In our current world, the land is full of plants with the ability to flower. These plants attract insects and other animals through flowering for pollination, and then produce delicious fruits with seeds wrapped in pericarp, attracting various animals to eat and spread the seeds. About 100 million years ago, one of these angiosperms brought their "gifts" to the ocean and became the only flowering plant in the ocean.
It is precisely because the seeds of these plants are wrapped that they are calledangiosperms, the Cenozoic Era is the era of angiosperms and mammals, which almost occupy most of the terrestrial ecological niches.
However, if we pull the time back to 150 million years ago, the situation is completely different. At that time, the dinosaurs that ruled the earth's land together with the dinosaurs did not bloom.Gymnosperms.
Gymnosperms and angiosperms separated from each other about 300 million years ago. About 150 million years ago, angiosperms began to emerge. In the following tens of millions of years, they completely defeated gymnosperms and became the king of the land plant world. Today, only about 1,000 species of gymnosperms are left.
No one knows why angiosperms are so successful, and why gymnosperms are so vulnerable to them. Darwin called it an "abominable mystery."
But no matter what, angiosperms do have some outstanding features.They grow very fast, and by wrapping the seeds, they spread farther.——Because the spread of wind, water, and animals has become easier.
△A seaweed flower ©SHANEGROSS
This kind of angiosperms that came from land is a dimensionality-reducing blow to those primitive marine plants, so now seagrass has long been the most successful plant in the ocean. They are distributed in all shallow sea areas except Antarctica. Their number is even as much as that of corals, and they play the most important ecological role in the ocean.
they areseaweed, they will bloom, bear fruit and spread seeds in the ocean, experiencing a completely different way of survival from their terrestrial relatives.
So how exactly did seagrass adapt to the ocean? Can flowers on the seabed attract animals to help pollinate them?
△Distribution of seagrass-green part
What kind of plant is seaweed?
Native plants in the ocean—some speciesseaweedand our protagonist todayseaweedThey look very similar. Without the "invasion" of seaweed, shallow seas would probably be covered with these seaweeds.
Seaweeds are algae. They are very primitive and do not have roots, stems, leaves and other plant tissues. They use a fixator to fix themselves to the seabed, but this structure does not absorb nutrients. They transport nutrients into the body through diffusion;
△The difference between seaweed and seaweed
Seagrasses, like terrestrial flowering plants, divide labor and transport nutrients through roots, stems, and leaves to achieve rational utilization of survival resources.
What's more, seagrass grows quickly and has a strong reproductive capacity just like prairie grass, so seaweed does not have much ability to compete with it. Just because of the need for photosynthesis, seagrass grows at a depth of usually between 1 and 3 meters in shallow seas (the deepest species reaches 58 meters).
However, what is interesting is that seagrass has not diversified. This may be related to the fact that the geographical isolation of the ocean is not obvious enough. Until today, there are only about 72 species, including only four families.
Normally, only one or a few types of seagrass grow in each region. Only in a few tropical areas does the diversity of seaweed appear larger - but the most area has only 14 species.
Seagrasses have extremely strong asexual reproduction capabilities. Like grasses on land, they will constantly sprout new shoots through their roots to clone themselves., creating “clone meadows” composed of genetically identical individuals.
Because the well-developed root system of seagrass is rarely damaged in the ocean, the oldest and largest known plants are all from the same seagrass species - called Posidoniaoceanica.
The largest and oldest known seagrass, its "clone meadow" is 200,000 years old and dates back to the Ice Age of the late Pleistocene. Its width spans 15 kilometers, which is much crazier than bamboo on land.
△Flowers of the seaweed Cymodoceanodosa
How are undersea flowers pollinated?
In addition to asexual reproduction, as angiosperms, reaching the bottom of the sea does not prevent them from flowering, bearing fruit, and carrying out sexual reproduction. However, the problem that must be solved for flowering is pollination.
In the past, scientists generally believed that seagrass pollination was entirely determined by water flow, because for a long time no animals were found that could be attracted to their flowers.
It was not until recent research that it was discovered that the pollen grains of a kind of seaweed, Thalassiatestudinum, are food for some small shrimp-like crustaceans.
When these critters eat seagrass pollen, they also help pollinate them, just like insects on land.
However, for most seagrasses, no marine animals have been observed to help pollinate them.
Seagrass has the largest male flower pollen grains among flowering plants, reaching 5 mm in size, while ordinary pollen grains are usually less than 0.1 mm.
Moreover, these pollen grains will gather into filamentous clumps and move under the action of ocean currents, which will help them hang on the pistils of female flowers, thereby completing pollination.
△The fruit of the seaweed Posidoniaaustralis
Once pollinated, they develop seeds, which are usually neutrally buoyant and can float great distances in the ocean.
In addition, these seeds usually contain a large amount of nutrients. Some types of seeds carry nutrients that can even be used until the seedlings develop in the first year. Some types of seeds also have their own photosynthesis capabilities.
In short, the survival rate of seagrass seeds is very high compared to terrestrial flowering plants.
△The evolution of plants from their origin in the ocean to land, the diversification of land plants, and the subsequent return of seagrass to the ocean
at last
Land plants evolved from ocean algae about 450 million years ago, and seagrass has returned to the ocean.
It is very difficult for plants to fully transition from terrestrial environments to marine environments, and to do so requires special ecological tolerances such as high salinity, low light tolerance, underwater carbon capture capabilities, different pathogen defenses, structural flexibility and underwater pollination, to name a few.
But seagrasses did, and they may have done it three more times—with three lineages all entering the marine environment independently.
Of course, during their long ocean lives, some of them also returned to freshwater environments, and it is believed that this situation evolved independently more than 200 times.