When brook trout (Salvelinus fontinalis) transition into small fish capable of swimming and leaving their gravel nests in the rivers of North America, they face numerous survival challenges. They must navigate obstacles, find food, and, crucially, avoid predators—including adults of their own species. Understanding how the fish’s sensory system detects these dangers and opportunities is key. The lateral line, a sensory system essential for detecting such stimuli, adapts during the trout’s development. This fine-tuning is influenced by the environmental conditions the fish experience as they grow, including temperature changes anticipated by global warming models, which are already occurring, according to a study published in the Journal of Morphology in July.
“The water is dense enough for the animal to sense objects purely through changes in flow caused by those objects,” explains zoologist Pedro Rizzato from the Institute of Biosciences at the University of São Paulo (IB-USP). “It’s like the fish’s sixth sense,” he says, referring to a distant form of tactile ability that can even discern texture differences. The receptors, known as neuromasts, are distributed on the fish’s skin and may also be found within channels beneath it. Each scale that covers the opening of a channel has a pore above and another below, forming a visible line from head to tail—hence the name “lateral line.”
This structure varies across species. The trout studied possess a less developed lateral line compared to what might be expected for fish that begin swimming at such a young age. Fish from temperate regions tend to have more receptors than those from tropical areas, and recent research has revealed variations in the structure of these sensors. “In still waters, surface receptors are more advantageous, whereas in turbulent waters, receptors tend to be concentrated within channels beneath the skin,” Rizzato explains. He also notes an intriguing hypothesis—still under debate—that this structure helps filter out water turbulence and reduce excessive sensory input.
The zoologist’s recent work has further explored how the distribution of neuromasts changes throughout the trout’s development, in collaboration with two American evolutionary biologists from the University of Rhode Island, in the United States: Aubree Jones, who conducted the research as part of her doctoral work, and her advisor, Jacqueline Webb. “I had already been influenced by [Webb’s] research, and after I presented my findings at a conference, they approached me, and we began this partnership,” Rizzato explains.
Trout, part of the salmon family, are commercially significant fish, which further intensifies interest in understanding them. The species in question inhabits temperate regions of North America and typically migrates upstream to reproduce. Its life cycle is closely tied to the changing seasons, and its development is notably slow.
According to Rizzato, in the early stages of life, the neuromasts are fully active. As the young fish detach from the riverbed and move into the water column, the current accelerates, and the receptors become engulfed by an invagination that transports them into channels. This process takes eight months from fertilization—at least six months after hatching in these particular fish. The development is slow: in some species, it can take just a month.
To investigate this process, the group studied fish ranging from newly hatched fish, approximately 1.5 centimeters (cm) in length, to juveniles nearing a year old and measuring around 8 cm. Adults of the species can grow to over 40 cm. Their observations employed a range of methodologies, including microscopic sectioning, the study of fish preserved in alcohol, scanning electron microscopy, computed tomography, and diaphanization, a technique that renders surface tissues transparent while staining bones and cartilage in distinct colors, facilitating their differentiation. Another method involved uses fluorescent dyes that specifically bind to receptors on the lateral line. “The dye attaches to the active receptors, causing them to glow,” explains Rizzato. “This allowed us to provide a detailed description that had not previously been provided for any fish species.”
Natural experiment
The fish in this study possess a unique trait that distinguishes them from the natural behavior of their species: they live in reservoirs, where they cannot migrate as they would in a river where they must swim upstream during the breeding season and then move downstream as they mature. This situation, which has occurred as the result of human intervention, piqued the interest of evolutionary biologist Aubree Jones. “I wanted to understand how dams affect fish migration,” she explains. Before comparing life in still water to that in rivers, she saw an experimental opportunity to explore how temperature influences the fish’s development.
Jones set up three experimental conditions in which the fish were raised at the region’s average river temperature, as well as at temperatures 2 degrees Celsius (°C) and 4 °C higher. “We found that the structure of the neuromasts remained intact under these conditions,” she notes. However, the size of the neuromasts was smaller, and development progressed at a faster pace. These findings are part of her doctoral dissertation, defended in September 2023, though they have not yet been published.
According to Jones, these changes could lead to a mismatch between the young trout’s development and the environmental conditions they face. “The young trout are nocturnal, which helps them avoid being preyed upon by adult trout that feed during the day,” she explains. If sensory maturation accelerates without allowing enough time for the fish to reach a more robust size, it could make them more vulnerable to predation.
When Jones began her research about six years ago, she saw the experimental temperature changes as somewhat hypothetical. But she soon realized that these conditions were becoming a reality in some of the fish’s habitats. “Things are changing so quickly that it’s hard to generate meaningful results in time,” she laments. “A 4 °C temperature increase is already on the horizon.”
The problem could be more severe than it seems, warns biologist Adalberto Val from the National Institute for Amazonian Research (INPA). “Climate change encompasses more than just rising temperatures; the increase in carbon dioxide has a profound effect on water, leading to acidification.” Fish live in the water, breathing through their gills, and their epidermal sensors are highly sensitive to changes in acidity. “Oxygen intake must occur at the correct pH,” the researcher explains. These shifts in temperature and acidity—and their impacts on fish—are precisely the focus of Val’s research, as he discussed in an interview published in September (see Pesquisa FAPESP issue nº 343).
Val is particularly concerned about the effects of these changes on Amazonian fish species. After all, these fish evolved in an equatorial environment, where conditions fluctuate little between seasons. By studying how fish from temperate regions, more accustomed to environmental fluctuations, adapt to such changes, researchers could gain insights into how the lateral line might be remodeled in tropical environments. “It would be interesting to make such comparisons for the sake of understanding,” he says.
The story above was published with the title “The sixth sense of trout” in issue 347 of january/2025.
Scientific article
JONES, A. E. et al. Development of the cranial lateral line system of Brook Trout, Salvelinus fontinalis (Teleostei: Salmonidae): Evolutionary and ecological implications. Journal of Morphology. vol. 285, no. 8. e21754. aug. 2024.
