According to a recent study that was published in Science of The Total Environment, future ocean warming and marine heatwaves may have an effect on the growth and development of clownfish in their early life stages.
The famous coral reef fish was raised in captivity by a group of marine biologists from the Okinawa Institute of Science and Technology (OIST) in water that was either 28°C or 31°C. Okinawa’s current summer seawater temperatures are 28°C, while contemporary marine heatwaves can reach temperatures of 31°C, which are consistent with the IPCC’s estimates of a 3°C warming by 2100.
The clownfish, Amphiprion ocellaris, was observed for 20 days after hatching, a critical stage of development that has received little attention in relation to climate change. They discovered that the larvae developed more quickly, had higher metabolic rates, and displayed alterations in the activity of specific genes under warmer temperatures.
“Most climate change studies have focused on adult or juvenile clownfish,” said Billy Moore, first author and Ph.D. student in the OIST Marine Climate Change Unit, led by Professor Timothy Ravasi. “By focusing on the larval stages, we can get a more complete picture of how clownfish could be impacted by warming oceans and climate change.”
Clownfish go through seven developmental stages over the course of their first 20 days, each of which is distinguished in the wild by unique alterations in location and behavior. Clownfish larvae are transported from their native reef and into the open ocean as soon as they hatch.
Most climate change studies have focused on adult or juvenile clownfish. By focusing on the larval stages, we can get a more complete picture of how clownfish could be impacted by warming oceans and climate change.
Billy Moore
The clownfish reach stage four after about ten days, at this point metamorphosis takes place and the fish begin to take on their recognizable orange-white hues. At around day 14, the clownfish, now in the fifth stage of development, find a new reef, and at stage six, seek out and settle into an anemone host.
In order to establish the clownfish larvae’s developmental stage, the researchers scanned, measured, and experimented with the larvae over the course of 20 days. The research group discovered that larvae raised at 31°C reached developmental milestones including metamorphosis around two days earlier than those raised at 28°C. Also, they discovered that larvae had greater average metabolic rates, which probably contributed to the faster growth rates.
However, predicting how faster development could impact the survival of clownfish under future warming is very difficult.
Moore explained, “On one hand, the majority of clownfish do not survive the larval period due to predation, so shortening this time could increase their chances of survival and settlement. On the other hand, faster development may mean the clownfish disperse to new reefs less effectively, and the larvae may not be able to take in sufficient food to meet their increased energy demands.”
The research team also investigated the activity of clownfish genes at each stage of development.
“Overall, we saw a really strong molecular response to warming,” said Moore. “At every stage, many genes more than 450 either increased or decreased in activity.”
When the researchers looked into the functions of genes that had altered activity, they found that in clownfish raised at 31°C, genes associated with heat stress were more active from the earliest developmental stages.
Researchers discovered a drop in the activity of genes involved in neurotransmission in stage seven, suggesting that warmer temperatures may influence changes in clownfish behavior.
The clownfish’s epigenome may have changed as a result of being raised in warmer environments, according to the researchers’ findings. The epigenome, Moore explained, refers to chemical alterations made across the genome, either directly to the DNA or to proteins associated with the DNA. These chemical changes, known as epigenetic modifications, affect how the DNA is packaged into chromosomes, which determines how active genes are.
In the clownfish raised at 28°C, genes involved in epigenetic modification were highly active in stages three and six. However, in clownfish raised at 31°C, the team did not observe the same activity, suggesting that these epigenetic modifications may be reduced at higher temperatures.
“We are now beginning experiments to study how the epigenome differs between clownfish raised at 31°C compared to 28°C. This could have really important implications as epigenetic changes laid down in the early stages of development impact how individuals respond to changes and stresses in the environment during their adult life,” said Moore.
