This article is reproduced from the research of the Academia Sinica, and Pan-Science is the implementation unit of publicity and promotion.
- Interview writing / Hanbo
- Art Design / Lin Xun’an, Cai Wanjie
Fish otoliths and paleontological research
Fish otoliths are calcium carbonate crystals in the ears of bony fish, which can be sampled from fresh raw fish or fossils excavated from the ground. Academia Sinica’s “Research” has an exclusive interview with Lin Qianxiang, assistant researcher of the Biodiversity Research Center in the academy. He is a rare paleontologist in Taiwan. Before his Ph.D., he had recorded more than 1,000 species of living fish otoliths, and he continued to invest in fish. Research on otolith fossils. What information can be obtained by analyzing fish otolith fossils? From the morphology of fish otoliths, the species, body size and group structure of fish can be determined, and the life history of fish can be seen by analyzing the isotopes accumulated in the otoliths.
From comparative anatomy to fish otolith research
Assistant Researcher Lin Qianxiang of the Biodiversity Research Center of Academia Sinica, when he was studying in the Department of Life Sciences of National Cheng Kung University, realized that he did not like fields such as biomedicine, cells, nerves, physiology, etc. Therefore, he decided to take the “Comparative Anatomy” exam when preparing for the graduate school entrance examination. There is only one research institute in Taiwan that takes this subject: the Institute of Zoology, National Taiwan University.
After Lin Qianxiang entered the National Taiwan University Animal Institute, he became a rare Taoist disciple under the apprenticeship of Professor Tao Xizhen. Professor Tao Xizhen, now retired, has taught vertebrate anatomy at Taiwan University for more than 40 years and is a rare paleontologist in Taiwan. Professor Tao Xizhen’s signature research is fish fossils, while Lin Qianxiang is particularly interested in fish otolith fossils.
Since the end of the 19th century, some scholars have studied fish otolith fossils. Lin Qianxiang first came into contact with fish otoliths from a course in the Department of Earth Sciences when he was in college. There are several researchers in Taiwan who are good at fish otoliths. They are all disciples of Professor Zeng Wannian from the Institute of Fisheries Science of National Taiwan University. They all focus on modern fish, while Lin Qianxiang looks at ancient fish.
Before Lin Qianxiang went to the University of Bari (Università degli Studi di Bari Aldo Moro in Italy to study for a doctorate), together with Professor Zhang Zhiwei of the National Museum of Marine Biology, he published the “Otolith Atlas of Taiwan Fishes”, which recorded more than With 1,000 species of fish otoliths, it is already a small and accomplished researcher. But he still chose to go abroad for further studies and devoted himself to the field of fish otolith fossils.
After in-depth research on fish otolith fossils in the Mediterranean Sea and the Northeast Atlantic Ocean, and upgrading his knowledge, Lin Qianxiang can be regarded as the latest generation of experts in this field. He can finally answer the doubts that could not be solved before, and also found that there are many new topics waiting to be explored!
What is fish otolith and what is it used for? Do people also have otoliths?
Fish otoliths are the “stones” in the ears of teleosts. Of course, they are not real stones. The main component of fish otoliths is calcium carbonate, which can be sampled from fresh live fish and fossils can be excavated from the ground. There are usually 3 otoliths on each side of the inner ear of a fish’s head: sagittal, stellate, and gravel. Most commonly used for analysis, the largest volume is the sagittal stone.
When fish grow from small fry, otoliths also gradually grow together; otoliths are connected with the auditory nerve and are the auditory parts of fish; the evolutionary history and living environment of different fishes are affected by different functional requirements, and different otolith forms will be derived .
The shape of fish otoliths is mainly related to species. According to the morphological characteristics of otoliths of different fish, which species can be identified; and the closer the blood relationship is, the more similar the otoliths of fish are, and the differences in otoliths can also be compared. Determine the closeness, distance, and evolutionary relationship between fish.
Lin Qianxiang said that the identification of species by the shape of fish otoliths is similar to the teeth of mammals, just as mammal classification experts can judge cats, leopards, horses, wolves, sheep, hippos, humans and other animals based on the shape of teeth.
