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In the underwater survival game Subnautica there is a fish called Crashfish that, upon being too close to it, swims out of its plant to chase you and eventually explode. The explosion leaves no remains of the fish, it is completely vanished.

The plant in which the Crashfish lives is called a Sulfur Plant. As you can read on its Wiki page there is no mention of the plant having anything to do with the Crashfish other than that it just lives in the plant.

How can a fish that blows itself up prosper enough to reproduce on a grand enough scale to persist as a species?

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    $\begingroup$ The "games" tag refers to games within a fictional world. (See the tag information.) Your question appears to use a real-world game as background, in which case the "games" tag does not apply. $\endgroup$
    – user
    Jan 25, 2018 at 8:14
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    $\begingroup$ Curiously, this Q is in the VTC queue, but with no votes. Frankly, the question feels off-topic to me, but for the life of me, I can't figure out a valid reason why. VTCers, if you vote to close, please post an explanation. Thanks! $\endgroup$
    – JBH
    Jan 25, 2018 at 8:36
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    $\begingroup$ Change the title to "explodes". A pufferfish blows itself up and lives to tell the tale. $\endgroup$
    – Harper - Reinstate Monica
    Jan 25, 2018 at 10:48
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    $\begingroup$ Is the fish allowed to live in 'colonies'? If so, one only has to look at honeybees. $\endgroup$
    – Bas Jansen
    Jan 25, 2018 at 12:06
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    $\begingroup$ @Aify, you know, that's gotta be it. I'm really learning to hate that VTC reason because 99.9% of the questions asked here are by definition fishing for ideas in a way that is opinion-based. I'd be willing to sell my first born child if we could simply admit to ourselves that a creative site is opinion based by definition and drop the reason altogether. $\endgroup$
    – JBH
    Jan 25, 2018 at 17:59

12 Answers 12

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Eusociality

Like bees, the fish are eusocial. They live as colonies with the majority of the individuals being asexual workers. The exploding fish are a specialised caste of defensive individuals that sacrifice themselves for the good of collective. Possibly, they are a later end state of individuals from other castes which transition to the "exploding" caste when they start getting old and sick.

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    $\begingroup$ While all of the answers are valid and some super interesting, to me this is the best and most plausible explanation. Thanks Jack! $\endgroup$
    – Cartman
    Jan 25, 2018 at 14:56
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    $\begingroup$ There is an actual real life example of this among ants. en.m.wikipedia.org/wiki/Camponotus_saundersi $\endgroup$
    – Myrdden Wyllt
    Jan 25, 2018 at 15:45
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    $\begingroup$ Small nit for accuracy, bee colonies are not mostly drones. "Drone" is the word for a male of the species, which happens to be haploid (spawned from an unfertilized egg). The "workers", the large majority of the individuals, are underdeveloped females (diploid, spawned from a fertilized egg). Also, with bees, their jobs for the colonies is very dependent on age, so that would be very believable for this fish. $\endgroup$
    – user458
    Jan 25, 2018 at 17:54
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    $\begingroup$ In the article on the exploding ants there's a link to an article on this behavior, which mentions termites which age into this ability: en.m.wikipedia.org/wiki/Autothysis $\endgroup$
    – Morgen
    Jan 27, 2018 at 4:08
  • $\begingroup$ @Myrdden Wyllt - Why don't you post that as an answer. $\endgroup$
    – Eric
    Jan 29, 2018 at 16:09
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When the female fish blows up, the explosion sprays all of its fertilised eggs and some stick to the animal that triggered the explosion, or the thorn apart pieces of flesh, and gain their nutrition from it later. If another fish eats the meat, no problem, they will just continue their existence as parasites within the animal's intestines and leave through evacuation once they are big enough to survive on their own. The male of the species might not even do this blowing up thing, it might look entirely different and might not live in the same plant.

