Ocean Love Story Series丨Anglerfish——This Sticky Love

Author: Scientific Scraps Reviewer: Zhang Wei, Associate Professor of Marine Biology, Shandong University

"Clingy" love

In the previous episode, we mentioned that the couple A and B lived together until old age, which is a model of childhood sweethearts who love each other until death. In contrast, the couple we are going to talk about today can be said to be a model of clingy lovers. How clingy are they? Once they are together, they will never separate again.

The picture below is our protagonist, the anglerfish.

The black anglerfish looks fierce at first glance. Image source (wikimedia)

Deep Sea Hunter Anglerfish

In fact, anglerfish is a large family with many species, distributed in the sea at depths of hundreds to thousands of meters.

The protagonists of our love story live in a relatively deep ocean, where light is scarce and it is pitch black. It is not easy to find food and reproduce in such an environment.

But the anglerfish has its own way of dealing with it.

The most distinctive feature of the anglerfish is the lantern-shaped light organ on its head. Interestingly, the anglerfish itself cannot emit light. Instead, two types of Vibrio bacteria coexist in its lantern-shaped light organ, which are responsible for the light emission[1].

In the dark deep sea, small fish and shrimp may be attracted by the light above the anglerfish's head. But hidden under the starlight is the ruthless bloody mouth of the anglerfish, and the small fish will be swallowed before they can react.

The big mouth under the "lantern" is always ready to swallow the small fish caught on the hook. Image source (Wikimedia)

Hard to find place

Living in the deep sea, besides the difficulty of finding food, it is also extremely difficult to find a partner. The fish coming and going cannot see each other clearly, so who knows whether the one swimming towards them is a ferocious predator or the coveted opposite sex?

To this end, male anglerfish have evolved an unparalleled sense of smell. They can smell the scent of female fish in the dark sea and then move themselves to "stick close".

The male fish's "sticking" is not just that simple.

For male fish, it is not enough to just find the female fish, they must fertilize the female fish to complete their mission. In order to stay close to the female fish and not lose them, the male fish came up with a solution, which is to bite the female fish in one bite, so that they will never lose them.

Once the male fish of some species have latched onto a female, they never let go, releasing an enzyme that connects their bodies to hers.

Although this is also a type of "spending the rest of your life together", it doesn't sound very romantic, but rather a bit heavy.

But the question arises again, how does the male fish eat when it has merged with the female fish?

At this time, the male fish no longer need to hunt for food themselves. They directly drink the blood of the female fish and use the nutrients in the female fish's body to sustain their own lives.

More extreme is that at this time, the male fish no longer needs eyes, so the eyes degenerate, no longer need to swim, and the fins can be discarded. Even some internal organs can be discarded. In this way, the male fish merges into a part of the female fish's body.

But the male fish retains a very important organ, the testicles. After all, this is where sperm is produced, and the ultimate purpose of the male fish doing all this is to fertilize the female fish. After merging with the female fish, the male fish can continuously provide sperm to the female fish.

For humans, if there is another person on their body, they will definitely have difficulty in moving. However, female anglerfish do not have this problem. Female anglerfish are generally much larger than male anglerfish. For example, the female Melanocetus johnsonii can grow to 60 times the size of the male[2]. It is like a 1.7-meter-tall girl bitten by a boy less than 3 centimeters long. The female fish will not have difficulty in moving because of the male fish on her body.

The anglerfish's bonding method sounds quite strong, but it is also a helpless move to adapt to the special environment of the deep sea. This strong bonding method has a name, called sexual parasitism.

The male fish is like a parasite, parasitizing the female fish. In this marriage, the male fish can only follow one female fish wholeheartedly, after all, after becoming one, there is no possibility of regretting the marriage. But female fish are different, a female fish can parasitize several male fish at the same time, and let these male fish provide sperm for its eggs.

Of course, it needs to be explained that sexual parasitism is very rare in the entire biological world. Even among anglerfish, only some anglerfish choose this strategy.

Most anglerfish are fertilized externally, meaning the female releases eggs into the water, and the male only needs to take the opportunity to release sperm to fertilize the eggs.

For these anglerfish, the males do not need to merge themselves into the females' bodies. They can bite the females, wait for the right time, and then just pat their butts and walk away after they've ejaculated.

Love makes fish "let down their guard"

Let's go back to sexual parasitism. Although we are seeing a heavy taste of love, scientists have realized a problem: Why doesn't the immune system of female fish attack male fish? Why doesn't it eliminate them?

If foreign matter enters the human body, the immune system will take action and eliminate it.

In addition, if you want to perform an organ transplant surgery on a human body, you also need to do a good job of matching in the early stage. If the matching is suitable, it is generally necessary to limit the immune function of the transplant recipient. Even so, rejection reactions may still occur. For female anglerfish, there is a fish mixed in their body, so why does the immune system have no reaction at all?

To find out, scientists sequenced the genomes of 13 species of anglerfish.

They found that some genes related to immune response were missing in anglerfish that exhibited sexually parasitic behavior, causing these fish to lose the ability to produce T cells and antibodies[3].

T cells and antibodies are extremely important for the immune system of animals. If this happens to humans, it will be a very serious immunodeficiency disease, and such people are likely to die within one year after birth.

Why can anglerfish stay healthy? Scientists speculate that other immune cells in their bodies may work hard to compensate for the lack of antibodies and T cells. But we still know too little about this deep-sea fish, and scientists still need more research to draw conclusions.

Perhaps, humans can uncover more mysteries about immunology from these fish. This strong-tasting fish may also be able to save people with immunodeficiency diseases or help humans overcome difficulties in organ transplantation.

However, the anglerfish's practice of losing some of its immunity for the sake of reproduction can, to some extent, be considered as removing its "defense" because of love.

The sexual parasitism of anglerfish is definitely bizarre, but the next two couples may continue to subvert your worldview. In the next episode, we will continue to tell a new ocean love story.

References:

[1] Baker, LJ, Freed, LL, Easson, CG, Lopez, J. V, Sutton, TT, Nyholm, S. V, & Hendry, TA (2019). Diverse deep-sea anglerfishes share a genetically reduced luminous symbiont that is acquired from the environment. Elife, 8:e47606, 1–21.

[2] https://www.smithsonianmag.com/smart-news/Anglerfish-Drop-Their-Immune-Defenses-to-Find-Love-180975458/

[3] Swann JB, Holland SJ, Petersen M, et al. The immunogenetics of sexual parasitism[J]. Science, 2020, 369(6511): 1608-1615.

This article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.