Do animals that are not fish also have otoliths? In fact, there are still otoliths in the ears of terrestrial animals such as humans, but the function is not hearing. The function of human hearing is replaced by the ear bone formed by calcium phosphate. The otoliths of terrestrial animals are very small, and their main function is to maintain the balance of the body. For example, the otoliths of human beings are small.
Fish get smaller and smaller?Recreating ethnic composition with otoliths
Fish otoliths can be used to identify species, but this is only the most basic purpose. Although classification is important, Lin Qianxiang emphasized that the artificially recognized classification system is constantly changing, reflecting the changes in our biological cognition; the continuous accumulation of new knowledge and new concepts will change the old framework, and that is the more meaningful knowledge progress.
A fish’s otolith grows with its body, so there is a positive correlation between the two volumes: the bigger the otolith, the bigger the fish. Collecting otoliths from many individuals of the same fish allows the body size composition of the population to be assessed.
Tracking the composition of groups over time can help grasp the evolution of the fish; if the fish are generally smaller compared to a few years ago, one can suspect habitat destruction or “overfishing” — overfishing. The staff of Lin Qianxiang’s laboratory will regularly go to Daxi Fishing Port and other places, and learn a lot of information from Xiazayu.
However, Lin Qianxiang mentioned that although the size of otoliths is related to body size, the growth rate is not necessarily. Even if the final otolith is the same size, it may grow slower or faster, that is, late maturity or early maturity; in the absence of other information, the shape of the otolith alone may not be able to distinguish the growth history, and it must be sliced and ground to read its age information. .
“Sciaenidae” is a group that Lin Qianxiang has studied in depth. The edible large yellow croaker and small yellow croaker belong to this group. Both are greatly affected by the large-scale fishing by humans, and there are also many fish farms along the coast of China. The big yellow croaker and the small yellow croaker are different fish. Even if the big yellow croaker shrinks in size, it is still the big yellow croaker. Lin Qianxiang observed from otoliths that “small large yellow croakers” appeared due to overfishing and artificial breeding, and their current age structure is also different from ancient times.
What fish are there by the reef?A comparison of ancient and modern through thousands of years
Otoliths can identify different types of fish, and a batch of samples can be used to identify how many species there are and the relative proportions (abundance) of various fish, which is the “community structure”. Environmental changes, natural or human-induced, and overfishing, which reduce the proportion of some fish and increase the proportion of others, may affect the colony structure.
During his postdoctoral research, Lin Qianxiang traveled to the Caribbean to study fish otoliths, comparing ancient and modern cluster differences. The preservation and search of ancient fish otoliths is also a university question.
After the death of fish, otoliths, teeth, bones, scales and other structures have the opportunity to become fossils and remain, and the chemical composition of different structures is different, and each has a suitable preservation environment. The composition of otoliths is calcium carbonate, which is easier to preserve together with shells and foraminifera of the same genus calcium carbonate; however, fish bones made of calcium phosphate and scallops made of calcite are different from the suitable burial conditions for otoliths and are not easy to deposit in the formation. see together.
Lin Qianxiang’s postdoctoral research topic is to study the composition and changes of coral reef fish in the Caribbean Sea. Coral reefs have always been rich in fish life, and they must leave a lot of otoliths after they die, but it is technically difficult to remove the otoliths from the coral reef limestone. Fortunately, a 7,000-year-old ancient lagoon in the Dominican Republic was blocked from the sea, and it has not yet formed a limestone reef, which has contributed many samples of otoliths and the remains of various marine life.
After comparison, it was unexpectedly found that the diversity of modern fish groups is greater than that of ancient times. The inference is that due to changes in the surrounding environment, some fish that are not related to the coral reefs have also moved into the vicinity, not that the coral reef fish population is originally so diverse. From this point of view, reefs can also be used as a source of fish otoliths to study the structure of nearby fish groups.
Stratigraphy, Archaeological Sites and Tuna Maw: Understanding Clustering Structures in Different Time and Space
If you look for otoliths in more ancient times, you will have the opportunity to learn about the state of fish in more ancient times, and even see the evolution of the same type of fish in different ages.
Lin Qianxiang analyzed the otolith fossils from the Niupu area of Chiayi in the early Pleistocene (1.22 to 1.95 million years ago) and the otolith fossils in northern Taiwan in the late Miocene (5 to 8 million years ago). Recently, another batch of marine core samples has been obtained, which can investigate the diversity and abundance of fish in the western Pacific over the past 460,000 years.
