By analyzing video records of over 100,000 individuals across the Pacific, Indian and Atlantic Oceans, Christopher D H Thompson and Jessica Meeuwig from the University of Western Australia found that big fishes living in the open ocean, like tunas, prefer to scrape on sharks, rather than on other fish species.

"Tunas were quite orderly, lining up behind the shark and taking turns to brush against the tail. Rainbow runners were unruly, forming a school around the back half of the shark and darting out in turns to bump against its body," the researchers said, when detailing different scraping behaviors between various fish species.

They discovered that pelagic fishes, being hosts to a diverse array of parasites but with few options for removal, tended to scratch their heads, eyes, gill covers and lateral surfaces -- areas most heavily impacted by parasite damage.

Parasites can negatively affect the fitness of their hosts by draining resources and diverting energy from growth, reproduction and other bodily functions, according to the study. As shark skin is made up of small tooth-like structures called dermal denticles, it serves as an ideal surface to scratch against for removing parasites and dead skin, which can help increase the fitness and survival of other fishes.

In an article released by the Conversation on Thursday, Thompson and Meeuwig expressed their concern over the rapid decline of shark populations, mentioning that some species have shrunk by up to 92 percent off the Queensland coast of Australia.

"The continued decline of shark populations could have knock-on effects through the loss of relationships such as those we describe," the marine experts warned.

"Marine protected areas have been shown to conserve behaviors in sharks and fishes. The introduction of more of these areas could help restore and preserve these behaviors," they added.

Xinhua

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