In my fantasy world, Mainspring, the top of the oceans' food chain is the maculatum, a species of giant eel which originated on another plane/dimension. Maculatum reproduce asexually and wthout any intention: they constantly excrete an oily fluid slightly heavier than water which, upon pooling on the seafloor in sufficient quantity, congeals into something like an egg. When the egg hatches, several three-inch-long maculatum emerge and immediately begins hunting smaller creatures (and each other). The primary drive of a maculatum is its constant hunger, as it gets 10% longer and thicker every week, compounding indefinitely until the square-cube-law or its own caloric needs crush it into ineffectiveness and, eventually, death.

My question is what the general upper limit for size would be on one of these oily eels given that constant growth, and whether it would typically be starvation or collapse that killed them first.

Other information that may be relevant:

  • Despite their biology being alien to Mainspring, maculatum get energy from their prey at the same rate a mundane eel would.
  • Aside from some exceptions notable enough to have personal names instead of species names, the largest creatures in the oceans are whales and giant squid (and possibly maculatum).
  • The materials of the maculatum's body have the same relative strength as that of a moray eel, and they employ the same hunting methods.
  • When a maculatum dies its body denatures into the same oily fluid it is always excreting (edit: @datacube mentioned that having it nutrientless would lead to total ecosystem collapse).
  • The oily fluid tastes bad but is highly nutritious, and is therefore favored by scavengers, who will occasionally hunt young or adolescent maculatum.
  • The only humanoids who tend to mess with maculatum are merfolk who harvest the oily fluid for its use as a memory-enhancing drug.
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    $\begingroup$ Not an answer to the question, but under the given constraints your sea life would pretty quicky be erased and be just, eels eating smaller eels until collapsing and being reborn ans thousands of smaller eels again, since obviously no other creature would eat it or its remains, and the remains become new eggs.... $\endgroup$
    – datacube
    Jul 27, 2023 at 13:21
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    $\begingroup$ darn it, if I had a nickel for every time the Great Multiplanar Leak of the Second Age introduced an invasive species that would wipe out the entire ecosystem without outside intervention, I'd have two nickels. Which isn't a lot, but it's weird that it happened twice. $\endgroup$ Jul 27, 2023 at 13:33
  • $\begingroup$ I've made some changes according to your concerns, @datacube, thank you for your input! $\endgroup$ Jul 27, 2023 at 13:47
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    $\begingroup$ worldbuilding.stackexchange.com/questions/317/… i think this answer might be similar enough to your question to be helpful. $\endgroup$
    – NimRad
    Jul 27, 2023 at 18:14
  • $\begingroup$ So the oily castoff seems similar/same to marine snow. except denser, if sufficiently dense it will mostly end up on the abyssal plain. which if the young are forming down there that will really hamper the growth. $\endgroup$ Jul 28, 2023 at 2:15

2 Answers 2


8 Metres. But Eel is a Bad Body Plan.

Like your Big Flat Eel, both crocodiles and sharks are suspected to continue growing throughout their lifetime. Though the growth slows down as they age. Wikipedia says the largest specimens are about 20 feet long. Of the two, the shark is heavier, with one great white shark named Deep Blue being estimated at 20 feet long and 2 tonnes.

The presumption here is that modern seas don't contain enough food to support and animal much larger than Deep Blue.

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Let's take a Conger eel and scale it up to 2 tonnes. This paper suggests a 1 metre (3 feet) conger weighs about 4 kilos

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Scale that up to two tonnes by multiplying by 500. To multiply the mass by 500 you multiply each dimension by $\sqrt[3]{500} \simeq 8$. So 8 metres of eel weighs about 2 tonnes.

(That's not much more than the shark and the eel is also thinner than the shark. Hmm. . . I wonder how reliable the shark estimate is.)

8 metres is only a touch longer than this oarfish found washed up on the shore near San Diego, California, in September 1996.

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The issue is now that oarfish is a relatively slow filter feeder. It is not an apex predator. The apex predator Deep Blue eats whales and sealions. Sealions are fast and nimble.

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The 8m eel can maybe hit a high top speed. But it is not nimble. It cannot keep up.

I suggest your eel has either sonar or electroreceptors. It is an ambush predator, like real eels, and hunts in murky seas by lunging at them before they have time to react.

The numbers aren't all that impressive, are they? For me the issue is not the size but can a long lanky creature move rapidly enough to be an apex predator?

Oarfish cannot lunge, by the way. They swim by undulating the crest of fins on the back.

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If this was my world, I'd say my ocean contains something big an relativel slow for the eel to eat. Like some Dunkleosteus:

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and the eel is the length of the mighty Megalodon:

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These bad boys are about 60 feet long.

Since the eel eats armored fish, it is also well-adapted to eat ships, which are a little bit like armored fish, and their crews.

  • $\begingroup$ There is enough food to make 16 meters and 45 tones sperm whales possible though, orcas are also quite chunky. $\endgroup$
    – LazyLizard
    Jul 28, 2023 at 11:17
  • $\begingroup$ As a further suggestion, giant or colossal squid are thought to barely move. They use their scale to be efficient, eating whatever comes too close. The eel might use a similar strategy, though with it's requirement of increasing 10% a week makes that incredibly unlikely. $\endgroup$
    – Trioxidane
    Jul 28, 2023 at 12:43
  • $\begingroup$ @Trioxidane I am thinking "top of the ocean's food chain" and "barely move" are hard to reconcile. $\endgroup$
    – Daron
    Jul 28, 2023 at 13:15

Apex Predators aren't 'Apex' until they're fully grown.

Using an example I gave for another question consider the Salt Water Crocodile (salties). It's the apex predator of it's ecosystem and no other creature in that ecosystem comes even close to be able to challenge it's dominance. Yet 99% of salties never make it to adulthood.

Just like your creature young salties are preyed on by multiple species since they're only a few inches long at birth. Birds, fish, mammals, lots of other animals prey on young salties.

And it would be even worse in your world for a young 'Maculatum'. As you note they are only 3 inches long at birth and the ocean is a 'fish eat fish' world. An oceanic environment is hyper competitive, Basically (in broad terms) anything smaller than you is potential prey, anything bigger than you is a potential threat.

Add in other adult Maculatum who regard juveniles as potential competitors for territory and/or a free meal and you start to see why so many fail to make it to maturity. Beyond that if, as you say the 'oily fluid' from which all Maculatum arise is highly nutritious even if it tastes 'bad' there will be some smaller ocean floor species (bacteria, molluscs worms) etc that will soon adapt to eat it.

EDIT: An upper size on the organism is also imposed by the amount of protein available for it to consume per cubic volume of water given a specific feeding style. Unless stated otherwise if Maculatum were for instance apex hunters like killer whales there's a bottom limit on how small their pray species can be once they are fully grown (minnows are not on the menu). If they are filter feeders there's an upper limit on the prey size (salmon etc are off the menu).

Over time an equilibrium has to be reached between the number and average size of apex predators and the amount of food available to sustain them. Initially when a new predator is introduced into an ecosystem there would be severe oscillations in predator/prey populations (cycles of boom/bust) that would slowly decline over thousands of years until an equilibrium was reached between predator and prey. That or the system collapses and the predator goes extinct due to a lack of food.

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    $\begingroup$ Though good points, it does not answer the question at all. $\endgroup$
    – Trioxidane
    Jul 28, 2023 at 9:58

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