How does “flesh and bones made of water” swim?

Author: Li Chenyang

People leave traces behind, wild geese leave traces behind, but jellyfish are sad little animals - in the strata, they usually leave nothing behind.

"Most jellyfish are composed of 97% water, making them difficult to fossilize. This makes the origin and evolution of jellyfish behavior a long-standing and difficult question for the academic community," Han Jian, a researcher at Northwest University, told China Science Daily.

But by chance, Han Jian found the key to solving this problem. He and his team discovered that the swimming behavior of jellyfish may be a "by-product". The relevant paper was recently published in the journal eLife.

Rare jellyfish fossil discovered

In 2016, Han Jian and others accidentally saw an imprint similar to the mouth of a bowl in the published Chengjiang Biota fossils. There were radial lines below the mouth of the bowl, and the shape was very similar to that of modern jellyfish.

Through careful identification, they confirmed that this is the oldest swimming jellyfish known to date.

The Chengjiang Biota in Yunnan records the Cambrian ecological landscape about 518 million years ago, and the ancestral types of most modern animal phyla can be found here.

But before this discovery, no trace of jellyfish had ever been found here.

"Scarcity makes things valuable. This is a very rare research material," said Han Jian.

Why this jellyfish was preserved in a fossil is not an easy question to explain. Scientists speculate that the Chengjiang fauna experienced a rapid burial stage, and many organisms encountered violent muddy turbidity sedimentation before and after death. Under the combined effects of physicochemical conditions and anaerobic microorganisms, the soft tissues did not have time to completely degrade and were converted into various minerals, such as inorganic carbon, pyrite, aluminosilicate, calcium phosphate, etc.

However, these fossils have a major flaw.

"Modern jellyfish propel themselves in swimming by contracting and displacing the coronal muscles at the edge of the umbrella, generating reaction force. But the jellyfish fossils of the Chengjiang Biota do not leave any evidence of these muscles," said Han Jian.

The ancestor of jellyfish that can't swim discovered

The light of luck has once again descended on southern Shaanxi.

Researchers discovered jellyfish fossils that preserve various soft tissue structures in the Cambrian Kuanchuanpu Formation (about 535 million years ago), which is older than the Chengjiang Biota. Similar to modern jellyfish, muscle bundles with a diameter of 1 to 4 microns are discretely distributed around the edge of the inner umbrella.

But these jellyfish also exhibited a characteristic that was different from modern jellyfish: they were wrapped in a sheath of their own secretion both before and for a long time after hatching.

This structure looks very familiar to scientists.

Dr. Song Xikun, who participated in the study and is a researcher at the State Key Laboratory of Coastal Marine Environmental Sciences at Xiamen University, said: "Chitin sheaths and calcium skeletons are widely found in the polyps of living cnidarians. We also found such protective structures in the advanced polyp fossils found in the late Cambrian period."

"These early Cambrian jellyfish with sheaths are most likely a specialized polyp structure," said Song Xikun. "They should have been benthic and could not swim freely."

Scanning electron microscope photos of fossils from the Early Cambrian Kuanchuanpu Formation in southern Shaanxi showing circular muscles (AD) and systematic fossil reconstruction (E)

So what is the relationship between "jellyfish" and "hydra"? It can be said that "you are in me and I am in you", which is very interesting.

According to modern biological system classification, the phylum Cnidaria includes the superclass Corals and the superclass Jellyfish, among which the superclass Jellyfish includes the class Cruciozoa, the class Cubozoa, the class Scyphozoa and the class Hydromedusa.

These animals look different in shape, some like branches and vines, some like transparent fish tanks, but they can actually be roughly divided into two forms: the polyp in the fixed stage and the jellyfish in the swimming stage.

Taking Scyphozoa as an example, their life process can be roughly divided into: fertilized eggs, floating larvae, fixed polyps, disc-shaped larvae with swimming ability and jellyfish bodies that can swim and prey.

If you look closely, the polyp can be roughly seen as an inverted jellyfish, although their internal morphology and structure may be different.

How do jellyfish swim?

Ancient fossils are like pieces of a puzzle, gradually completing the mystery of how jellyfish swim.

Most of the small-shelled fossil animals in the first Cambrian period lived benthic and surface dwellings, and early jellyfish were no exception.

In the late polyp stage of embryonic development, they have a cone-shaped or tower-shaped sheath and live on the sediment at the bottom of the water.

The circular muscle system hidden in the sheath can be used for eating and breathing by squeezing and contracting the jellyfish bell.

By the third period of the Cambrian period, jellyfish were already able to swim.

This roughly locks down the time period when jellyfish swimming behavior is likely to occur.

"Jellyfish that broke free of their sheaths and were able to swim freely probably appeared in the second period of the Cambrian period, but there is no fossil evidence yet," said Dr. Wang Xing, the first author of the paper from the Qingdao Institute of Marine Geology.

"According to Darwin's theory of evolution, any complex life phenomenon is the result of long-term evolution. We speculate that the swimming behavior of jellyfish is just a byproduct of benthic jellyfish feeding and breathing."

Han Jian said, "During the long evolution, benthic hydroid jellyfish gradually acquired internal conditions such as rhythmic nerve impulses, balance stones that sense gravity, coronal muscles with strong elasticity, thicker mesoglea, and deeper hypopygial cavity, and were reborn. At the same time, suitable external conditions, such as increased oxygen content in seawater and other prey in the water, helped them get rid of the constraints of the sheath and become swimming jellyfish, starting to swim freely in the sea."

Related paper information:

https://elifesciences.org/articles/74716

Editor | Zhao Lu

Typesetting | Guo Gang

Source: China Science Daily