Ostracods: Fossil time machines into past and future ecosystems

Written by Emma Cieslak-Jones

Ostracods are a class of microfossil from the subphylum crustacean. The name originates from the Greek ‘Ostrakon’ which translates to ‘shell’ and refers to their appearance which is bivalved meaning the carapace (shell) is made up of two components joined by a hinge. The small, calcified carapace preserves well within the fossil record making them excellent indicators for paleobiological studies.

While ostracods are known in the fossil record they can also be found alive today. They can be analyzed to help understand long-term impacts that humans have had on ecosystems as they can be studied both before and after the appearance of mankind. A temporal comparison can be made to identify ecosystem changes and the main environmental drivers.

Here, we discuss such a study looking at Hong Kong, where the monsoon climate and pollution are key factors. Here, the situation is like an accelerated version of what the future could hold for other tropical and subtropical coastal regions with increasing human-induced ecological degradation, making the results of this study extremely important. The research is being carried out by Moriaki Yasuhara — paleobiologist and associate professor at the University of Hong Kong and Yuanyuan Hong — ostracod paleontologist and post-doctoral fellow from the University of Hong Kong.

Above: An image of ostracod slide by Keyence digital microscope. Image credit: Jiamian Hu, Yuanyuan Hong, and Moriaki Yasuhara.

How are ostracod fossils used for conservation paleobiology?

A main aim of conservation paleobiology is to understand biotic responses to past, present, and future anthropogenic and climatic changes using fossil records. Because biological monitoring generally postdates the onset of different human activities (e.g., on-going warming, pollution, overexploitation) that are of concern to scientists and society, fossils records provide “time machine” for scientists to travel back to deeper time; to early- and pre- human periods that biological monitoring does not cover, and allow us to study long-term human impacts, humanecosystem interactions as well as natural baseline of the ecosystem of our planet.

Ostracods are one of these fossils, which are a class of small crustaceans, typically hundreds micron or around one millimeter in size. Their soft bodies are protected by bivalve-like, calcified valves. These ostracod valves are well preserved as fossils, which are useful for quantitative conservation paleobiology, because of their small size, high abundance, and high species, phylogenetic, morphological, and functional diversity. Most of ordinary marine organisms do not have such good fossil records if any. So, we use ostracods as a model system or “window” to look into past ecosystem and biodiversity. For example, we compared fossil (= natural baseline) and present-day ostracods quantitatively, to investigate changes in species, communities, and ecosystems. As the result, we were able to detect and identify changes in Hong Kong’s ecosystem and the main environmental drivers, that were the monsoon climate and pollution.

Below: Scanning Electronic Microscopic images of lateral view and internal view for ostracod specimens A1&2) Loxoconcha zhejiangensis: right valve (RV), juvenile; B1&2) Neocyprideis sp.: left valve (LV), adult; C1&2) Neocytheretta faceta: RV, adult; D1&2) Neomonoceratina delicata: LV, adult. Scale bar = 100 μm. Image credit: Yuanyuan Hong and Moriaki Yasuhara.

In your 2021 study published in Anthropocene, why did you choose to study sediments in Hong Kong?

Hong Kong is one of the most urbanized coastal areas in the world. Human activities, including rapid industrialization, sewage discharge, trawling, dredging, land reclamation, and ship traffic have led to pronounced physical and chemical changes in Hong Kong’s coastal ecosystems. Situated at the mouth of one of the largest rivers in Asia, the Pearl River, Hong Kong is sensitive to monsoons, resulting precipitation change, and other climate variations related to river discharge as well as pollutants via this river. In addition, Hong Kong is situated at the northern edge of the Coral Triangle, a biodiversity hotspot wellknown for the highest marine biological diversity on Earth. The intersection of high biodiversity and elevated anthropogenic and climatic stressors makes Hong Kong’s coastal waters a conservation priority and a model for evaluating ecosystem-level changes in the tropics. The current Hong Kong situation is the plausible future of many other tropical and subtropical coastal regions with increasing human-induced ecological degradation.

