Despite the risk of being eaten, tunas scratch themselves on sharks

(Thompson et al. PLoS One, 2022 https://bit.ly/3TPCVPM)

Hygiene is important… even for fish. That’s why tunas scratch their gills, eyes and fins on sharks.

A team of researchers from the University of Western Australia studied data from underwater stereo video systems (BRUVS) and found that tuna makes lines to scratch on shark skin.

The fitness and survival of living organisms is influenced by the symbiotic relationship between different species.

However, scientists still don't understand very well the behavioral interactions of pelagic fish.

For example, fish that live away from coastal areas and reefs have few opportunities to get rid of parasites.

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CAUGHT ON CAMERA
Now, the team led by Christopher Thompson examined BRUVS lift survey records from 36 locations around the world between the years 2012 and 2019.

These surveys were collected as part of a program to study the ecological characteristics of pelagic fish communities in the Atlantic, Pacific, and Indian Oceans.

Different types of bony fish were captured trying to scratch on sharks or their relatives .

The tunas used these sharks as a scraping surface:

• the blue shark (Prionace glauca)
• the silky shark (Carcharhinus falciformis)
• the salmon shark (Lamnidae)
• the mako shark (Isurus oxyrhinchus)

At the same time, fish tried to scratch on the blue shark in 58% of the cases.

Pelagic fish are often prey for sharks, but in 84% of cases, the fish preferred to scratch on the sharks, ignoring their relatives. They were even ready to stand in line.

This behavior has been documented in southern bluefin tuna (Thunnus maccoyii) and yellowfin tuna (Thunnus albacares).

These fish always approached the sharks from behind, at the level of the center of the caudal fin.

When the distance between the tuna and the shark was less than one body length of the first, the fish froze, waiting for the stroke of the shark's tail.

Then, the tuna quickly approached the shark, closing its gills and spreading its fins.

After that it arched its body in order to scratch on the shark with the largest possible body area.

Then the fish turned quickly in the other direction and moved away from the shark. Finally, the tuna stood in the tail of a line of other tunas waiting for their turn.

Rainbow mackerel (Elagatis bipinnulata) turned out to be less organized in this regard.

They formed a flock near the shark's tail and from time to time swam randomly closer to the shark, exposing various parts of the body.

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THE EXCEPTIONS
Only the skipjack tuna (Katsuwonus pelamis) did not want to scratch the sharks.

Thompson’s team attribute this to the fact that skipjacks are significantly smaller than the rest of other tuna: its body length is only 22.6%-25.7% of the body length of sharks recorded.

Studies of the stomach contents of sharks have shown that 75% of shark prey is made up of creatures whose body size is between 20% and 36% of the body length of a shark.

Unfortunately for the skipjack tuna, they fall into this range of size. Also, the rest of the tunas were only half the size of the sharks.

The caught fish scratched on their relatives much less often.

The tuna preferred to approach their relatives from behind and above, choosing the blind zone of the fish they were going to scratch on.

Even so, in more than half (75%) of the cases, the target fish avoided the impact by trying to evade or swim away.

Scientists attribute this unwillingness to help relatives because the same species of fish is exposed to the same parasites.

Accordingly, the parasites that one fish is trying to get rid of, can immediately attach to another during friction.

Even more, the friction-damaged mucous layer on the skin of the fish also reduces the protection against pests and parasites.

Obligate ectoparasites tend to be very host-specific, which means that sharks may not share parasites with tuna at all.

The shark skin is made up of small structures called dermal denticles that are similar to teeth.

The abrasive properties of denticles provide an ideal surface for fish to get rid of ectoparasites and dead skin cells.

Source:

• PLoS One: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0275458
• NewScientist: https://www.newscientist.com/article/2343044-tuna-use-sharks-as-back-scratchers-despite-risk-of-being-eaten/

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