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    $\begingroup$ This would work better for me if the crashfish was asexual ... and born pregnant like a Tribble. The little bounders would literally live to explode. $\endgroup$
    – JBH
    Jan 25, 2018 at 8:21
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    $\begingroup$ I think sexual reproduction works fine with this, the "exploders" are fertilised, mature females with developed eggs. $\endgroup$
    – Jack Aidley
    Jan 25, 2018 at 9:23
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    $\begingroup$ @JBH, they can just be one of the many fish that change sex as they get older, start male, grow into female, explode. $\endgroup$
    – Separatrix
    Jan 25, 2018 at 10:17
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    $\begingroup$ @Seperatrix: That sounds like a particularly odd catchphrase for some reason. $\endgroup$
    – Joe Bloggs
    Jan 25, 2018 at 11:07
  • $\begingroup$ Parasites... brilliant! Makes me think of what happens to the poor capertillars in my 'mater garden $\endgroup$
    – WernerCD
    Jan 25, 2018 at 12:13
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Salmon

What do salmon do? They are born... grow up... drop eggs/sperm... After which, it doesn't matter because they have already seeded the future.

Cycle of life

Make the fish have the same basic life cycle: Birth. Growth. Procreate. BOOM!

Once the eggs are fertilized - most likely in/around the plants that they later protect... there is no reason for them to be OTHER than protecting the plants.

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    $\begingroup$ Birth. Growth. Procreate. BOOM! I wish I had an industrial metal band so I could make a song with that for a title. +1. $\endgroup$
    – The Square-Cube Law
    Jan 25, 2018 at 13:47
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    $\begingroup$ Real world example, +1. Some octopuses and spiders directly sacrifice themselves for their young. Many other parental animals also behave in similar ways, as well as those who may just have a symbiotic relationship a plant. OP's fish is on guard duty. Either for its young, its home, or both. $\endgroup$
    – Mazura
    Jan 26, 2018 at 0:22
  • $\begingroup$ @Mazura Yeah, I could add other examples and flush out the "protection" aspect... Salmon don't do that... but I'm sure they would ruin some bears day if they had the opportunity. I was thinking more of the life cycle aspect and a use for the fish AFTER planting seed. $\endgroup$
    – WernerCD
    Jan 26, 2018 at 0:31
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    $\begingroup$ @Mazura sometimes even humans do that… $\endgroup$
    – Holger
    Jan 26, 2018 at 11:07
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The blowing capability is a recessive feature carried by b allele, meaning that only individuals having the bb pair in their genome will blow up. Bb or BB individuals will not blow up, but will benefit from the protection offered by bb individuals and will be able to transmit the feature to future generations.

The individual will perish, the species will thrive.

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    $\begingroup$ +1 A similar (real-world) example of this is the sickle-cell trait. Those with one abnormal allele are resistant to malaria, without many of the problems of getting full sickle-cell anemia. Those with two abnormal alleles get sick, but are also far more resistant to malaria, and as such their existence helps the overall population by reducing the spread of the disease. As such it's a good trait for the population to have, despite it being a detriment to those who specifically have two abnormal alleles. $\endgroup$
    – Bilkokuya
    Jan 25, 2018 at 12:54
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Although the answers here are all logical, I would like to throw in another concept: animal learning.

Some plants are highly toxic and kill animals eating them. This doesn't directly protect them, as animals only realize they can't eat that after the plant has been eaten. This protects them because animals learn that they can't eat those.

A more animalistic approach:

For information, what I'm gonna say here comes from a book (actually a light novel: "Kumo desu ga, nani ka?") and has no example in our world.

In a book I read, there is a tribe of apes that live amongst strong monsters called "revenge monkeys". Those apes are not strong by themselves, but when one of them gets killed, all the other in a 20km radius will try to avenge him, whatever the cost. As a result, the other species learned not to mess with them.

