The serra is a large air-breathing fish, about 10m long, with a long serrated crest coming out of its forehead. It is predatory, and lives similarly to a shark. However, it has one unique feature, that being that it often attacks ships by attempting (and if the ship is made of wood, often suceeding) to saw through the base of the ship, sinking it. What use would this behaviour have for the serra, and why would it evolve?

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    $\begingroup$ Large scale cookie cutter shark? Cookie cutter sharks are small, opportunistic,. Just scale them up and it would be close to what you are describing. $\endgroup$ Commented Apr 27, 2021 at 21:17
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    $\begingroup$ "Air-breathing fish"? Isn't that a contradiction? Could you clarify what this means? Thanks $\endgroup$
    – Binyomin
    Commented Apr 27, 2021 at 21:30
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    $\begingroup$ @Binyomin It is a fish that has lungs and breathes air, much like many fish both extinct and alive today $\endgroup$ Commented Apr 28, 2021 at 11:43
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    $\begingroup$ @Binyomin Lots of fish breath air, especially large fish in river environment (see arapaima), being able to breath air provides a lot of advantages when the water does not have that much oxygen. In the ocean not being able to breath air, and being forced to take oxygen from water puts down ward pressure energy you can use, so it can limit speed/size/intellect, hence why vertebrates constantly evolve to go to the water, air breathing aquatic organisms can use more energy than their non air breathing counter parts. A fish evolving to do the same in the ocean would not be out of the question $\endgroup$
    – Krupip
    Commented Apr 28, 2021 at 18:57
  • $\begingroup$ Sounds very sawfishy. marineconservation.org.au/sawfish may have helpful information. $\endgroup$
    – wingnut
    Commented Apr 29, 2021 at 4:41

6 Answers 6


Because the creature evolved to attack from beneath its prey, which was something similar to a whale.

From below, a boat looks like a long, taper-ended object much like a whale silhouetted against the sky above the water. In fact, boats, being dark below the water, would stand-out more than whales, most of which have evolved light coloured bellies to camouflage against the light above.

All I can say is that the creatures that sink boats (a recent invention in evolutionary terms) have not evolved or learned to distinguish between the two.

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    $\begingroup$ The serrated crest might be how it incapacitated its prey. $\endgroup$ Commented Apr 27, 2021 at 21:13
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    $\begingroup$ @JamesMcLellan That's a better way to put it, though thoughts like " it depends on the shape of the crest, maybe it serves to carve off a piece of the prey which can be eaten" flashed through my head - I've sort of left it open, without adding suggestions. $\endgroup$ Commented Apr 27, 2021 at 21:18
  • $\begingroup$ @JamesMcLellan I also had thoughts about a complex evolution of a whole ecosystem involving the addition of iron-containing blood from the "prey" animal, which might increase algal bloom, thus increasing the proliferation of animals which could be predated upon by the OP's creature, but I don't have the time the spend referencing this in an answer at present. $\endgroup$ Commented Apr 27, 2021 at 21:36
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    $\begingroup$ Ships are full of tasty snacks, why would it care to distinguish? $\endgroup$
    – Separatrix
    Commented Apr 28, 2021 at 10:02

It is itchy.

It is so hard to get a good scratch out in midwater. Water is not scratchy at all. More just splashy. The bottom is soft and muddy and it just makes you itch more. If something comes floating along that is hard and big enough to offer that good scratching, you gotta scratch!

itchy sharl


That is how it is with your fish. It is itchy. When a boat comes it wants to scratch and boat wood offers first rate scratching.

Often one fish will be satisfied with a few minutes of scratching and the damage to the boat is reparable. But that scratching sound travels a long way underwater and if there are any other itchy fish in the vicinity they know what it means. Scratching time!

