Thousands of kilometers for the behemoths of the seas
Let's start our trip to California. Bordered by the Pacific Ocean, it runs along the west coast of the United States for several thousand kilometers to Mexico (Baja California) and its peninsula which protects the Gulf of California, also known as the Sea of Cortes, which a part is classified as World Heritage of Humanity by UNESCO, in particular the newly baptized Île Jacques Cousteau. It is in these warm waters that each year in the winter period, we can see nearly 30 gray whales (Eschrichtius robustus) arriving after swimming about 000 kilometers from the Bering Strait. Why such a long trip? This area of the North Pacific, between Siberia and Alaska, has waters teeming with plankton, the main food of whales. They can thus eat at will, and store a large mass of fat, a real "parka" allowing them to survive in these polar temperatures. But it is also the energy reserve that will allow them to accomplish this long journey south. Because it is unimaginable to give birth in these icy waters, the calves would not survive. It is therefore in this Sea of Cortes, that the gray whales come to reproduce, and give birth to their young, every year, from the month of December. A few months later, usually in April, when the calves have enough strength, they take to the road all together, north, until the next winter!
Long traveler ...
Now let's jump from an ocean to the east, towards the Sargasso Sea, near Florida. It is in this region that eels are born, these serpent-shaped fish that we find in our rivers. Let's start at the beginning… The hatching of the eggs gives rise to small transparent planktonic larvae, called Leptocephali, which are “sucked in” by sea currents. Or more precisely, by the best known of all, the Gulf Stream. Here they are on their way north. There are actually two species, corresponding to the American eel (Anguilla rostrata) and the European eel (Anguilla anguilla). The first sees its larvae travel for several months to the east coast of the United States; the second, for about a year to Europe. Once there, the larvae metamorphose into glass eels, resembling small transparent eels. They will then be able to begin a slightly different migration, leaving the marine environment, to go up along rivers and streams. It is in these fresh or brackish waters that they will spend most of their life, up to 20 years for females. It is only after this long period as yellow eels that they will return to the sea to reach their place of birth. These are then sexually mature silver eels which will reproduce between 400 and 600m deep in the Sargasso Sea. This will be their last trip… They will die after giving birth! The cycle can start again ...
From fresh water to sea water and vice versa… Not so simple!
Perhaps some of you have had the experience, during the great existential questions of your adolescence, of offering a freshly caught sea fish a beautiful jar of "tap" water, in the hope of tame it… And the first trick that you probably taught it was to swim “stomach up” (position of the dying fish!)… obviously!
Switching from fresh water to sea water, and from sea water to fresh water, like the eel, cannot be done instantly. It is the result of a physiological and physical metamorphosis that can last several weeks.
The eel is a so-called “catadromous” fish, meaning that it spends most of its life in freshwater, and migrates to the sea to reproduce ('cata' in Greek means' down ').
A fish living in the sea, and migrating in rivers to reproduce, is said to be 'anadromous' ('ana' in Greek means 'upwards'). The best known is of course the salmon, which can be seen leaping in small streams during the breeding season.
In the shoes of a migrant
Let's continue our tour of the oceans ... heading to southern Australia, not far from Melbourne. Each year, it is possible to attend scenes worthy of the greatest science fiction films.
Rising from the depths, thousands, or rather tens of thousands of giant spider crabs (Leptomithrax gaimardii), assemble, forming a veritable armored army. In a few hours, the bottoms are completely covered with a carpet of several layers of these crabs with long claws ... After wearing their shell for a whole year, it's time to change them. The union being the force, here they are safer to molt: the risk when the crustacean gets out of its old shell is to be attacked, while the news is not yet hard. Of course, when a large predator, such as a stingray, passes by, some will get caught.
This gathering is also an opportunity for males and females to meet and reproduce. As well enjoy ! Once the operations are over, everyone slips away… back to the depths, until next year…
Merry Christmas Crab!
Let's get lost now, still in the vast Australian territory, but a little further north, on an island called Christmas… discovered of course on December 25, in the year 1643, in the south of Indonesia. About two-thirds of the island is classified as a national park, largely thanks to a crustacean ...
The red crab (Gecarcoidea natalis) is a surprising occupant of the island. Freed from the marine environment, he lives on a plateau covered with forest, at an altitude of over 300m. It is a humid environment, but limits its efforts at times when humidity is the highest. When the air is too dry, it takes refuge in a burrow.
But when the wet season arrives, in November, the red crabs, estimated at more than one hundred million individuals, begin their annual pilgrimage: you have to reach the beach to reproduce and lay eggs! This is how they invade the roads and streets in their path. Real red tides on dry land! And the journey can take almost a week. The males are the first to leave, to dig the burrows that will serve as a love nest. The females follow them and lay their eggs in the water after reproduction. Each individual, after having passed through the water to rehydrate their gills, begins their long walk again to the forest. The larvae, after growing in the sea for a month, become young crabs, which come out of the water to join the adults ...
When everyone is bent on sardines!
The food chain… this is undoubtedly what most connects species to each other in nature. Imagine for a moment one of the most theatrical scenes that the ocean can give us ...
Cape of Good Hope, South Africa. Each year at the same season, an incredible scene occurs without us really understanding why. Sardinops sagax sardines congregate along the coasts, by the millions, forming veritable dark masses, similar to immense submarines. They are carried by the cold currents which go up towards the Indian Ocean. The opportunity is too good. Rarely is the dish served so easily. The schools of dolphins arrive, and start hunting ... this is the beginning of what is called the "Sardine Run". They surround the sardines, break the bench into several balls, bring them up to the surface, and rush into the pile to catch them. Northern gannets hit the surface at lightning speed. These missiles from the sky enter the fishing trip… and will be quickly joined by sharks, sea lions, and the largest animal on the planet, the blue whale. All came for the sardines and mix with each other in the greatest indifference. The ball shrunk visibly ... little by little the sardines disappear, attacked from all sides.
When they reach the first warm currents of the Indian Ocean, it will be time to turn back. Few survivors will return to the waters of the Cape ... which suggests that the reason for this migration is purely sexual ...
The biggest migration on the planet
And if finally all these events that we have just described were only the hidden face of the iceberg… Although spectacular, and not always elucidated, they become a drop of water in the ocean facing the biggest planet migration. As large as it is discreet… and yet with such a well-regulated frequency.
Every night, the phenomenon begins again ... as soon as the sunlight disappears, the phytoplankton, which needs solar energy so much during the day, begins its slow migration to the depths, in search of nutrients.
Meanwhile, zooplankton and some large animals like squids, which do not need light and live at greater depths, rise to the surface to find food while remaining in relative safety. It would be too dangerous to come there during the day.
This is how each night, these nycthemeral migrations (a nycthemeral is a day-night cycle. The word comes from the Greek nukhthêmeron, a word made up from nuktos meaning night, and hêmera meaning day) occur in all seas and oceans of the planet, displacing incalculable biomasses, they are so immense! At the first light of dawn, everyone will have taken their place ...
All species need to move to ensure their sustainability. Even the sedentary people have found a way to colonize new places, in particular by the dissemination of larvae in open water, which sail with the sandstone currents. Migration is also very often vital ... Let us not forget, to feed and reproduce, these are indeed the only obligations of the living world!