Are you still worried about your body shape? Look at how others have resisted the mass extinction by relying on their extra flesh!

Produced by: Science Popularization China

Author: Gu Ming Di Lian (popular science creator)

Producer: China Science Expo

When you think of seashells, what do you think of? Grilled oysters, steamed scallops, spicy stir-fried razor clams, or boiled hairy clams?

In most people's minds, seashells are closely associated with food, but this is not because you are greedy, but a common impression of humans. Humans eat more than 16 million tons of mollusks every year, and 70% of them are bivalve shellfish.

Since the early humans in East Africa who ate shellfish on the shore, the meat of bivalve shellfish has become a food source for humans. They filter the water, absorb a large amount of phytoplankton and zooplankton that other organisms cannot use, and transform them into nutritious, tender and crisp shellfish meat, which is then delivered to human tables.

The oysters are plump and full of meat in their shells.

(Photo source: veer)

Early brachiopods and bivalves

However, it is actually very lucky for humans to be able to eat such abundant seashells. For more than 200 million years since the emergence of life, the main filter-feeding animals in the ocean were not bivalve shellfish, but another type of creature called "brachiopods."

At first glance, brachiopods and bivalves look very similar. Both have two calcareous shells with flesh inside, and it seems that there is not much difference in eating either one, but their difference lies precisely in the flesh.

The shell of bivalve molluscs has a well-developed soft body, with a mantle covering a rich variety of muscles, internal organs and water pipes. The thin and tough mantle, the sweet adductor muscle, the rich internal organs and the crisp water pipes together make up the delicious taste of the molluscs.

Brachiopods are just the opposite. Their shells are very thick, but there is very little meat inside . An inedible, hard arm and the filtering tendrils growing on it take up as much as two-thirds of the internal space of the shell, while the muscles and internal organs are squeezed into the corners, and the proportion of the edible part is pitifully small.

Comparison of brachiopods (left) and bivalves (right), showing the difference in meat quantity. Note that the gray crown and cilia that make up the largest proportion of the brachiopod's body are not edible.

(Image source: Quora)

Marine animals grow flesh not for human consumption, but for their own survival. Flesh contains physiologically active cells, and the more flesh there is, the faster the metabolic rate. Bivalves with thick flesh have a metabolic rate that is three or even ten times that of skinny brachiopods, and can produce more offspring.

Scallop flesh and shell

(Image source: Wikipedia)

But compared to bivalves, brachiopods have the advantage of "seniority".

As early as the Cambrian period 540 million years ago, filter-feeding brachiopods appeared on Earth. They multiplied to a very large number over hundreds of millions of years, gaining a first-mover advantage. At this time, early bivalves were still burrowing in the bottom mud like little snails with two-valve shells, feeding on organic matter in the bottom mud. When they finally evolved syncelon with tight gill filaments and the ability to filter feed in the Ordovician period 480 million years ago, the brachiopods on the seabed had already multiplied to a very large number.

Even though the fleshy bivalves had a faster metabolism, they were still insignificant in the face of the overwhelming numerical advantage of brachiopods. However, this situation was changed forever because of a mass extinction event.

The changing status of life on Earth in extinction

At the end of the Permian period 252 million years ago, the largest extinction event of life on Earth occurred . A super volcano in Siberia erupted on a large scale. A huge amount of magma swallowed up all living things in the surrounding area. The released sulfur dioxide, acid rain brought by smoke and dust, and cold weather seriously damaged the marine environment. The world was frozen and the sea level dropped sharply. After the impact of sulfur dioxide, the carbon dioxide that followed harvested the last batch of organisms.

After this extinction event, bivalves soon surpassed brachiopods in number and dominated all parts of the world, but brachiopods never recovered their former prosperity and eventually could only live in dark and cold areas, becoming marginal elements of the ocean.

What caused the change in status between the two?

Simulating the end-Permian mass extinction, a massive volcanic eruption

(Image credit: José-Luis Olivares/MIT)

Naturally, people think of competition.

Brachiopods and bivalves have similar morphology and the same lifestyle, but bivalves have significant advantages in terms of flesh. Powerful muscles allow them to dig, paddle, and so on, to avoid enemies and find better food sources; water duct muscles can stir up strong water flow, filter more food, achieve faster growth and reproduction rates, and produce more offspring.

For this reason, since the 1860s, around 1860, scientists have assumed that the two are in a competitive relationship, and that the bivalves with physiological advantages have continuously expanded their distribution through competition, compressed the living space of brachiopods, and gradually "eliminated" the inferior brachiopods.

