Why do these fish hold their breath underwater? It's not because they're bored, it's to save their lives!

Produced by: Science Popularization China

Author: Su Chengyu (popular science creator)

Producer: China Science Expo

Fish living underwater seem to have no reason to hold their breath, just like humans living on land, who normally breathe in and out, and only hold their breath when they go into the water, or do some kind of training, or when they are bored. After all, mammals cannot breathe freely underwater, and they cannot survive without oxygen.

But there is a kind of fish that actually holds its breath underwater. Could it be that it does this out of boredom?

I don't like swimming, but I love holding my breath.

This fish is called Chaunax , a deep-sea fish that is found in Japan, the South China Sea, and the East China Sea, and lives at depths of 500 to 1,985 meters. Some of them look a bit ugly, like the one below. Because it looks a bit like a toad, it is also called a sea toad .

Chaunax suttkusi, a single stickleback handfish

(Image source: Wikipedia)

Does it look a bit like an anglerfish with a "small lantern" on its head? That's because they are indeed related. All fish in the family of handfish belong to the order Anglerfish.

Chaunax pictus, a species of handfish

(Image source: Reference [1])

When at the bottom of the water, the handfish does not swim continuously like other fish, but stays there, as steady as a rock, so as to ambush passing small fish.

Even when it moves, the handfish spends most of its time crawling on the bottom of the water using its pectoral and pelvic fins. It doesn't like swimming, so it swims slowly. Although scientists have occasionally observed some handfish swimming quickly when escaping, they seem to get tired quickly and can't swim after a while.

Single stickleback handfish crawling on the bottom of the water

(Photo credit: shorefishes)

Based on the "stable" living habits of the single-spined handfish and the cold and resource-scarce seabed environment in which they live, scientists have inferred that the single-spined handfish's metabolic level is not high.

Searching for fish in thousands of places, the fish are all in the video

So how do scientists know that this fish can hold its breath?

It is certain that scientists will not just catch a single-spined handfish for research. This is because this fish is difficult to observe and it is not easy to catch one. In addition, wild animals cannot be caught at will, as they are protected by laws and regulations.

So, we can only observe. Scientists found videos of single-spined handfish taken by remotely operated submersibles (ROVs) in the past and analyzed their breathing movements frame by frame, that is, the changes in the volume of their gill cavities. A total of 10 videos were found, and 4 single-spined handfish species were shown in the videos.

Four species of handfish in the video

(Image source: Reference [1])

By observing the video, scientists found that these four single-spined handfish will maintain a bulging, completely unchanged state for a long time, that is, holding their breath. However, their breath-holding time varies, ranging from tens of seconds to hundreds of seconds. The longest time is the Indian Ocean oarfish (Chaunacops coloratus) , which can hold its breath for up to 245 seconds.

The Indian Ocean handfish in the picture below, after holding its breath, continued to exhale slowly. After about 7 breathing cycles , it slowly reduced its volume, eventually reducing its volume by about 29%.

The volume of the Indian Ocean handfish changes over time

(Image source: Reference [1])

Unlike the Indian Ocean oarfish, another slightly uglier-looking redeye fish (Chaunax pictus) exhales all the air in less than three breathing cycles , reducing its volume by about 20% . The redeye fish in the video hardly moved and stayed on the rock.

The volume of red-eye fish changes over time

(Image source: Reference [1])

Take your breath! Otherwise your life will be in danger.

Evidence of the single-spined handfish holding its breath was found, but another question arose - why do these single-spined handfish hold their breath?

In 1879, scientist Hilgendorf investigated the living habits of the handfish and found that it could go for a long time without eating anything. According to Hilgendorf's research, it is normal for these fish to not have food throughout their lives, so they are basically not picky about food. As long as there is prey that can be swallowed in one bite, they will swallow it in one bite. Their menu is highly diverse. It can be said that the handfish is a typical hunter who waits for its prey.

During the breath-holding process, seawater fills the gill chambers of the handfish. They exhale slowly at a very low frequency (0.03-0.004Hz), and even when swimming, their breathing rate is not much higher than when they are motionless. Maintaining a low-frequency breathing or even holding their breath can keep the energy they consume at a low level.

A single-spined handfish lying on the seafloor

(Image source: NOAA)

This is also the reason why single-spined handfish need to hold their breath. They need to save some energy. If they exhale too quickly, they may really starve to death.

In addition to saving energy, scientists speculate that there is another benefit to holding breath for handfish: it helps them sense changes in the surrounding environment. No matter what kind of fish it is, there are lateral lines on both sides of its body , which are important sensory organs for fish to adapt to life in water. Without lateral lines, they would find it difficult to survive in water. Frequent breathing may increase the disturbance of the surrounding water, which in turn affects the lateral line's perception of predators or prey.

Single-spined handfish live at the bottom of the continental slope where the light is weak. They mainly rely on their lateral lines to sense the vibrations of the water and then perceive everything underwater. Therefore, the highly sensitive lateral line system is particularly important for deep-sea fish such as single-spined handfish.

Red-eye fish lying down

(Image source: NOAA)

In order to maintain a highly sensitive sensory system, handfish need to reduce their swimming and breathing rate, which is one of the reasons why they hold their breath.

Of course, another reason for holding their breath may be that they want to increase their size by holding their breath, making them look more threatening like pufferfish, so as to better protect themselves.

Pufferfish, will I, who is also "puffy", also hold my breath?

At this point, you may be curious, why don't we mention that pufferfish can also hold their breath?

For pufferfish, they don't make themselves bloated by holding their breath, but by swallowing water into their stomachs to expand themselves. When they reach the shore, they will vomit all the water.

The "drunk" pufferfish

(Photo source: Veer Gallery)

In 2014, scientists conducted a study on pufferfish. They placed the pufferfish in a transparent water tank and observed its breathing. They first poked the pufferfish with a tube to simulate a predator attack. As expected, the pufferfish swelled up immediately, and its breathing volume increased nearly five times during the swelling process.

In other words, the pufferfish is not holding its breath when it expands, but is actually panting. Not only that, scientists have also found that it takes several hours for these pufferfish in the experiment to return to their resting breathing rate.

So, while both the handfish and pufferfish grow larger, the handfish actually holds its breath, while the pufferfish just "drank too much."

Conclusion

Fish also need to hold their breath underwater. This may sound ridiculous, but for the handfish, holding their breath is their signature skill for survival.

Whether it is the sky, the grass, or the ocean, nature itself is a stage that adheres to the law of "survival of the fittest". In order to survive for a long time, fish must also learn to hold their breath... The wonders of living things are far beyond our imagination. It is in the process of exploring and understanding nature that humans can gradually understand the unique meaning of each life.

References:

【1】Long, Nicholas P.; Farina, Stacy C. (2019). Enormous gill chambers of deep-sea coffinfishes (Lophiiformes: Chaunacidae) support unique ventilatory specializations such as breath holding and extreme inflation. Journal of Fish Biology, jfb.14003–. doi:10.1111/jfb.14003 .

【2】McGee, GE; Clark, TD (2014). All puffed out: do pufferfish hold their breath while inflated?. Biology Letters, 10(12), 20140823–20140823. doi:10.1098/rsbl.2014.0823.