Cross-breeding, cryopreservation (freezing) of semen and creating a bank of reproductive cells and coral larvae are all examples of scientific strategies to enhance the resilience of coral reefs and protect these invaluable ecosystems from the negative effects of climate change.

– Due to climate change and other anthropological factors, we have already lost more than half of all coral reefs. Although corals have remarkable mechanisms of resilience, the rate of climate change (increasing water temperature and decreasing pH level) exceeds their natural ability to adapt – emphasises dr. Radosław Kowalski from the Department of Gamete and Embryo Biology of the Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences in Olsztyn.

The scientist investigates reefs near the Japanese island of Okinawa. He has been studying coral reproduction for more than a decade. – Back then, the topic of coral reef conservation was niche. Today, after a growing number of publications and projects, it is becoming clear that the problem is severe, and the need to find a strategy to protect these invaluable ecosystems supporting the diversity of marine life is indeed urgent – he adds.

The researcher points out that besides an increase in water temperature, a significant negative impact on coral reefs is also caused by a decrease in the ocean’s pH, i.e. its acidification. This is linked to an increase in carbon dioxide emissions into the atmosphere, which then dissolves in the oceans. – This is why, in recent decades, the natural pH level of the oceans has dropped from 8.2 to 8.1. However, this inconspicuous one-tenth means a one-third increase in ocean acidification! Corals build their skeletons from calcium carbonate, which is 'taken up’ from the water, but this mechanism stops at a pH of 7.9. In addition, water acidification reduces the reproductive capacity of corals – says Radosław Kowalski.

And although corals look more like plants, they are animals (invertebrates). They are found in equatorial regions.


Modern coral reefs, including the Great Barrier Reef in Australia, formed about 8-10,000 years ago. As the reefs are mainly found around the equator, where the annual temperature changes were not very pronounced, their biological cycle synchronised with the phases of the moon – depending on the species, they approach spawning on a specific full or new moon counting from the beginning of the year.

– This synchronisation allows these non-migratory animals to ensure contact with gametes (reproductive cells) of the same species by releasing them en masse into the ocean, usually during a single night. However, climate warming has resulted in temperatures during the coral spawning season that their physiology has not previously encountered. And while thermals did not previously play a major regulatory role in coral reproduction, we are now seeing a significant modulating effect. For this reason, unsynchronised coral spawning is now occurring annually on the reef – the researcher points out.

This is resulting in a noticeable increase in interspecies hybrids. An example is the crossing of species from deeper parts of the ocean, where there is cooler water, with those from shallower parts with warmer water. The result is a hybrid with mediated characteristics that is able to inhabit larger spaces. – This fascinating mechanism shows how corals try to adapt to change –  explains Radosław Kowalski.

Interspecies breeding also has considerable potential as a tool for scientifically assisted evolution, as combining the gametes of different coral species offers the possibility of obtaining a hybrid, e.g. with increased thermal tolerance (to changes in water temperature) or with specific desirable adaptive traits to changing environmental conditions.

Dr. Radoslaw Kowalski is conducting such research with a team of Japanese scientists from the University of the Ryukyus in Okinawa.

Recently, the researcher has been focusing on the cross-breeding possibilities of aquarium-reared species.
– From my observations, I have noticed that corals bred in aquaria are subject to enormous environmental pressure, so that there is a lot of natural selection, and only the strongest individuals survive. We want to cross naturally occurring species with those bred in aquaria and see if their hybrid will be more adaptable – he says.


Another strategy to protect coral reefs is the cryopreservation of coral sperm, i.e. their storage at ultra-low temperatures. This method enables the preservation of genetic material, acting as a genetic insurance policy that can be used in the future to restore and rebuild coral reefs.

– Creating a bank of gametes and coral larvae is one of the biggest challenges for scientists working on this topic. We are already able to cryopreserve sperm, but after thawing, the eggs are still needed for fertilisation. With larvae, there would not be such a problem, but here we still need to improve the method of freezing them – Radosław Kowalski points out.

The semen cryopreservation method can also be helpful in the process of crossbreeding between species, for example, by allowing semen taken from resistant individuals to be transported from aquaria to coral reefs, where it can be used to create more resistant individuals of a particular species.


– The state of coral reefs around the world is critical; these ecosystems are on the brink of collapse. Research into strategies to protect reefs is therefore needed as never before. However, I always stress that even the most cutting-edge scientific solutions are no substitute for our everyday actions, such as saving energy or water, which can make a real difference in protecting our planet – Radosław Kowalski concludes.

Data publikacji: 22.03.2024