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Context

I am disigning a place called the Poisoned Sea, see this question for description, there are clouds of purple plankton that eat the tough flesh of the dead God-Eaters and anything that enters the cloud of plankton.

  • The plankton are zooplankton
  • The plankton can consume an entire human body in roughly half a day
  • On average human that swims into a cloud on the plankton without protection will last 2 minutes before sustaining life long internal injuries and 5 minutes before being fatality injured ether due to damage to the spine, circulatory system, and/or respiratory system.
  • The plankton typically make large purple tides of themselves around recently deceased God-Eaters.
  • The plankton are eaten specialized filter feeders.

The Question

Are these Plankton reasonable in their feeding habits and speed?

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Generally, organisms have two possible modes of consumption. They could be mechanical masticators... jaws/teeth/whatsits that mechanically cut or chew. Or, they could chemically pre-digest their prey. In the real world, many organisms rely on both to some degree, though the bulk of the damage will come from one or the other.

Mechanical action tends to require some parity in size, if only because while that army ant's mandibles can take a nip out of you, it only lasts a second before your arm (which has a mass twenty thousand times hers) will smash down Hulk-style and the ant is no more. Even swarming behaviors of many tiny organisms tend to not tackle living prey (exceptions exist but are rare).

Chemical pre-digestion is trickier. They could, in addition to dumping some weird acids/enzymes that start dissolving your skin, also dump any number of other chemicals into the water which would disable their prey. Nerve agents and paralytics and so forth. But, supposing you don't want to do that, it's difficult to imagine a chemical agent that would start dissolving human skin so quickly that within five minutes the victim is dead. Sea water doesn't make it impossible for an acid to dissolve, but its sheer volume tends to dilute things quickly, and tiny organisms have tiny reservoirs anyway. So it'd almost certainly have to be enzymatic in nature.

Perhaps the best approach would be a combination of both... these things attach themselves like barnacles, and drill into your skin. Then, to mitigate dilution, they dump the enzymes directly into these (sealed) wounds. If this were to cause them to release and reposition themselves, the resulting wound might have more surface area for more to attach. Repeat that three times a minute for the next few minutes, and a person might start to look more like a chum bucket. Maybe in such a time frame, they could open up an artery or a vein.

Though, if I wanted to make this sound more plausible, I'd probably back off on the "5 minutes and you're dead" stuff. Sure, with medieval medical science, pulling a person out of the water after 5 minutes might be a death sentence and even within a day or two (even today, if large areas of skin are destroyed, things can be dicey), but they'll still be speaking. Or more likely screaming "kill me now".

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5 minutes before being fatality injured ether due to damage to the spine, circulatory system, and/or respiratory system.

Well, nothing plankton-sized is going to cause fatal traumatic injury in that sort of timescale.

What you'd need is some kind of secreted toxin. There are plenty of really unpleasant toxins produced by a range of different plankton... consider something like saxitoxin as an example of the upper range of toxicity in real-world plankton toxins.

Swimming around in an neurotoxic plankton bloom is likely to be quite bad for you in fairly short order. Permanent damage can occur without death, including damage to brain functionality, and disruption of organ functionality.

The plankton can consume an entire human body in roughly half a day

This is quite a tall order, because people are Quite Large relative to the size of your average zooplankton, they're made of quite a lot of stuff that's hard to digest, and they have an awkward surface-area-to-volume ratio that makes it difficult to use sheer numbers to gobble them up.

That's not to say that quite a bit of damage couldn't be done.

Myzocytosis is one mechanism used by some unicellular predators, where they have a special feeding tube that's injected into a target cell which is then drained of its tasty bits. Humans are covered with an inconvenient thick layer of dead cells which makes them awkward to consume this way, but there are all sorts of bits of them which are a bit more vulnerable and might be attackable. Imagine something that attacks the eyes and mouth, and various other orifices you might have lying around. It might feel like a very unpleasant burning sensation, but once the damage is done then the neurotoxin the beasties shed into the water can take effect much more quickly with the result that the victims might cough up a little blood and bleed out of their eye sockets and then just stop wailing and thrashing and expire quite quietly as their lungs stop working.


There's a fictional example of the sort of thing you want... Frank Schätzing's The Swarm includes an engineered bioweapon in the form of Pfiesteria homicida, a modified version of the real-world Pfiesteria piscicida which attacks with rapidly deadly neurotoxins. I can't find my copy right now, but I'll see if I can dig up some of the descriptive text from it.

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Cyanide.

One of the few toxins that has the fast knockdown you want is cyanide.

https://en.wikipedia.org/wiki/Cyanide_poisoning

Cyanide poisoning is poisoning that results from exposure to any of a number of forms of cyanide. Early symptoms include headache, dizziness, fast heart rate, shortness of breath, and vomiting. This phase may then be followed by seizures, slow heart rate, low blood pressure, loss of consciousness, and cardiac arrest. Onset of symptoms usually occurs within a few minutes. Some survivors have long-term neurological problems.

If hydrogen cyanide is inhaled it can cause a coma with seizures, apnea, and cardiac arrest, with death following in a matter of seconds. At lower doses, loss of consciousness may be preceded by general weakness, dizziness, headaches, vertigo, confusion, and perceived difficulty in breathing. At the first stages of unconsciousness, breathing is often sufficient or even rapid, although the state of the person progresses towards a deep coma, sometimes accompanied by pulmonary edema, and finally cardiac arrest.

Some plankton can produce cyanide!

https://www.biorxiv.org/content/10.1101/2020.01.04.894782v1.full.pdf

Cyano-assassins: Widespread cyanogenic production from cyanobacteria

...In this study, we tested whether cyanobacteria are able, not only to reduce, but also to produce HCN. The production of HCN was examined in 7 cyanobacteria strains from all five principal sections of cyanobacteria, both non-heterocytous and heterocytous, representing a variety of lifestyles and habitats. Twenty-eight (28) strains were found positive for HCN production, with universal representation amongst 22 cyanobacterial planktic and epilithic genera inhabiting
freshwater, brackish, marine (including sponges), and terrestrial (including anchialine) habitats.

Your plankton swarm things near them and start kicking out the cyanide. It is absorbed cutaneously and within a few minutes is also in the air in the vicinity. Persons who realize what is going on within the first few minutes can stumble out of the water and possibly recover though might still die. Once it is volatile, if there is no breeze death comes in seconds for those who inhale it, and so the plankton can claim would be rescuers of the first victim.

Once the victims are dead the plankton can feed at their leisure in the manner of their kind.

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Consider the jellyfish

Stings from certain jellyfish can cause cardiac arrest and death in relatively short order, not to mention considerable pain. If your plankton have developed similar potent toxins and swarm in large numbers, it's certainly believable that they could disable and kill humans with relatively low effort. It's not like jellyfish are super complex species, they don't have brains/hearts, they're hundreds of millions of years old, and they are 98% water.

Once the plankton have disabled the large prey, it'd be a easy for them to scavenge from their remains to get all the sustenance they need. And it's not like any other predators/scavengers are going to try and get near to steal their quarry, because the plankton could disable them too.

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