A particular species of shrimp lives between 2,000 and nearly 5,000 meters deep in temperate hydrothermal waters (15 to 20 ° C), according to our partner The Conversation.
Eyeless, decked out with a very large head,
Rimicaris exoculata
feeds on a bacterium with which it lives in symbiosis!
The analysis of this phenomenon was carried out by Marie-Anne Cambon, researcher in microbial ecology and symbioses specializing in the deep sea at Ifremer.
I am a shrimp with a very strange name:
Rimicaris exoculata
.
I was identified in 1986 by researchers on board a small submarine, at 3,600 meters deep in the middle of the Atlantic Ocean, along the Mid-Atlantic Ridge.
In red, the areas where Rimicaris exoculata has been spotted © Ifremer, CC BY-NC-ND
What was their surprise to discover me there, in the company of all my congeners, small white and black shrimps, even red, swarming along the walls of the “smokers” (hydrothermal vents) which spit out their hot and toxic fluid.
VIDEO:
Meeting with the deep sea shrimp (Ifremer / Youtube, 2020)
A shrimp aggregate from the Snake Pit site. We can distinguish red juveniles and large adults with white jowls © Ifremer / Nautile BICOSE2 2018, CC BY-NC-ND
When they saw us, they called it a "swarm" because we reminded them of the habitat of bees hanging on a branch.
We are very active, constantly swimming in the current which is established between this hot fluid which comes out of the chimney and the cold water containing the oxygen allowing us to breathe.
Rimicaris exoculata © Ifremer, CC BY-NC-ND
Since that date, at the beginning of the 1990s, so many adventures have been experienced alongside scientists who are trying to unravel the mystery of our way of life! They were challenged by our existence in the dark and the depths. And they soon wondered what we could find to eat, why our heads were so big, how we were doing to reproduce, etc.
It must be said that we are breaking some records.
Starting with this place where we live, located between 2,000 and nearly 5,000 meters deep in temperate waters (15 to 20 ° C);
sometimes we pass a little too close to the hot fluids outlets, emitted by hydrothermal vents, to the point of burning our shell ... In the dark, the cold (the seawater at the bottom is around 2 ° C) ), the small points created by hydrothermal vents allow us to develop and live in an organized society along the mid-Atlantic ridge and its thousands of kilometers.
In the red circle, detail of a shrimp having burned its shell on contact with fluids © Ifremer / Nautile BICOSE2 2018, CC BY-NC-ND
A big head and friendly bacteria
Raised to the surface aboard oceanographic vessels, so we began our laboratory adventures.
We first had to give us a name and it was
Rimicaris exoculata
, scientists realizing that we didn't have eyes, a classic move when we live in the deep sea, in complete darkness.
Instead, we have a V-shaped organ above the head which is still quite enigmatic.
Perhaps it plays a role in our perception of the environment, in the reading of infrared ... Who knows?
Then the researchers took a closer look at our large head, which is almost half our size. It is called the cephalothorax, the head having fused with the first metamer of our thorax (in other words, our tail). Suddenly, here we are with a very big, long and very swollen head on each side, a bit like a hamster having just swallowed its meal!
Using their electron microscopes, scientists discovered that we were hiding long microbial filaments as well as countless bacteria.
After long examinations, they understood that they were friendly bacteria, with which we live in symbiosis, like humans with theirs (those present on the skin or in the digestive system).
These feed us directly from the head!
Here, no classic digestion passage for more than 70% of our nutrition.
The deep-sea shrimp live in symbiosis with countless microbial colonies © MA Cambon / Ifremer, CC BY-NC-ND
Another of our characteristics is to be very efficient: we live on hydrothermal sites all along the Mid-Atlantic Ridge where the chemistry of the chimney fluids is very variable in terms of chemical compounds.
Our bacteria, or symbionts, will then use these chemical compounds such as hydrogen sulfide (H2S), hydrogen (H2), ferrous iron (Fe (II)) and sometimes methane (CH4) to fix carbon dioxide. (CO2) and manufacture the sugars, lipids and proteins that nourish us.
This is called chemosynthesis which, like photosynthesis, uses light as an energy source to make organic matter.
Here at the bottom of the ocean, chemistry plays the primary role of light.
A real little factory to activate our symbionts, well protected from predators sheltered by our shell.
Photosynthesis and chemosynthesis © MA Cambon / Ifremer, CC BY-NC-ND
Every ten days or so, this process is slowed down because of the minerals that are deposited on our bacterial filaments, giving us a black or red color, depending on their sulphide or iron content.
This phenomenon causes our moult.
Here we are again entirely white, subject to the need to acquire a new population of symbionts.
We also have a digestive tract, a stomach and a hepatopancreas that do not fail to intrigue scientists, because there are stones in large numbers as well as the mysterious symbiont bacteria.
But where do the larvae go?
© MA Cambon / Ifremer, CC BY-NC-ND
For some time now, scientists have been coming to see us regularly on board their little yellow submarine: they want to know more about our mores and customs… Where do we reproduce? Are there as many males as females? Where are our larvae and our young hidden? How do we live in such aggregates, where we can be up to 2,500 shrimp per square meter?
At certain times of the year, our aggregates are mostly made up of females.
In 2007, researchers were able to collect some of them with their eggs.
Females keep them under their abdomen between their pleopods (small legs) which hold them until they hatch.
This is where it all gets complicated.
Because where do the larvae go?
Scientists conducted observation campaigns in 2014, 2017 and 2018 to study us during the breeding season without being able to answer the question.
Female and her eggs © Ifremer, CC BY-NC-ND
It will undoubtedly take a little more time to find out where they go, as scientists have little knowledge of the ocean currents in the deep sea that lead us to who knows where. Knowing that these larvae must then return to an active site to resume their life cycle. But, there again, mystery.
And mystery also on how we find ourselves there in this infinitely large and dark.
One thing is certain, we are getting there!
Scientists call this "recruitment": here we are in large numbers, young red and spindle-shaped juveniles arriving on site.
At this point, we have not yet taken the big head.
It will form later, as growth moults and metamorphoses, a bit like the caterpillar becomes a butterfly.
We therefore arrive in large cohorts and we place adults on the outskirts to integrate the colony.
The cohorts of juveniles surrounding the adults © Ifremer / Nautile BICOSE2 2018, CC BY-NC-ND
If biologists try to understand our life cycle, microbiologists try to unravel this intimate relationship with our symbionts: how do we attract them?
How do we control them so that they do not completely cover us?
How do bacteria communicate with each other so as not to fight each other, but to cooperate?
How do our young people acquire these symbionts?
Our “IFREMER” file
So many questions still unanswered… The oceanic campaigns to come and study us are not about to stop.
The mystery of the deep sea holds many surprises and discoveries to push the limits of our knowledge and see that life is limitless.
Planet
How underwater video makes it possible to (discreetly) observe fish in their habitat
Planet
How algae are revolutionizing the food, cosmetics, pharmaceutical and energy sectors
This analysis was written by Marie-Anne Cambon, researcher in microbial ecology and secialized symbioses in the deep sea at Ifremer.
The original article was published on The Conversation website.
Declaration of interests
Marie-Anne Cambon received funding from UBO, Région Bretagne, Labex MER, IsBlue, ARN and Institut Carnot Ifremer-EDROME.
Animals
Sea
Biodiversity
Ocean
Video
The Conversation
Planet