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  • $\begingroup$ Welcome to WorldBuilding Nathan! Cool answer (and cute profile pic). If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Jan 25, 2018 at 10:59
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    $\begingroup$ Not just in fiction the same has been seen in chimps pnas.org/content/104/32/13046.short There is also a theory that humans were not hunted for the same reason. $\endgroup$
    – PStag
    Jan 25, 2018 at 12:37
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    $\begingroup$ @PStag circumstantial evidence for this is provided by the short list of animals which actively prey on humans consisting almost exclusively of animals which are really good at hiding after a successful kill. Apparently, rattlesnakes are under pressure to develop this trait: nbc26.com/news/national/… $\endgroup$
    – Morgen
    Jan 27, 2018 at 4:19
  • $\begingroup$ Thought exactly this, why some species of flies evolued to look like bees ? Because predators learned the hard way that eating bees is not a great idea. $\endgroup$
    – polku
    Jan 29, 2018 at 13:05
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    $\begingroup$ @polku Exactly! The incredible part is that, though for bees, they sacrifice themselves for the community, for some species they just have this tendency of killing very "intruder" even at the cost of their lives, and thus inspiring fear to all potential predators of their specy as a side effect. So this means this trait didn't come "to defend the specy", but a side effect of this trait is the defense of the specy! Same result, but the underlying meaning is quite different $\endgroup$
    – Turtle
    Jan 29, 2018 at 13:28
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I really liked Jack Aidley and L.Dutch's answers. However, I just wanted to add one more possibility: quorum sensing, whereby certain behavior is triggered by the density of a certain external stimulus.

In the case of the Crashfish, each fish constantly poops releases a hormone in the surrounding water, a bit like humans may release chemicals perceived as body odor by others. When the local concentration of the hormone in the water becomes higher than a certain threshold, the fish triggers a hormone response called the self-ignition response, enabling the explosive behavior. The explosion clears the local area from the hormone, thus having a "sedative" effects on the nearest Crashfishes.

In practice, if the colony is too successful, then there will be more hormone in the water, and there will be more explosions, taking down the population number. If the colony is not particularly successful, it may happen that no crashfish will blow itself up.

Hiding in the plant may just be a intermediate response to high hormone concentration, whereby the fish tries to find an area, by instinct hiding in the plant, with reduced hormone concentration before triggering the self-ignition capability.

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There is precedent in the animal world. There are many types of creatures in the deep seas that glow to attract predators so that, when eaten, the predators themselves now glow and they in turn can be eaten, disposing of the predator. The biological cost to individuals is high, but favorable to the species as a whole which is always the calculus of nature.

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    $\begingroup$ While really cool I'm not quite how exploding to kill interlopers and getting eaten to be spread (alive) by your predator's predator are related. $\endgroup$
    – user25818
    Jan 25, 2018 at 19:04
  • $\begingroup$ @notstoreboughtdirt Who said "alive?" $\endgroup$
    – wizzwizz4
    Jan 25, 2018 at 21:22
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    $\begingroup$ In your link the bacteria are alive at the end after being eaten by shrimp which are eaten by fish. $\endgroup$
    – user25818
    Jan 25, 2018 at 21:28
  • $\begingroup$ There is precedent in the animal world. $\endgroup$
    – The Square-Cube Law
    Aug 26, 2019 at 14:11
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I know you said there's no mention of the fish being related to the plant, however, it seems likely to me that the fish leaves spores after explosion that will cause a plant to grow.

It's highly possible that the "fish" part of "Crashfish" is a misnomer, and it's actually technically part of the plant.

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It is as simple as a matter of relative population sizes. In a balanced ecosystem, prey has to exist in large numbers lest they disappear, while predators should be comparatively few lest they exterminate.

But if most prey individuals blow their attackers, they simply shift the relative numbers in favor of their colony as a whole.

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It is the same as how wasps go out of their way to sting you but they prosper only because we do not mess with them. One fish blowing up would not mean much as long as it has already given birth to babies, and this species would have no natural predators because of well the exploding.

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My honest guess is that it would do that in order to protect its offspring ... I imagine that it already laid its eggs in that plant and that no animal is stupid enough to try and approach a kamikaze fish even the little ones ... you don't want to have a few of those explode in your mouth or stomach do you now?

Basically 1 dies to send a message to the rest of animals , we are crazy run for the hills .

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There is a species of explosive ant in Malaysia, Colobopsis saundersi:

A worker can explode suicidally and aggressively as an ultimate act of defense, an ability it has in common with several other species in this genus and a few other insects.

So this is alredy a thing in nature. So to answer the core of the question:

How can a fish that blows itself up prosper enough to reproduce on a grand enough scale to persist as a species?

There is no problem in a few fish exploding, as long as enough fish don't explode. This is not different from non-explosive behaviors. When a bird eats a distinctively colored butterfly and gets sick enough to puke the butterfly after that, the butterfly is already dead, but the remaining population is safe (from that particular bird).

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