In Finding Nemo, there is a group of clownfish living in coral reefs, the most famous of which is Nemo. In the movie, its father, Marlin, takes great care of it, and when it is lost, Marlin travels across the ocean to find it.
But the real clownfish, unlike in the movie, although they have egg protection behavior, they will not raise the little Nemos. Once hatched, the little Nemos would go to the ocean to live a planktonous life, grow to the size of a grain of rice, and then begin to find suitable coral reefs to settle on their own.
Lively coral reefs
In the vast ocean, coral reefs suitable for little Nemo to settle are only two-thousandths. So, how does it find the reef?
Listen to it, and maybe there will be clues. Healthy coral reefs are a lively place, not only colorful, but also diverse.
Coral Sea in Australia | WWF
The background sound of the coral reef sounds a bit like the sound of jumping on the tongue when I ate jumping candy as a child. Compared with the noise of this sound, the creature that makes this sound, the gun shrimp, appears to be very small.
Gun shrimp, about the size of a thumb | WHOI
Shrimp as the name suggests, the distinctive claw is its "pistol", the gun shrimp will close its "pistol" at an ultra-high speed, the squeezed water column impacts the surrounding water body to produce bubbles, the bubble bursts instantly, emitting a "gunshot sound", while accompanied by a shock wave, stun their prey. In addition, gun shrimp will use these sounds to communicate. Therefore, when many gun shrimp in the coral reef are hunting and talking, "crackling" becomes the background sound of the coral reef.
Gun Shrimp uses its "pistol" | References[1]
The other sound comes from the reef fish, and when they sing together, you feel as if you are in the midst of a hundred birds singing.
Choir of three species of fish: a bullfrog-like call from the double-spined proto-yellowgu, a hum from a member of the family, and a softer "ba-ba-ba" sound from batfish | References[2]
At this time, they are likely to be feasting, or mating and laying eggs. When night falls, the deep plankton migrate vertically to the surface of the ocean, and the fish that feed on them also turn on the night foodie mode, and the sound of predation is endless. During the spawning season, several species from the family Snapper, Codidae, Bassidae, and Totoabae like to gather in the right spawning grounds of the currents to stimulate everyone's mating enthusiasm with sound.
In addition to these, there is the sound of parrotfish nibbling on coral bones, the sound of sea urchins scavenging for seaweed on the reef, and the sound of "bar chirping" is almost like no one around... Anyway, these reef dwellers can be said to be non-stop for a moment, and they will always make some noises.
These "noisy" sounds happen to be the clues to little Nemo's return home.
Disappearing coral reef sounds
But for Little Nemo, the clues to the reef seemed to be blurring.
The main reason is that the coral reef is getting quieter and quieter. When coral reefs decline, the marine life that uses them as habitat also suffers. Without the inhabitants of the "noisy", the sound complexity, diversity and volume of the coral reef will be reduced.
Scientists traveled to Australia's Lizard Island in 2012 and 2016 to collect the sounds of coral reefs. They found that the sounds collected in 2016 were far less colorful than before.
Sounds of Decaying Coral Reefs: Sounds are collected from Australian Lizard Island, the first from 2012, the latter from 2016, feel what has changed |? Tim Gordon
From the hustle and bustle of 2012 to the silence of 2016: what happened to Lizard Island?
Unfortunately, it experienced the region's worst recession on record: it lost 40 percent of its reefs during Tropical Storm Ita in April 2014; it was followed by Tropical Storm Nathan in May 2015; and more than 60 percent of corals bleached due to extreme heat in early 2016.
Declining Lizard Island | Tim Gordon VS Healthy Lizard Island | Phoebe Smith
Lizard Island is just a microcosm, as global warming and rising sea temperatures have seen 54% of coral reefs worldwide experience moderate or greater decline.
Impaired fish hearing
Even if some of the coral reefs are still as healthy as before, the noise is still the same, and little Nemo's hearing is also problematic.
Almost all fish can hear sounds. Since the density of the fish's body is comparable to that of the surrounding seawater, they do not need to collect the external structure of the sound (unlike us on land, there are external ears and middle ears), and a pair of inner ears located next to the fish's brain in the cranial cavity are their hearing organs. There are three pairs of calcareous otoliths in the inner ear, close to the sensory epidermis, responsible for the production of hearing.
Left: The inner ear of the fish is located in the cranial cavity; Right: In addition to three pairs of semicircular canals that are very similar to ours (light blue), there are three pairs of otoliths (yellow), close to the sensory epidermis (red) | References[4]
The problem is that these calcareous otoliths are very sensitive to acid solutions. As humans burn fossil fuels, more and more carbon dioxide (CO2) is emitted, and a third of it is absorbed and dissolved by the oceans. The pH of the seawater thus drops, causing the acidic solution to corrode Nemo's otoliths.