However, this assumption based on conjecture and "common sense" has been questioned recently. With the increase of fossil evidence, scientists can observe this issue from a more grand perspective.

Recently, doctoral student Guo Zhen and Professor Chen Zhongqiang from China University of Geosciences (Wuhan) collaborated with Professor Mike Benton and doctoral student Joseph Flannery-Sutherland from the University of Bristol in the UK to reconstruct the changes in brachiopod and bivalve species before, during and after the Permian-Permian mass extinction event and in subsequent evolution based on the huge paleontological database (PBDB) and thousands of fossils.

Studies have shown that brachiopods suffered a severe blow in the mass extinction event, and although bivalves also suffered some losses, there was no large-scale extinction of major taxa above the order. Brachiopods suffered much more serious damage than bivalves in this mass extinction event .

After the mass extinction event, the number of bivalve species began to recover immediately, and a large number of new species evolved in the first 10 million years of the Triassic period. At the same time, brachiopods did not fall into despair, and they also re-evolved new types with great vitality. However, in the Olenekian Stage, 2 million years after the beginning of the Triassic period, brachiopods experienced another large-scale extinction, and the population that had just recovered declined again, and bivalves immediately took an absolute advantage.

However, bivalve dominance did not suppress brachiopods, which recovered in the Middle Triassic and Middle Jurassic, and became stable in the oceans during the Cretaceous, and even flourished further in the Cenozoic after the extinction of the dinosaurs, becoming extremely abundant in individual upwelling environments of the tropical continental shelf of the western South Atlantic, outnumbering bivalves and gastropods combined.

If the two were in competition, the diversification of bivalves would simultaneously suppress the recovery rate of brachiopods until they were eventually squeezed out of space by the large number of bivalves and became extinct. However, the opposite is true .

The curves showing the changes in the number of bivalves (orange) and brachiopods (purple) genera show that brachiopods suffered a severe blow at the end of the Permian period, but there were still periods of increase in the number of genera after the extinction, rather than a continuous decline.

(Image source: Reference [1])

A physiological advantage for resisting mass extinction

For most of the time, bivalves and brachiopods have a "prosperity-all-prosperity-and-loss-all-prosperity" relationship. Both need the same space and food resources, and when the environment changes, both will naturally prosper and decline at the same time. As filter feeders without brains and low mobility, although the two have competition, it is not as bloody as between predators. It is more of a hidden competition for food and space.

Normally, the ocean with abundant food resources can fully accommodate the two species. Bivalves and brachiopods can live in different areas or live together, enjoying the food in the ocean currents together, and occupy their own share of the seabed almost without affecting each other. However, if a mass extinction event occurs or the marine environment deteriorates, each can only rely on its own ability.

Unfortunately, the lack of flesh in brachiopods makes it difficult for them to resist mass extinction events. Most brachiopods are completely sessile, using fleshy stems or shell spines to fix to sediments or hard substrates, and live in an unchanging position.

The modern brachiopod Liothyrella neozelandica is densely distributed on the cliffs beneath the sea surface of New Zealand.

(Image credit: Ryan Photographic)

The muscular bivalves are mobile "burrowing specialists" that live primarily beneath the seabed, digging burrows at varying depths for protection and evolving straw-like structures called "water pipes" to bring food and oxygenated water into their shells while buried in the mud.

Other bivalves, notably scallops, have evolved ways to swim by rapidly opening and closing their shells using the adductor muscle, greatly increasing their ability to escape danger and predators.

The cockles’ muscular feet are strong, long, and sickle-shaped, allowing them to move quickly through the sand to avoid predators and harsh environments, while also giving them a crisp, sweet, tender and delicious flavor.

(Image source: Quora)

During the Olenekian Stage, shortly after the mass extinction event, the Earth's temperature rose sharply by about 8-10 degrees Celsius due to the strong greenhouse effect, which lasted for nearly 5 million years, causing the dissolved oxygen in the seawater to become extremely low, and a large number of marine life disappeared. Among them, brachiopods were particularly hard hit.

The reason is that they have no meat .

The thick shell hinders the diffusion of oxygen, and the small body can hardly drive the water flow and speed up breathing, let alone propel itself to move, so it can only stay fixed in place and suffocate to death.

In contrast, bivalves have thin shells and thick flesh, and their powerful muscles allow them to stir up strong water currents. Friends who have been sprayed with water by shellfish in shopping malls should have experienced this; many bivalves have many tiny pores on their shells that allow water to flow through and bring in fresh oxygen. In disasters, these abilities become their magic weapon to avoid being troubled by hypoxia.

Axe-shaped clam buried in the sand, revealing two long, muscular tubes used for breathing and filtering.

