on the adaptations that fishes have suffered, this week we are focussing on the biodiversity. In concrete, we are explaining crustaceans, squids, cnidarians (corals, jellyfishes and anemones), fishes and worms. 

INTRODUCTION

In 1840, the scientist Edward Forbes concluded that there wasn’t life under 550 meters depth. Nowadays, it is known that this is not true because recently it has been found a fish at 8,100 meters. It has been determined that the relative abundance of animals depends on depth. In fact, in general terms, the abundance decreases with depth, but this don’t exclude that there are a lot of species.

 

BIODIVERSITY

CRUSTACEANS

Amphipods are by far the most abundant crustaceans in the deep sea. They are small animals with the body compressed laterally and without a carapace, which feeds on carrion and live inside cavities made by themselves in the sea floor. These small animals are transparent, except for them eyes, which are red due to a pigment in the retina.

Deep sea amphipod. They are characterized by the presence of a transparent body with red eyes. (Picture from http://www.astronoo.com/es/articulos/bioluminiscencia.html)

Other deep sea crustaceans are stone crabs, with a carapace of 7.5 cm length and legs of about 15 cm; the armoured shrimp, one of the species that lives at 6,000 meters and has a length of 7 to 10 cm; and more.

DEEP SQUIDS

In spite of the general thinking that deep sea squids are all large, like the giant squid, which can achieve a length of 18 meters; the truth is that this is an exemption because there are some spices of just 4 cm. They hunt with the suckers in the tentacles and driving the prey to the mouth. Most of these squids are bioluminescent and can regulate the colour, the intensity and the angular distribution of the light.

The Humboldt or jumbo squid (Dosidicus gigas) lives in the western coasts of Central and South Amercia and can achieve a length of 4 meters, which feeds on fishes and practise cannibalism.

Humboldt or jumbo squid (Dosidicus gigas). They have bad reputation because they attack divers.

CNIDARIANS: CORALS, JELLYFISHES AND SEA ANEMONES

Differences between shallower cnidarians and deep ones are due to differences in the food distribution. In the deep sea, anemones and corals don’t have directly phytoplankton and zooplankton, and they depend on the nutrient rain from the shallower waters of the ocean. On the other hand, jellyfishes have a slow metabolism to survive in hard conditions. It supposes slower growth, but a longer life.

To give an example, this crown jellyfish inhabits between 200 and 2000 meters depth and can measure until 15 cm. It feeds on small crustaceans and organic matter. Its red colour let them be camouflaged in the environment. In addition, they are bioluminescent animals.

Crown jellyfish. Its red colour let them be camouflaged in the environment.

Deep-sea jellyfishes are voracious predators, but also can be a prey for some fishes. They produce light discharges to attract small animals. To dissuade predators, they expel a brilliant particles stream.

An habitual feature of deep-sea jellyfishes, but also present in other groups, is gigantism. It means they are bigger than their equivalents in the shallow ocean. The possible explanation to this could be that bigger animals are more efficient than smaller to get food when the environmental conditions are almost constant during long periods of time.

FISHES

Gonostomatidae fishes are the most abundant vertebrates in the Earth and live in the mesopelagic zone. Together with the lantern fishes, they represent a 90% of the captures in the pelagic trawling fishery. Deep-sea fishes usually have a length between 2,5 – 10 cm and a thin and soft body, but there are exceptions.

There are some examples here:

• Anglerfish: These fishes inhabit in the deepest parts of the oceans and present the optimal colouration to absorb the few light that arrive and, in this way, to be camouflaged. They present a light in the end of the antenna, which let them to capture preys.

Anglerfish

• Spiny lantern fish: Because of its silvery body, this fish is not much vulnerable since its contour can’t be seen clearly. In addition, spiny lantern fish presents a bag in the eye with bioluminescent bacteria.

Spiny lantern fish

• Pelican eel: This animal can measure 2 meters long. Its enormous mouth are connected directly to the stomach.

Pelican eel

• Tripodfish: Tripodfish has long prolongations in its pelvic and caudal fins, which let them put on the sea floor, while it is waiting for its prey.

Tripodfish

• Black swallower: This small fish has the ability to dilate a lot its stomach and, in this way, it can swallow preys bigger than itself.

Black swallower

 

MARINE WORMS

Deep-sea worms can be from microscopic to measure 2 meters long and are one of the most abundant and different invertebrates. They can be of different groups: polychaetes, tubular worms, sipunculids and equiurids. They live partly or totally buried in the sediments.

Tubular worms usually live in big groups near to thermal springs and present red bright gills as a consequence of a high level in hemoglobin to absorb oxygen. In addition, they can retain sulfurs, which will be used for symbiotic bacteria.

Tubular worms. They use the sulphur produce in the thermal springs thanks to symbiotic bacteria.

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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.

 This post is under a Creative Commons licence:
Licencia Creative Commons Atribución-NoComercial-CompartirIgual 4.0 Internacional.