Tunas line up for scratching on sharks

Despite the danger of being eaten, tuna of various species, for personal hygiene, prefer to scratch their gills, eyes and fins on sharks, and not on their relatives. Scientists studied data from underwater stereo video systems (BRUVS) and found that tuna even line up to scratch on shark skin. Article published in PLoS One.

The fitness and survival of living organisms is significantly influenced by the symbiotic relationship between different species. Sometimes this interaction takes the most bizarre forms – for example, giant pandas use horse manure to protect themselves from the cold. However, the behavioral interactions of pelagic fish have so far remained poorly understood. As hosts of well-known and newly discovered parasites, fish that live away from reefs and coastal areas have few opportunities to get rid of pests.

Christopher Thompson of the University of Western Australia and colleagues examined BRUVS lift survey records from 36 locations around the world between 2012 and 2019. These data were collected as part of a broad program to study the ecological characteristics of pelagic communities in the Pacific, Indian and Atlantic Oceans. A variety of bony fish got into the frame, which, with varying degrees of success, tried to scratch their relatives or sharks. The sharks chosen by the fish as a scraping surface were the blue shark (Prionace glauca), the silky shark (Carcharhinus falciformis), the salmon shark (Lamnidae) and the mako shark (Isurus oxyrhinchus). At the same time, fish tried to scratch the blue shark in more than half of the cases (58.5 percent).

Pelagic fish are often prey for sharks. However, in 84 percent of cases, the fish preferred to scratch on the sharks, ignoring their relatives. For the sake of scratching about a predator, the fish were even ready to stand in line.

This behavior has been documented in yellowfin tuna (Thunnus albacares) and southern bluefin tuna (Thunnus maccoyii). These fish always approached sharks from behind, at the level of the center of the caudal fin. When the distance between the shark and the tuna was less than one body length of the latter, the tuna froze, waiting for the stroke of the shark’s tail. After that, the tuna quickly approached the shark, closing its gills and spreading its fins, and arched its body in order to cling to the shark with the largest possible body area. Then the tuna turned sharply in the other direction and quickly moved away from the shark, after which it stood in the tail of a chain of other tunas waiting for their turn to scratch. 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.

Only the skipjack tuna (Katsuwonus pelamis) did not want to scratch the shark. Scientists attribute this to the fact that skipjack tuna is significantly smaller than the rest: its body length is only 22.6-25.7 percent of the body length of sharks recorded on records. Studies of the stomach contents of sharks have shown that 75 percent of shark prey is made up of creatures whose body size is between 20 and 36 percent of the body length of a shark – which means that skipjack tuna falls into this range. At the same time, the rest of the tunas were only half the size of the shark they chose to scratch on: on average, their body length was 54 ± 2.85 percent of the shark’s length.

Fish scratched about their relatives much less often. The fish preferred to approach their relatives from behind from above, choosing the blind zone of the one they were going to scratch on. Even so, in more than half (75 percent) of the cases, the target fish avoided the impact by trying to evade or swim away.

Scientists attribute this unwillingness to help relatives to the fact that the same species of fish is exposed to the same parasites. Accordingly, those parasites that one fish tried to get rid of during friction can immediately attach to another. Moreover, the friction-damaged mucous layer on the skin of the fish also reduces the protection against parasitic organisms.

Obligate ectoparasites tend to be very host-specific – meaning that sharks may not share parasites with tuna at all. In addition, shark skin, used as sandpaper in the pre-industrial era, is made up of small tooth-like structures called dermal denticles. Its abrasive properties provide an ideal surface for fish to get rid of ectoparasites and dead skin cells.

Climate change and human activities significantly affect the life and reproduction of fish. Scientists suggest that the extermination of sharks can negatively affect the population of fish that are the object of fishing, as it disrupts existing symbiotic relationships.

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