How can the ear bones of fish help scientists determine the change of a coral reef’s fish community over time?

Reconstructing reef fish communities using fish otoliths in coral reef sediments.

By:  Chien-Hsiang Lin, Brigida De Gracia, Michele E. R. Pierotti, Allen H. Andrews, Katie Griswold, Aaron O’Dea

Summarized by Erica Fancher, a geology major at the University of South Florida. Currently, she is a senior. Although she is undecided about what field she wants to enter, she plans to attend graduate school later when the time is appropriate. When she is not studying geology, she enjoys spending time outside, reading, and being with her family. 

What data were used? In this study, the authors wanted to determine if fish otoliths (ear bones) found in coral reef sediments were an effective way of recreating the diversity of ancient fish communities. The authors stated that little is known about how the fish communities among coral reefs has changed over time, and there has not been a viable way of conducting the research. Fish otoliths were thought to be too rare to provide any significant data to research like this, until the authors sorted through coral reef sediment and found a significant number of otoliths. Juvenile otoliths were found to be indistinguishable between each other, so the authors were unable to use them in their study.

Methods: Before the authors could conduct a study, they first had to see if otoliths were a viable option for collecting data. Their hypothesis was: are scientists able to use the ear bones of fish to effectively see a change over time in ancient fish communities? To do this, the authors collected coral reef sediment from the two regions of Caribbean Panama and the Dominican Republic. Both regions had coral reefs from the mid-Holocene (the last 11,700 years) and sub-recent periods that were used in the sampling; this gave the scientists a total of four sediment samples. After the sediment was dried and weighed, they used mesh screens of varying sizes to find otoliths, the ear bones of fish. Using specific references and other guides, they sorted over 4,000 otoliths into their respective families and sometimes were able to identify the genus. After the otoliths were sorted, the authors compared the diversity and species richness among the sample reef regions and time periods.

Results: The authors concluded that using fish otoliths was an effective way of seeing a change in diversity among fish communities in coral reefs. Due to the high amount of predation in fish communities, there was an abundance (over 5,000) of otoliths found, 4,000 of which were able to be properly sorted and used to identify change in communities over time. 

The number of otoliths identified from each sediment sample. (Note that otolith counts are log-transformed.)

Why is this study important?  This study will help scientists have a clear understanding of not only coral reefs, but of the diversity that surrounds them. This research can now be replicated and applied to further investigations. Although the data of juvenile otoliths cannot be used due to indistinguishable similarities, there is still a significant number of otoliths available for study due to the natural predation among fish. Research of this nature is important in paleontology because it forged a new way of gathering information on a topic that was not well known beforehand. In this field, scientists often have limited material to work with with and the fossil record is biased because of how difficult it is for organisms of the past to become fossilized. Because of this study, the authors made available information that scientists will now be able to use in further research on similar topics. 

The big picture: Because of the outcome of this study, scientists are now able to use the information gained to apply more knowledge of paleoecology, climate change, and patterns of diversity to not only coral reefs, but the fish community around them. Coral reefs and the fish communities that inhabit them have a mutually beneficial relationship that allow each to survive and flourish. If scientists wish to continually study coral reefs and their importance to Earth’s oceans, understanding how the fish are affected through time is a vital piece of knowledge as well. As sorting guides and references improve, our taxonomic resolution will become better. This study, and all science studies in general, are a vital process in the science community because without new hypotheses and new methods of testing those hypotheses, science would not progress. This process allows new ideas to reach countless amounts of people and can spread data and methods that could further another’s research creating a ripple effect of knowledge.

Citation: Lin, C. H., De Gracia, B., Pierotti, M. E., Andrews, A. H., Griswold, K., & O’Dea, A. (2019). Reconstructing reef fish communities using fish otoliths in coral reef sediments. PloS one14(6), e0218413.

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