For Nemos, "insensitive ears" doesn't just mean they can't hear the reef's signals. And with that comes a lurking crisis.
Three species of reef fish that eat juveniles: brown snapper of the family Apiaceae, Griss female of the family Leptodondae, crescent fish of the family Bulbophyllaceae, | Reef Life Survey, Philippe Bourjon, Leonard Low
According to statistics, 60% of juvenile reef fish fall into the belly of hunters during the period of settlement. Little Nemo, who is hearing impaired due to ocean acidification, will have a harder time distinguishing the sounds of these predators, which will greatly increase its chances of being ingested.
Marine noise disturbance
In addition to carbon dioxide, too much ocean noise can also interfere with Little Nemo's tracking of sound cues, and even cause chaos in its hormone secretion.
Sound travels faster and farther in seawater than in the air, so the problem of "noise pollution" is more serious in the sea. Coral reefs are mostly adjacent to land, and the noise generated by human marine activities (such as nearshore engineering construction, ship transportation, fishing activities, etc.) can travel thousands of meters in seawater, which will seriously interfere with the daily life of marine life.
Global distribution map of tropical shallow reefs (blue+) and major ports located within 4 km of the reef (red asterisk) | References[5]
To see what effect the noise would have on the clownfish, the scientists placed horns near the anemones where the clownfish lived, playing the sound of motorboats. It turned out that the noise-disturbed clownfish preferred to hide in the anemone, went out to hunt less, and became very aggressive, attacking smaller companions who lived in the same anemone.
Further studies found that their cortisol, testosterone, and 11-one testosterone levels were all elevated. Cortisol is also called "stress hormone," and Suzanne Mills, the scientist who conducted the study, speculated, "which means that the noise of the motorboat will make the clown fish feel stressed, and under the pressure, their stress response is also affected, and if there is a predator, the consequences can be fatal." ”
Clownfish and motor boats | Frederick Zuberer
How to change the status quo
In fact, in addition to providing clues for juvenile reef fish, the sound of coral reefs can also provide a new dimension to human coral reef monitoring systems. For example, what kind of biodiversity can we read about this coral reef? Where do fish flock to spawn? Are some of the remote reefs in good condition? ......
Stephen Simpson uses a hydrophone to record the sound of fish | BBC documentary: The Secret Sounds of Fish
But does the scientist's job stop at monitoring sound? Of course not, in fact, Stephen Simpson's team is exploring whether sound material can be turned into a practical tool, proposing a "sound enrichment" strategy.
They collected nearly 1 ton of dead coral bones and piled up 36 bones. Of these, 12 were experimental groups that played the sounds of healthy reefs at night (juveniles prefer to return to the reef at night to settle down) with an underwater megaphone, while the remaining 24 were used as "silent" controls.
After 6 weeks, the bone piles of the experimental group full of healthy coral reef sounds were favored by more juvenile fish than in the "silent" control group, not only doubling in numbers, but also halving the diversity of species, and increasing the number of fish of all trophic levels (algae eaters, plankton eaters, invertebrate eaters, fish eaters, and everything).
Tim Gordon places a loudspeaker | on the reef Harry Harding
Lost sounds, we can replay. Can the extra noise be eliminated?
Fortunately, noise pollution is one of the easiest environmental pollution to reverse: spatially, ships and projects can be kept away from important breeding grounds and feeding grounds; time- can avoid the season when juveniles return to coral reefs; and technically, ships can use a new generation of engines with less noise, and nearshore engineering construction can use air curtains to reduce noise.
Gas curtain principle: Taking the offshore wind power project in the figure as an example, a hose is laid around the piling machine, and the compressed air is released from the small hole on the pipe, forming a dense bubble "curtain" around the noise source, cutting off the propagation of sound waves | Continental AG
In the long run, global mitigation of warming and ocean acidification is critical to the protection of coral reefs, but addressing this problem is not achieved overnight. We can only start by improving the environment in local areas, reducing the impact of near-shore engineering, reducing water pollution, and unsustainable fishing practices (overfishing, frying fish, electric fish, poisonous fish, etc.) to make coral reefs better able to respond to global change.
Close to us, there are also beautiful corals: in May 2020, Shenzhen Da'ao Bay photographed the ovulation of wild corals for the first time | Dive in love with Zhan Xiaofeng
The prototype of Little Nemo: Amphiprion ocellaris | Lily Romero