Representational Image.

(NOAA)

What if a school of rainbow runners, on a trip in the middle of the ocean, somehow finds itself stuck with tiny crustaceans that nibble and feed on their skin? How would these fishes scratch the parasites off, with neither limbs nor cleaning stations — wrasses, shrimps or reef surfaces found near the coast — to help them scratch out?

As running back to the coast isn’t an option, these fishes do the unimaginable, i.e. seek out their predators: the sharks!

In a new study published in the journal PLOS One, researchers have found video evidence of fishes in open oceans using sharks to scrape parasites off their bodies.

After poring over thousands of hours of video footage spanning across three ocean basins — Pacific, Indian and Atlantic — the first records of scraping interactions involving tuna and rainbow runners with blue sharks and mako sharks were documented.

This scratching behaviour is mainly employed to get rid of parasites that can hamper the health of fishes and divert their energy from growth, reproduction, and other metabolic functions.

Those rubbing shoulders with the sharks were mostly bony fishes that preferred scraping their head, eyes, gill cover and lateral surfaces — areas where parasites most commonly trouble them and impact their overall fitness.

There were also differences in how these fishes approached the sharks. Tunas were the ideal bunch, orderly lining-up behind the shark and taking turns to brush against the tail. But when rainbow runners approached the shark, chaos ensued, as they would form a school around the back half of the shark and dart out to bump against its body.

But why pick one of the deadliest predators in the world? That’s because a shark’s skin feels like sandpaper (in pre-industrial times, it was actually used for that purpose). It is covered in tooth-like placoid scales, also called dermal denticles, and offers a perfect scratching surface (provided we forget about their owners’ predatory behaviour).

However, not all fishes think these sharks are similar to the friendly ‘Bruce’ from ‘Finding Nemo’. The study showed that smaller fish were less likely to scrape on bigger sharks, perhaps to avoid being turned into a meal.

While the study shows how sharks indirectly help ensure a healthy fish population, it also raises an important question. With the population of sharks dwindling in the global ocean (some species have declined by up to 92% off the Queensland coast of Australia), what would happen to ocean health if they are gone? It is likely that there would be a net loss of fitness in the bony fishes?

This study was carried out in the open oceans, which are one of the largest habitats on the planet and also the most challenging to study. The researchers intend to continue sampling offshore waters and remote regions to discover more such intriguing interactions.

Meanwhile, we must also do our bit and take the initiative to ensure the conservation of such vital species. Creation of more marine protected areas (MPA) can be a way to restore and preserve the exciting ways in which nature functions.

The study was published in the journal PLOS One and can be accessed here.

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