What key impacts from human activity did you observe on benthic ecosystems in that region?

One is pollution. Hong Kong is known as one of the largest coastal cities in the world and thus has one of the world’s busiest ports. Copper, lead, and zinc are components included in most historical and contemporary marine antifouling paints and are known to be among the most toxic metals to marine invertebrates. The central part of Hong Kong has also been seriously affected by metal pollution via the discharge of sewage, industrial inputs, and surface runoff from the urban environment.

Our result indicated that these metals from antifouling paints and/or other urban and industrial sources have substantially altered marine benthic environments in Victoria Harbour, which is the major port and surrounded by two big downtown areas, Tsim Sha Tsui and Causeway Bay. Hong Kong’s eastern waters are much less polluted than the central and western waters. However, the eastern-water ecosystem was found to be much more sensitive to climate change. Future anthropogenic warming may reduce global ocean circulation, that may enhance the East Asian Summer Monsoon.

So, organisms in Hong Kong’s eastern waters may confront enhanced discharge from the Pearl River and resulting lower salinity, higher turbidity, and muddier environments in the near future. Our research indicates that these environmental changes will especially affect rare species. Most species in highly-biodiverse tropical and subtropical places like Hong Kong are rare, so the anticipated changes may have a profound impact.

Tell us more about the ostracod micropaleontology research going on at the Yasuhara lab and current opportunities.

Yasuhara Lab works broadly on biodiversity and paleontology. We recently dub this line of research using fossils to understand past, present, future biodiversity and ecosystems as Time Machine Biology. We of course most often use microfossil Ostracoda as the model system.

A major new direction is deep-learning based automation of detection, identification, and measurement, that will speed up our process and allow us to use bigger data to tackle fundamental questions, for example on conservation paleobiology in Hong Kong. Tropical biodiversity has been one of our main interests for the past 5-10 years.

We are getting good ostracod data from various tropical regions such as the Coral Triangle, Caribbean Sea, and Tethys Sea as old as Eocene to the present.

Hopefully these studies will bring us to a new and better understanding of tropical diversity and the reason(s) of their high biodiversity.

We are also interested in deeper time biodiversity, starting a project on Ordovician latitudinal diversity with a new PhD student. We are always open to ostracod lovers and more broadly, enthusiastic paleobiologists.

Various Mphil, PhD, and postdoc opportunities are available from the University of Hong Kong, Hong Kong Government, and other external grant agencies. Please join us!

The effect that humans and the climate have had (and continue to have) on ecosystems in Hong Kong can be assessed using the ostracods found there. By analyzing the pre and post impacts, a model can be created that can be applied to other tropical and subtropical coastal regions to help limit further human-induced degradation and ultimately, provide solutions to counter the negative impact.

Hong Kong provides an example of what happens to ecosystems after extensive pollution and climatic effects. It can be used as an educational resource to predict the future and mitigate against potentially disastrous effects. Just as fossils can be a time machine to the past, case studies like Hong Kong can be a time machine for the future.

Moriaki Yasuhara Moriaki Yasuhara is a paleobiologist and associate professor at the University of Hong Kong. His recent research has focused on the tropical biodiversity, the spatio-temporal dynamics of large-scale biodiversity patterns, and their mechanisms behind. He is also interested in microfossil-based conservation paleobiology and paleontology of the Ostracoda in general.

Yuanyuan Hong Yuanyuan Hong is an ostracod paleontologist especially focusing on the Anthropocene and Quaternary, currently working as a post-doctoral fellow in the University of Hong Kong. Her primary research interests are on conservation paleobiology and marine paleoecology. Recently, she has been developing a new research direction, deep-learning-based automation in micropaleontology.

Website: moriakiyasuhara.com

Twitter: @DrMoriartyY

Moriaki Yasuhara email: yasuhara@hku.hk

Yuanyuan Hong email: oocirclr@gmail.com