Last week, Subnautica was released, and I haven’t stopped thinking about it since. For those of you who are unaware, Subnautica is an exploration and survival game set in the ocean of an unexplored alien world. You play the only surviving crew member of the Aurora, a massive spaceship that was hit by a mysterious energy pulse and crash-landed on the surface of the planet.
As a biologist, one of my favorite parts of Subnautica has been exploring the environment and seeing the diverse types of plant and animal life present, which run the gamut from small, harmless fish that you can catch and eat to voracious apex predators that will catch and eat you. There is, however, one fish that stuck out to me in particular due to its unique behavior: the crashfish.
Crashfish are small, red, pufferfish-like organisms which have a symbiotic relationship with sulfur plants. Sulfur plants act as homes for crashfish to live in, while feeding off whatever excrement and secretions the crashfish produces. Plants using nutrient-dense poop to get nutrients is nothing new, as anyone living near farmland will undoubtedly tell you, but to find a plant giving free housing to their fecal suppliers you’ll have to travel to Borneo, where a unique species of pitcher plant has evolved to act as housing for roosting bats.
But I’m not here to talk about bat toilets, I’m here to talk about how the crashfish defends its territory: it charges at invaders and violently explodes. Now, exploding creatures is a common occurrence in video games, but the crashfish struck me as odd in Subnautica due to how much effort has clearly gone into making realistic creatures and environments. Surely an animal that defended its territory by killing itself would never be able to survive in the wild, right?
Wrong. While rare, this behavior has been found in several different animals. It’s called autothysis, or suicidal altruism, and can be found in several species of ants and termites. The carpenter ant, Camponotus saundersi, often uses this to fight off other ants and even curious entomologists. Upon bursting, the carpenter ants release a sticky goo that coats their attacker and rapidly hardens, trapping them. Similar goo-spraying methods are used by a number of termite species, which use the goo to block off tunnels leading to their nests. Other species, such as Neocapritermes taracua, release a toxic spray that paralyzes their foes on contact.
So explosive suicide is clearly a viable method of home defense, but does it work for crashfish? I would say no. All the animals that use autothysis are eusocial, animals that live and work together that are mainly made up of sterile workers that build and defend a few reproductive individuals. Self destructing is an effective strategy in eusocial systems as the colony can easily replace any members they lose, but for animals that aren’t eusocial, such as the crashfish, dying does nothing but make them dead.
So why is the crashfish so eager to die? I couldn’t tell you. Maybe they’ve already mated and they’re defending their eggs. Maybe there’s a hidden tunnel system connecting all the sulfur plants into a massive eusocial hive and we’re seeing the guardian caste. Maybe they aren’t animals at all but part of the sulfur plant used to filter nutrients from the environment that can detach and explode when the plant is threatened. Whatever it is, hopefully the developers will let us know in the next update. Until then, my headcanon is on the hidden tunnels.