A new Israeli study has revealed that there are reef fish that beat their fins all night to inject oxygen-rich water into the coral in which they live
Marit Sloin, Haaretz, Walla News
Margin coral fish outside the coral branches, during the day. They never seem to sleep. Photo: Emzia Ganin
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A chance observation that occurred in the waters of the Gulf of Eilat led to the discovery of a new type of symbiosis between fish and coral in the Red Sea, and was published a week ago in the "Editor's Choice" section of the journal "Science". A group of researchers from the Inter-University Institute of Marine Sciences in Eilat noticed during one of their night dives an unusual phenomenon: fish that live permanently near the corals and stay among their branches throughout the night were behaving in a strange way. Instead of sleeping motionless, as most reef fish do, the fish beat their fins hard.
"This strange phenomenon intrigued us and we decided to check what is behind it," says Prof. Ametsia Ganin, head of the Department of Evolution, Systematics and Ecology at the Hebrew University, who headed the group. "At first we thought that the flapping of the fins was in response to the light of the flashlight with which we illuminated the fish during the night dive. To test this, we asked Mordechai Ohaviya, the optical development person in our laboratory, to build an underwater video system with infrared lighting that the fish cannot distinguish, and thus we could watch on the video screen in the laboratory the natural movement of the fish on the reef at all hours of the day.
It soon became clear that the fish were not resting for a moment. They flap their fins relentlessly throughout the night, until they emerge from the coral at dawn. The processing of the video data also showed that the fish in the group divide the coral area into zones, so that each fish has its own zone and all parts of the coral are exposed to the movement of the fins. Most of the movement is done when the fish stays in place, and thus it mainly causes the water to move and not the fish to move.
"Such a considerable investment of effort on the part of the fish and the existence of the phenomenon in different species of fish testified to a significant ecological gain for the fluttering fish, thanks to which this strange behavior developed during evolution," says Ganin. But what is this profit?
Within the world's coral reefs, there are complex and close interactions between different creatures: symbiosis (cooperation in which each party benefits), parasitism, competition and predator-prey relationships. Several small fish species, among them the marginal coral, the striped coral and the greenish chrome, organized in social groups of 20-2 individuals, live in close proximity to the corals in a way that indicates the existence of symbiosis. Each group of fish has its own coral, near which it resides for many months, maybe even years. The fish find crazy hiding places among the coral branches. On the other hand, it is not clear what the fish gives the coral. "In 1995, we found that a coral with fish living between its arms grows much larger than a coral that lives without fish, which means that the fish undoubtedly give something to the coral," Ganin says.
The coral is an animal that feeds on plankton (tiny crustaceans and other tiny creatures that float in the water and are carried by the currents) and organic matter produced in the process of photosynthesis by single-celled algae that live within the coral tissue. "At first we thought that the fish fertilizes the coral environment with its secretions and thus increases the photosynthesis rate of the algae. It is also possible that the fish protects the coral from crazies. However, these two hypotheses were not tested, and beyond that, we did not see a connection between them and the tremendous effort we saw the fish put into flapping its fins throughout the night," says Ganin.
The team of researchers, which included graduate student Rotem Goldschmid, doctoral student Roi Holtzman from the Hebrew University, and fish movement expert Prof. Danny Weiss from the Technion, proposed a new hypothesis: the fish may be flapping their fins to "ventilate" the coral by increasing the flow of water over it. "We measured the currents between the coral branches with and without fish and saw that the fish greatly increase the flow of water between the coral branches. We also found that the coral significantly slows down the flow of sea water inside it - most of the water moves around it and does not enter between its branches," says Ganin.
"We connected one to another and came up with this hypothesis: it is known that the coral is an animal that consumes oxygen. During the day he has no problem because the algae in his body carry out photosynthesis and provide him with a lot of oxygen. But at night, when there is no light, the photosynthesis process does not take place, but the coral and the algae continue to breathe and consume the oxygen in the environment. When the sea currents are weak, the water between the coral arms does not exchange and the oxygen concentration there drops rapidly. The movement of the fish at night can solve the problem. It is possible that the rapid flapping of the fins causes an efficient exchange between the oxygen-poor water between the coral branches and the oxygen-rich water outside it, and thus the fish can ensure a high oxygen supply to the coral and to themselves."
To test the hypothesis, the researchers placed corals in tanks with seawater and measured the oxygen concentration in their environment. "We saw that at night, without the fish, an acute oxygen deficiency was created between the coral branches," Genin says. "The concentration of oxygen in the water around the coral reached 30% of its concentration in the open water. But, when we put the fish in the tanks, they flapped their fins and maintained a constant oxygen concentration, which reached 80% or more of its value in the water outside the coral. The agitation of the water on the surface of the coral also increases other biological processes, such as the supply of fertilizers to the algae, the disposal of waste materials and sand and increasing the chance of contact with prey. Until now it was not understood how corals thrive even in areas where the sea currents are weak like in the Eilat area; Now we have solved the riddle."
Ganin points out that to the best of his knowledge this is the first documentation of a symbiosis based on changing physical conditions in the living environment of the partners. The symbiosis situations known so far are mostly based on biological processes, such as food supply, protection from predators and elimination of competitors and parasites.
Do the fish that flap their fins all night sleep? Everything depends, of course, on the definition of the concept of sleep, says Ganin. The accepted definition refers to sleep as a state in which the level of motor activity and the possibility of sensing environmental signals decreases significantly. These two conditions are not met in the fish studied - their motor activity is high and they sense their position inside the coral all night long and possibly also the position of their neighbors. It seems therefore that these fish do not sleep all their lives. Such a lack of sleep has been known so far in fish living in the open sea (such as tuna fish), but not in coral reef fish.
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