In the open ocean, life is a struggle for survival, and threats come in many forms, from predators to parasites. While close interactions between species, which benefit both individuals, have been recorded, those among oceanic dwellers remain mysterious. But now, researchers have found that some of the large pelagic fish use sharks to scrape off their parasites.
Tuna in the ocean (Marc Taquet / CC BY 4.0)
This groundbreaking research documents the behaviour in several oceanic fish species spanning three oceans. It describes the first recorded instances of fish using sharks and other fish to brush and scrape against, with the goal of freeing themselves of parasites.
Parasites
Parasites are known to drain energy from their hosts. They deplete the resources they need to pursue their lives, as well as for growth and reproduction. Those fish who live in the open ocean are particularly vulnerable to a wide variety of parasites, but they have few options to remove them.
The fish in the study were most commonly seen scraping their head, eyes, gill covers, and their backs, which are the areas where parasites tend to accumulate and cause the most damage. Interestingly, different species showed different scraping preferences. Tunas, for instance, tended to scrape themselves on sharks' tails, while rainbow runners scraped themselves on a variety of locations on both sharks and their own species.
Deciding who to scrape on
The study also found that the size of the fish doing the scraping correlated with the size of the "scrapee." Larger fish were more likely to scrape against sharks than other fish, suggesting that they are taking the risk of predation into account. Smaller fish appeared to be more cautious in their choices, possibly due to the danger that they could become prey.
This vital action under threat
These findings suggest that scraping behaviour may serve as a vital means for oceanic fish to reduce their parasite loads, potentially improving their fitness, and sharks are particularly useful because of their rough skin. However, there is a downside: with shark populations in decline globally, this once-beneficial interaction could be disrupted. The loss of sharks could further stress vulnerable fish species by reducing the fitness benefits associated with this unique behaviour.
This new discovery highlights the complexity of oceanic ecosystems and the often-overlooked relationships that help maintain the balance of marine life. As human activity continues to impact the oceans, understanding these interactions becomes more important than ever for protecting marine biodiversity.
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