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    $\begingroup$ (+1) Insightful, funny, creative and plausible. I wish I could upvote it more. $\endgroup$
    – Rekesoft
    Commented Apr 28, 2021 at 9:17
  • $\begingroup$ The accepted answer seems more realistic, but this is so funny (and not entirely impossible) that it deserves to have all these upvotes! $\endgroup$ Commented Apr 30, 2021 at 11:27
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    $\begingroup$ I'll note that "itchiness" itself is an evolved sensation. So typically, there is some benefit beyond the sensation itself. In this case, I would say that the probably benefit would either be cleaning (stuff gets caught on the serrations) or sharpening (removing the outer layer of the serration, much like cats scratch stuff to sharpen their claws). $\endgroup$
    – Brian
    Commented Apr 30, 2021 at 14:14
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    $\begingroup$ This answer is better than mine in so many ways. I'm thinking of changing my name to Sancho Panza, or more aptly Ahab. $\endgroup$ Commented May 1, 2021 at 1:57

It is playing/trying out its might/territorial

We can look at Orcas. They have been seen attacking boats, or at least bumping the hull or the rudder. It is believed some attacks about a year ago were done by some 'teenage' orcas. It was most probably just a way for them to play. Play often has some deeper meanings, which in this case is having a certain power over other objects or creatures.

The Serra would attack to see what power it has over these objects in the water.

An alternative is simply that they are territorial. Something is in their waters. It doesn't matter if it's edible or not. It needs to be attacked so it'll know its place.


They are laying eggs

The serra are air-breathing, as stated, and live or frequently return to the delta of a large river (such as the Amazon) to lay their eggs on naturally formed floating islands that drift out to sea from time to time. They saw up through the islands from beneath to lay their previously fertilised eggs in an environment where there is plenty of food available for the young when they hatch. (How fertilisation occurs prior to egg laying is out of scope for this answer and a private matter for the serra.)

This practice:

  1. Allows the young serra to survive the early stages of their life when their lungs are too undeveloped to manage long periods of submersion.
  2. Allows the older serra to reproduce without risking becoming beached by venturing into shallow waters.
  3. Ensures that the young serra are in a position where they can immediately enter the salt water as soon as they have matured sufficiently.
  4. Protects the immature serra from the majority of predators that would threaten them on the land or in the water.

Unfortunately for human mariners, the serra haven't kept up with evolution and see ships as potential breeding opportunities.

Tuning the threat: Their ship-attacking behaviour can be made:

  • Seasonal - the serra are conditioned to only attempt to reproduce during the time of year when flooding routinely sends floating islands out to sea;
  • Geographically limited - the serra return to a certain area near the coast where floating islands are most common to reproduce and ignore ships in other areas; and/or
  • Frenzied - there's a shortage of floating islands and whenever one appears the serra mob it to ensure that their genetic legacy continues.

It tries to mate

When a female serra looks for a mate, they swim on the surface over the territory of a suitable male. The male then approaches the female from below, attempting to penetrate it. The serrated crest on its head is its genitalia.


Competing or prey species are vulnerable to heavy attacks from below, and as other commenters have said, it doesn't know any better when it attacks ships.

Alternatively, while it's true that it was adapted for attacking other megafauna, the Serra is smart - it's learned that ships have edible little bipeds inside. Like a piñata. There's significantly less to eat than if it were a real animal, but ships - especially wooden ones - are sitting ducks in comparison to the speeds a shark would be capable of.

Or, maybe, the Serra is a keyhole species that lives coastally and constructs some kind of shelter, fish trap, or other desired structure from deadwood and trees within a shark's reach?

Perhaps the Serra lives in a structure, natural or otherwise, but has adapted to attack and destroy the homes of other sharks to display dominance, establish or defend its territory, etc., and is also capable of using this destructive potential on what it perceives to be food or a threat.

  • $\begingroup$ You'll do well here with such a devious mind (the deadwood thing), welcome to worldbuilding. Please enjoy our tour and refer to the help center as needed to get the hang of our ways. Welcome to worldbuilding. $\endgroup$ Commented Apr 28, 2021 at 23:26

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