(Image source: Wikipedia)

At the same time, brachiopods were still struggling with hypoxia, not only did their populations die out in large numbers, but the survivors were also forced to reduce their size to adapt to the low-oxygen environment. Intolerance to high temperatures and hypoxia ultimately led to catastrophic consequences: brachiopods with long-floating larvae became completely extinct during this mass extinction event, which almost destroyed their ability to repopulate the Earth.

For animals with poor mobility, long-penetrating larvae are the top priority for benthic dispersal. They can float in the water for a long time and be carried by ocean currents for long-distance dispersal, reaching distant places that adults can never reach.

Bivalves have such larvae. During a drift period of one month or even more than one year, the larvae can float up to nearly 4,500 kilometers with the water current, which is equivalent to crossing a quarter of the Pacific Ocean! This allows bivalves to take root in more survivable areas, resulting in their widespread global geographical distribution.

However, after the Permian mass extinction event, the only surviving calcareous brachiopods were those with short-lived larvae. They used their own yolk as nutrition, did not eat, and only floated in the water for a few days before settling down, fixed to the bottom and no longer moving. The short floating period means that the offspring can usually only settle within a few thousand meters of their parents, or even closely connected to them.

The advantage of this type is its exclusivity - it forms dense groups in a small area, can occupy almost all the food resources there, and eliminate competition from other filter feeders. However, this also leads to serious "involution" between groups, and they compete with each other for food resources. Shell deformities often occur, and parents will oppress their offspring and suffocate them to death.

More importantly, the difficulty for larvae to spread over long distances severely limits their distribution range. Every time a new round of extinction comes, brachiopods with a narrow distribution range are likely to suffer a devastating blow. The two are intertwined, resulting in the dominance of bivalves in the oceans in the future.

After the extinction events, most of the calcareous brachiopods could only live in cold water and low light environments , which were relatively stable and less affected by extinction events, but poor in nutrition. This means that they have become marginal molecules in the ocean and have almost no chance of recovering their former prosperity.

Meanwhile, bivalves have continued to grow in numbers, filling every corner of the world’s oceans, their rich, juicy flesh helping them survive numerous mass extinction events and feeding generations of humans as the most readily available source of meat.

Fried clams are the most common and most produced shellfish in my country.

(Photo source: veer)

Conclusion

From shells found on coastlines and ruins of civilizations around the world, to the glowing sea asparagus in the hands of the Romans, to the elegant oysters sucked in "My Uncle Jules", to the continuous farms along the coastlines of modern civilization, countless bivalves have played an important role in history, providing countless foods, shells and pearls to the world for tens of thousands of years. Without this mass extinction event, all of this would no longer exist.

So you see, this mass extinction event was a catastrophe for the creatures at that time, but it was not necessarily a bad thing for us humans. Nature is so wonderful.

References:

[1]Guo, Z., Flannery-Sutherland, JT, Benton, MJ et al. Bayesian analyzes indicate bivalves did not drive the downfall of brachiopods following the Permian-Triassic mass extinction. Nat Commun 14, 5566 (2023).

[2]Sun Y, Joachimski MM, Wignall PB, et al. Lethally hot temperatures during the Early Triassic greenhouse[J]. Science, 2012, 338(6105): 366-370.

[3]Gould SJ, Calloway C B. Clams and brachiopods—ships that pass in the night[J]. Paleobiology, 1980, 6(4): 383-396.

[4]Liow LH, Reitan T, Harnik P G. Ecological interactions on macroevolutionary time scales: clams and brachiopods are more than ships that pass in the night[J]. Ecology letters, 2015, 18(10): 1030-1039.

[5]Kowalewski M, Simões MG, Carroll M, et al. Abundant brachiopods on a tropical, upwelling-influenced shelf (Southeast Brazilian Bight, South Atlantic)[J]. Palaios, 2002, 17(3): 277-286.

[6]Kowalewski, M., Hoffmeister, AP, Baumiller, TK & Bambach, RK Secondary evolutionary escalation between brachiopods and enemies of other prey. Science 308, 1774–1777 (2005).

[7]Valentine JW, Jablonski D. Larval adaptations and patterns of brachiopod diversity in space and time[J]. Evolution, 1983: 1052-1061.

[8]Stanley S M. What has happened to the articulate brachiopods[C]//Geological Society of America Abstracts with Programs. 1974, 6(7): 966-967.

[9]Rhodes MC, Thompson R J. Comparative physiology of suspension-feeding in living brachiopods and bivalves: evolutionary implications[J]. Paleobiology, 1993, 19(3): 322-334.