Growing up, many of us have experienced the influence of social dynamics on our personal style. Interestingly, similar social pressures also affect the appearance of young clownfish, scientifically known as anemonefish. A recent study conducted by the Okinawa Institute of Science and Technology (OIST) has unveiled the social influences and biological mechanisms behind the loss of white stripes in tomato anemonefish. The research highlights how the presence of older fish accelerates the disappearance of these distinctive stripes in younger fish.
Dr. Laurie Mitchell, the first author of the study published in PLOS Biology and a member of OIST’s Marine Eco-Evo-Devo Unit, explains, “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.”
Understanding the Role of Social Hierarchies
Many fish species, including the tomato anemonefish, operate within strict social hierarchies. Typically, only one breeding pair occupies a host anemone, while younger fish assume subordinate roles. These younger fish communicate their status through visual cues, such as size and color patterns.
Dr. Mitchell notes, “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. 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.”
Research Methodology and Findings
To explore this phenomenon, researchers set up cameras to observe young fish in various environments—some with older fish in host anemones, others with empty or fake anemones. Surprisingly, they discovered that the presence of adult fish accelerated the loss of bars. “This was very counterintuitive at first as we know that the extra bars are used to signal their subordinance,” highlights Dr. Mitchell.
The researchers propose that this accelerated bar loss is influenced by the complex social hierarchy of these fish. Freshly hatched anemonefish initially venture out to sea but eventually need to settle in an anemone. If the anemone is already occupied by adult fish, the young fish may want to appear non-threatening to avoid confrontation and secure their place in the social structure. Conversely, young fish at unoccupied anemones might retain their bars longer to appear less threatening to adult passersby.
Cellular Mechanisms Behind Color Changes
Beyond environmental factors, the study also delved into the cellular processes involved in bar loss. The focus was on iridophores, the cells responsible for the white coloration of the bars. Under microscopic examination, researchers observed 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.
Gene expression analysis revealed that genes associated with cell death, such as caspase-3, were highly expressed during bar loss. Additionally, changes in thyroid hormone production were noted in the presence or absence of adults, suggesting a potential hormonal link between social perception and bar loss.
Evolutionary Insights and Biodiversity
To comprehend how bar loss evolved, 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.
This association could be a protective mechanism. In larger groups, size differences within social hierarchies are minimal, reducing the risk of dangerous conflicts. However, in smaller groups, the disparity between large adults and small subordinates could make young fish vulnerable to fatal attacks.
By examining the evolution of developmental flexibility, the researchers aim to uncover 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,” concludes Dr. Mitchell.
