Life-history and connectivity shape the viromes of a migratory and river-resident galaxiid
This study reveals that host life-history strategies, particularly longevity and cumulative environmental exposure, significantly shape viral diversity in freshwater galaxiids, as the river-resident *Galaxias gollumoides* harbored a far richer virome than its amphidromous counterpart *Galaxias maculatus*, challenging the assumption that migration drives higher viral transmission.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Fish move through rivers in ways that shape their entire lives. Some species, like the īnanga, are born in freshwater but drift out to the ocean as tiny larvae before returning to the rivers to grow and reproduce. This journey connects different populations, mixing their genes and allowing them to travel across vast distances. Other fish, like the Gollum galaxias, spend their entire lives in the same small streams, never leaving the freshwater they were born in. These river-dwelling fish are often isolated from one another by waterfalls or man-made barriers, living in small, separated groups. Scientists have long wondered how these different lifestyles affect the invisible world of viruses that live inside and around these animals. While migration helps fish find food and mates, it also exposes them to new environments and potentially new germs. The question is whether the fish that travel far carry more viruses than the ones that stay home, or if the opposite is true.
To answer this, researchers in southern New Zealand set out to compare the viral communities of these two very different fish. They focused on the īnanga, a widespread migratory species, and the Gollum galaxias, a non-migratory species that lives in isolated headwater streams. The team collected hundreds of fish from twenty-seven different locations across the region, capturing them both above and below barriers that might stop fish from moving up or down the river. They gathered tissue from the gills and livers of the fish, which are common places for viruses to hide, and used advanced genetic sequencing to read the viral RNA present in the samples. This method allowed them to identify which viruses were present without needing to grow them in a lab, providing a direct look at the viral "virome" of each fish.
The results were surprising and overturned the researchers' initial expectations. They had predicted that the migratory īnanga, with their long journeys and contact with many different environments, would carry a wide variety of viruses. Instead, the migratory fish showed almost no signs of infection. Out of twenty-eight groups of īnanga tested, only a single group contained a fish-associated virus. In contrast, the river-resident Gollum galaxias, which live in isolated pockets, showed a higher presence of viruses, though they were still relatively uncommon across the sampling range. Twelve out of twenty-six groups of these fish carried at least one virus, but most pools contained only a single viral family, with only one pool containing two families. The most common viruses found in the Gollum galaxias belonged to a family called Circoviridae, which dominated their viral landscape.
The study also revealed a crucial pattern related to age. The researchers looked at young, post-larval fish of both species and found no detectable viruses in any of them. This suggests that these fish do not pick up these specific viruses as babies in the ocean or in the early stages of their life in the river. Instead, the viruses appear to accumulate over time. The Gollum galaxias live for several years, giving them a long window to encounter and harbor viruses from their environment. The īnanga, which typically live for only about a year, simply do not have enough time to build up a similar viral load. This finding points to the length of time a fish spends in its environment as a major driver of its viral diversity, rather than the distance it travels.
Furthermore, the researchers examined whether the barriers in the rivers, such as waterfalls or weirs, affected the spread of viruses. They compared fish caught upstream and downstream of these obstacles and found no significant difference in the viral communities on either side. This indicates that the physical barriers blocking the fish's movement were not the primary factor shaping which viruses they carried. The differences in viral makeup were driven more by the species themselves and how long they lived than by the specific geography of the river or the ability to move freely.
Ultimately, this research highlights that the life history of a fish—how it moves and how long it lives—plays a fundamental role in determining the viruses it carries. The study suggests that the migratory lifestyle of the īnanga does not necessarily lead to a higher risk of viral infection, while the long, stationary lives of the Gollum galaxias allow them to accumulate a richer and more diverse set of viruses, despite viruses being generally uncommon in both species. These insights help scientists understand how viruses spread in freshwater ecosystems and how the unique behaviors of different species influence their health. As researchers continue to explore the hidden viral worlds of New Zealand's native fish, they are learning that staying in one place for a long time might be just as significant for viral exposure as traveling across the world.
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