Growing up, you probably changed your style based on your social influences. It turns out, such pressures affect the appearance of young clownfish (anemonefish) too. A new study from the Okinawa Institute of Science and Technology (OIST) has revealed the social influences and biological mechanisms controlling bar loss in tomato anemonefish, showing how the presence of older fish changes the speed at which young fish lose their additional white vertical stripe.
Dr. Laurie Mitchell of OIST's Marine Eco-Evo-Devo Unit , first author on the new PLOS Biology publication says, "This research helps us better understand how animal color patterns have evolved to be developmentally flexible to suit unpredictable environmental conditions. It gives another level of insight and appreciation for how and why fish color patterns can form and change within a single lifetime."
Co-author Professor Vincent Laudet, head of the Marine Eco-Evo-Devo Unit adds, "Pigmentation traits like these white bars are often treated as simple visual markers, but they in fact carry rich biological meaning. This study shows how by combining ecology, evolution, genomics, and developmental biology, we can move beyond describing color patterns to understanding what they actually do."
Why do anemonefishes drop bars?
Many fish species have strict social hierarchies. For tomato anemonefish, usually only one breeding pair can occupy a particular host anemone, with younger fish taking a subordinate role in the social structure. These younger fish signal their subordinance through visual cues, such as size and color pattern.
"We've previously shown that anemonefish count bars to recognize each other . So, we know that bars, the white vertical stripes characteristic of clownfish, are essential in communication," notes Dr. Mitchell. "What's interesting is that around a third of anemonefish species have evolved to have more white bars in early development, only to lose some in a relatively short time span as they transition into adulthood. We wanted to understand how and why this change occurs."
To investigate, the researchers set up cameras to record the young fish in different environments, some with older fish in host anemones, and others with empty, fake or no anemones. Surprisingly, they found that the presence of adult fish sped up bar loss. "This was very counterintuitive at first as we know that the extra bars are used to signal their subordinance," highlights Dr. Mitchell.
The researchers believe that this time difference is caused by the intricate social hierarchy of these fish. Freshly hatched anemonefish head out to sea for a short period in their early development but eventually need to find an anemone to call their home. If this anemone is already occupied by adult fish, they will likely want to look unthreatening to avoid confrontation, until they are accepted at the bottom of the social hierarchy. However, they won't be at the lowest rank of the ladder forever. The researchers suggest that once these fish have been accepted in their society, they may lose their additional bar to cement their place before the next fish comes along.
Conversely, young fish at unoccupied anemones may keep their bars for longer to look less threatening to adult passersby. Dr. Mitchell speculates, "We can't be fully sure yet of the exact reasons that these fish lose their bars slower, but we think it's basically an insurance policy. If a marauding adult invades their anemone, there's less chance they'll be evicted if they keep their young, two-bar appearance."
Programmed cell death changes fish patterns
Alongside environmental drivers, the team were also interested in the process at a cellular level. Focusing on iridophores, the cells that cause the white color on bars, they examined cell structures under the microscope. What they found was mass cell death. "The cells shrink, their membranes wrinkle, and their nuclei fragment. These dead cells are not replaced by new iridophores. Instead, the white bar is replaced with their characteristic orange skin," explains Dr. Mitchell.
Most cellular pathway studies are on human tissues, or well-characterized model organisms, with very limited information about apoptosis (cell death) pathways in anemonefish available. However, gene expression analysis at different stages of fish growth found that genes known to be involved in cell death, such as caspase-3, were strongly expressed during bar loss. Thyroid hormone production-related expression changed in the presence or absence of adults, hinting at the potential hormonal link between social perception and bar loss.
The evolution of adaptable color patterns
To understand how bar loss developed, the researchers reconstructed the evolutionary history of this trait. "Our analysis found that bar loss in different species doesn't date back to one common ancestor. The main link between these species is social — they live in smaller groups," says Dr. Mitchell. "We don't fully understand this association yet, but it could be a protective mechanism. In larger groups, there are smaller size differences between different levels in the social hierarchies, so fights between these fish will be less dangerous. But in small groups, with only big adults and a few little subordinates, one bite could be fatal to the young fish."
By examining the evolution of this developmental flexibility, the researchers hope to reveal new insights into the origins of biodiversity. "While we've focused on changes in individual lifespans, the environmental drivers and genomic patterns of these changes are often similar at an evolutionary level. In the long term, these adaptive responses may evolve into fixed differences between species. Therefore, such studies can bring us closer to unlocking the mysteries of our diverse reef ecosystems," says Dr. Mitchell.
