Peer Pressure: A Faster Stripe Loss in Baby Clownfish
Have you ever changed your style to fit in with your friends? Well, it turns out that young clownfish (anemonefish) do something similar. A recent study from the Okinawa Institute of Science and Technology (OIST) has uncovered the fascinating social influences and biological mechanisms that control the loss of white vertical stripes in tomato anemonefish. This research reveals how the presence of older fish can accelerate the process of young fish shedding their distinctive stripes.
Dr. Laurie Mitchell, from OIST's Marine Eco-Evo-Devo Unit, explains, "This study helps us understand how animal color patterns can adapt to unpredictable environments. It provides valuable insights into the development and changes in fish color patterns during their lifetime."
Professor Vincent Laudet, the head of the same unit, adds, "Pigmentation traits like these white bars are more than just visual markers. They carry significant biological meaning. By combining ecology, evolution, genomics, and developmental biology, we can go beyond describing color patterns and understand their functional significance."
Why do anemonefish lose their bars?
Tomato anemonefish have a strict social hierarchy, with only one breeding pair typically occupying a host anemone. Younger fish take on a subordinate role, signaling their position through visual cues like size and color patterns. Interestingly, around a third of anemonefish species have evolved to have more white bars in early development, which they later lose as they mature.
Dr. Mitchell notes, "We've previously shown that anemonefish use bars to recognize each other. The extra bars are a way for them to communicate. What's intriguing is that the presence of adult fish speeds up the loss of these bars."
The researchers found that the presence of adult fish in host anemones accelerated the loss of bars, which was counterintuitive given that the extra bars signal subordinance. They hypothesize that this phenomenon is linked to the intricate social hierarchy of these fish. Young anemonefish need to find an anemone to call home, and if the anemone is occupied by adults, they may want to appear less threatening to avoid confrontation. However, once accepted into the social hierarchy, they might lose their extra bars to establish their position.
Conversely, young fish at unoccupied anemones may retain their bars longer to avoid attracting attention from adult fish. Dr. Mitchell speculates, "We're still unsure of the exact reasons, but it's like an insurance policy. If an adult invades their anemone, they're less likely to be evicted if they maintain their two-bar appearance."
Cellular Changes and Bar Loss
The researchers also investigated the cellular level, focusing on iridophores, the cells responsible for the white color on bars. They observed mass cell death under the microscope, where cells shrink, membranes wrinkle, and nuclei fragment. These dead cells are not replaced, and the white bar is replaced with orange skin.
Gene expression analysis revealed that genes associated with cell death, such as caspase-3, were highly expressed during bar loss. Thyroid hormone production-related genes also showed changes in the presence or absence of adults, suggesting a potential hormonal link between social perception and bar loss.
The Evolution of Adaptable Color Patterns
To understand the evolutionary history of bar loss, the researchers reconstructed the trait's evolutionary timeline. They found that bar loss in different species doesn't date back to a common ancestor but is associated with living in smaller groups. Dr. Mitchell explains, "We're still unraveling this connection, but it could be a protective mechanism. In larger groups, social hierarchies are less distinct, making fights less dangerous. However, in small groups, with big adults and few subordinates, a bite could be fatal."
By studying the evolution of this developmental flexibility, the researchers aim to uncover the origins of biodiversity. Dr. Mitchell concludes, "Our focus on individual lifespan changes has led us to similar environmental and genomic patterns at an evolutionary level. These adaptive responses may become fixed differences between species over time, helping us understand our diverse reef ecosystems."