However, Lin Qianxiang reminded that although fish otoliths are easy to use, they can only reproduce part of the information of ancient fish schools. In some environmental conditions, otoliths will not remain at all, and otoliths of certain fish are not easy to remain (for example, pufferfish belong to the order Pufferfish, which are very small). It is still necessary to match teeth, bones, scales and other materials, plus other materials. Information about the ancient fishes, and the ecosystems they belong to, can be more complete.
Archaeological sites associated with human activities may also retain fish otoliths. The Nanke archaeological site group in Taiwan started from the Nanguanli East and Nanguanli ruins nearly 5,000 years ago and lasted for thousands of years. Analysis of the fish otoliths unearthed in the ruins can deduce the resource utilization of the ancients, and the environment at that time. The fish otoliths unearthed earlier were very many, but the number of fish otoliths has decreased significantly later; is this a change in the way resources are used, or is it caused by overfishing? After analysis, Lin Qianxiang’s team found that it had nothing to do with overfishing, mainly because the changes in the coastline and landforms led to changes in the way resources were utilized.
Another study took materials from the belly of bigeye tuna in the Indian Ocean, analyzed the otoliths in the belly of the tuna, and studied the composition of the small fish eaten by the bigeye tuna. The genetic analysis method “DNA barcoding” (DNA barcoding) can also tell the type of fish eaten, but Lin Qianxiang said that only otoliths can clarify the size of the small fish that the tuna eats, and thus know the size of the fish. The tuna, the food fish eaten is also relatively large.
Ingredient analysis: An in-depth look back at a fish’s life history
In addition to morphological information, otoliths grow with the fish to death, and also record the life history of a fish.
The otoliths of fish are formed gradually and contain the trace elements that entered the fish at that time. Comparing the successively formed parts of the otolith and the changes of various components, we can understand the living environment of the fish in different periods. For example, heavy metals are detected in fish otoliths, which means that there is heavy metal pollution in the water at that time.
A stable isotope is an isotope that has a very long half-life and can be considered to persist without decay. Various stable isotopes are retained in the otolith after entering the fish, reflecting the state at that time. For example, analyzing the ratio of oxygen-18 and oxygen-16 stable isotopes can estimate the water temperature at that time; and cross-comparing the stable isotopes of different elements such as carbon, oxygen, and boron can obtain environmental information such as salinity and pH value.
In addition to calcium carbonate, otoliths also have a very low proportion of protein. Both carbon and nitrogen in protein originate from food, so stable isotopes of carbon and nitrogen can record the source of fish food intake. Big fish eat small fish, small fish eat shrimp, shrimp eat algae… This is called the “trophic level”, and the stable isotope nitrogen (d15N, hereinafter referred to as nitrogen 15) will accumulate as the trophic level increases. Eating meat, the relative proportion of nitrogen-15 will be higher than eating only plants and algae.
Analysis of nitrogen 15 can assess which trophic class the fish is eating from (or the target). Some fish have different feeding objects in the juvenile stage and adult stage, and the change can be seen from the difference in nitrogen 15 .
Ideally, by synthesizing the stable isotopes of various elements in the otolith, it is possible to know information about a fish at different stages of life, such as dietary composition, water temperature, living in fresh or sea water, salinity, pH value, and whether there is pollution around.
Recreate the sounds of the ancient ocean, looking forward to more new research rookies to join!
In addition to the above research, Lin Qianxiang’s most ambitious idea may be “reproducing the sounds of the ancient ocean”. The otolith is the auditory structure of the fish, and its shape reflects the hearing of the fish. The sound is fleeting and does not leave any records, but if the connection between the shape of the otolith and the sound can be found, there may be an opportunity to deduce the sound it could hear at that time based on the shape of the otolith of ancient fish. This theme has high potential and is still in the preliminary stage.
There are many people in Taiwan who are passionate about fossil collection and research. Lin Qianxiang mentioned in the interview that he welcomes students who are curious about fish and marine life to join the marine paleontology laboratory he presides. Research is ongoing, especially for material from Taiwan and the Western Pacific. As long as researchers are serious and committed, they have the opportunity to realize themselves.
Further reading
